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<chapter id="scsi">
<title>Common Access Method SCSI Controllers</title>
<para><emphasis>This chapter was written by &a.babkin;
Modifications for the handbook made by
&a.murray;.</emphasis></para>
<sect1 id="scsi-synopsis">
<title>Synopsis</title>
<para>This document assumes that the reader has a general
understanding of device drivers in FreeBSD and of the SCSI
protocol. Much of the information in this document was
extracted from the drivers:</para>
<itemizedlist>
<listitem><para>ncr (<filename>/sys/pci/ncr.c</filename>) by
Wolfgang Stanglmeier and Stefan Esser</para></listitem>
<listitem><para>sym (<filename>/sys/dev/sym/sym_hipd.c</filename>) by
Gerard Roudier</para></listitem>
<listitem><para>aic7xxx
(<filename>/sys/dev/aic7xxx/aic7xxx.c</filename>) by Justin
T. Gibbs</para></listitem>
</itemizedlist>
<para>and from the CAM code itself (by Justing T. Gibbs, see
<filename>/sys/cam/*</filename>). When some solution looked the
most logical and was essentially verbatim extracted from the code
by Justin Gibbs, I marked it as <quote>recommended</quote>.</para>
<para>The document is illustrated with examples in
pseudo-code. Although sometimes the examples have many details
and look like real code, it is still pseudo-code. It was written
to demonstrate the concepts in an understandable way. For a real
driver other approaches may be more modular and efficient. It
also abstracts from the hardware details, as well as issues that
would cloud the demonstrated concepts or that are supposed to be
described in the other chapters of the developers handbook. Such
details are commonly shown as calls to functions with descriptive
names, comments or pseudo-statements. Fortunately real life
full-size examples with all the details can be found in the real
drivers.</para>
</sect1>
<sect1 id="scsi-general">
<title>General architecture</title>
<para>CAM stands for Common Access Method. It is a generic way to
address the I/O buses in a SCSI-like way. This allows a
separation of the generic device drivers from the drivers
controlling the I/O bus: for example the disk driver becomes able
to control disks on both SCSI, IDE, and/or any other bus so the
disk driver portion does not have to be rewritten (or copied and
modified) for every new I/O bus. Thus the two most important
active entities are:</para>
<itemizedlist>
<listitem><para><emphasis>Peripheral Modules</emphasis> - a
driver for peripheral devices (disk, tape, CDROM,
etc.)</para></listitem>
<listitem><para><emphasis>SCSI Interface Modules </emphasis>(SIM)
- a Host Bus Adapter drivers for connecting to an I/O bus such
as SCSI or IDE.</para></listitem>
</itemizedlist>
<para>A peripheral driver receives requests from the OS, converts
them to a sequence of SCSI commands and passes these SCSI
commands to a SCSI Interface Module. The SCSI Interface Module
is responsible for passing these commands to the actual hardware
(or if the actual hardware is not SCSI but, for example, IDE
then also converting the SCSI commands to the native commands of
the hardware).</para>
<para>Because we are interested in writing a SCSI adapter driver
here, from this point on we will consider everything from the
SIM standpoint.</para>
<para>A typical SIM driver needs to include the following
CAM-related header files:</para>
<programlisting>#include <cam/cam.h>
#include <cam/cam_ccb.h>
#include <cam/cam_sim.h>
#include <cam/cam_xpt_sim.h>
#include <cam/cam_debug.h>
#include <cam/scsi/scsi_all.h></programlisting>
<para>The first thing each SIM driver must do is register itself
with the CAM subsystem. This is done during the driver's
<function>xxx_attach()</function> function (here and further
xxx_ is used to denote the unique driver name prefix). The
<function>xxx_attach()</function> function itself is called by
the system bus auto-configuration code which we do not describe
here.</para>
<para>This is achieved in multiple steps: first it is necessary to
allocate the queue of requests associated with this SIM:</para>
<programlisting> struct cam_devq *devq;
if(( devq = cam_simq_alloc(SIZE) )==NULL) {
error; /* some code to handle the error */
}</programlisting>
<para>Here SIZE is the size of the queue to be allocated, maximal
number of requests it could contain. It is the number of requests
that the SIM driver can handle in parallel on one SCSI
card. Commonly it can be calculated as:</para>
<programlisting>SIZE = NUMBER_OF_SUPPORTED_TARGETS * MAX_SIMULTANEOUS_COMMANDS_PER_TARGET</programlisting>
<para>Next we create a descriptor of our SIM:</para>
<programlisting> struct cam_sim *sim;
if(( sim = cam_sim_alloc(action_func, poll_func, driver_name,
softc, unit, max_dev_transactions,
max_tagged_dev_transactions, devq) )==NULL) {
cam_simq_free(devq);
error; /* some code to handle the error */
}</programlisting>
<para>Note that if we are not able to create a SIM descriptor we
free the <structname>devq</structname> also because we can do
nothing else with it and we want to conserve memory.</para>
<para>If a SCSI card has multiple SCSI buses on it then each bus
requires its own <structname>cam_sim</structname>
structure.</para>
<para>An interesting question is what to do if a SCSI card has
more than one SCSI bus, do we need one
<structname>devq</structname> structure per card or per SCSI
bus? The answer given in the comments to the CAM code is:
either way, as the driver's author prefers.</para>
<para>The arguments are:
<itemizedlist>
<listitem><para><function>action_func</function> - pointer to
the driver's <function>xxx_action</function> function.
<funcSynopsis><funcPrototype>
<funcDef>static void
<function>xxx_action</function>
</funcDef>
<paramdef>
<parameter>struct cam_sim *sim</parameter>,
<parameter>union ccb *ccb</parameter>
</paramdef>
</funcPrototype></funcSynopsis>
</para></listitem>
<listitem><para><function>poll_func</function> - pointer to
the driver's <function>xxx_poll()</function>
<funcSynopsis><funcPrototype>
<funcDef>static void
<function>xxx_poll</function>
</funcDef>
<paramdef>
<parameter>struct cam_sim *sim</parameter>
</paramdef>
</funcPrototype></funcSynopsis>
</para></listitem>
<listitem><para>driver_name - the name of the actual driver,
such as <quote>ncr</quote> or <quote>wds</quote>.</para></listitem>
<listitem><para><structName>softc</structName> - pointer to the
driver's internal descriptor for this SCSI card. This
pointer will be used by the driver in future to get private
data.</para></listitem>
<listitem><para>unit - the controller unit number, for example
for controller <quote>wds0</quote> this number will be
0</para></listitem>
<listitem><para>max_dev_transactions - maximal number of
simultaneous transactions per SCSI target in the non-tagged
mode. This value will be almost universally equal to 1, with
possible exceptions only for the non-SCSI cards. Also the
drivers that hope to take advantage by preparing one
transaction while another one is executed may set it to 2
but this does not seem to be worth the
complexity.</para></listitem>
<listitem><para>max_tagged_dev_transactions - the same thing,
but in the tagged mode. Tags are the SCSI way to initiate
multiple transactions on a device: each transaction is
assigned a unique tag and the transaction is sent to the
device. When the device completes some transaction it sends
back the result together with the tag so that the SCSI
adapter (and the driver) can tell which transaction was
completed. This argument is also known as the maximal tag
depth. It depends on the abilities of the SCSI
adapter.</para></listitem>
</itemizedlist>
</para>
<para>Finally we register the SCSI buses associated with our SCSI
adapter:</para>
<programlisting> if(xpt_bus_register(sim, bus_number) != CAM_SUCCESS) {
cam_sim_free(sim, /*free_devq*/ TRUE);
error; /* some code to handle the error */
}</programlisting>
<para>If there is one <structName>devq</structName> structure per
SCSI bus (i.e. we consider a card with multiple buses as
multiple cards with one bus each) then the bus number will
always be 0, otherwise each bus on the SCSI card should be get a
distinct number. Each bus needs its own separate structure
cam_sim.</para>
<para>After that our controller is completely hooked to the CAM
system. The value of <structName>devq</structName> can be
discarded now: sim will be passed as an argument in all further
calls from CAM and devq can be derived from it.</para>
<para>CAM provides the framework for such asynchronous
events. Some events originate from the lower levels (the SIM
drivers), some events originate from the peripheral drivers,
some events originate from the CAM subsystem itself. Any driver
can register callbacks for some types of the asynchronous
events, so that it would be notified if these events
occur.</para>
<para>A typical example of such an event is a device reset. Each
transaction and event identifies the devices to which it applies
by the means of <quote>path</quote>. The target-specific events normally
occur during a transaction with this device. So the path from
that transaction may be re-used to report this event (this is
safe because the event path is copied in the event reporting
routine but not deallocated nor passed anywhere further). Also
it is safe to allocate paths dynamically at any time including
the interrupt routines, although that incurs certain overhead,
and a possible problem with this approach is that there may be
no free memory at that time. For a bus reset event we need to
define a wildcard path including all devices on the bus. So we
can create the path for the future bus reset events in advance
and avoid problems with the future memory shortage:</para>
<programlisting> struct cam_path *path;
if(xpt_create_path(&path, /*periph*/NULL,
cam_sim_path(sim), CAM_TARGET_WILDCARD,
CAM_LUN_WILDCARD) != CAM_REQ_CMP) {
xpt_bus_deregister(cam_sim_path(sim));
cam_sim_free(sim, /*free_devq*/TRUE);
error; /* some code to handle the error */
}
softc->wpath = path;
softc->sim = sim;</programlisting>
<para>As you can see the path includes:</para>
<itemizedlist>
<listitem><para>ID of the peripheral driver (NULL here because we have
none)</para></listitem>
<listitem><para>ID of the SIM driver
(<function>cam_sim_path(sim)</function>)</para></listitem>
<listitem><para>SCSI target number of the device (CAM_TARGET_WILDCARD
means <quote>all devices</quote>)</para></listitem>
<listitem><para>SCSI LUN number of the subdevice (CAM_LUN_WILDCARD means
<quote>all LUNs</quote>)</para></listitem>
</itemizedlist>
<para>If the driver can not allocate this path it will not be able to
work normally, so in that case we dismantle that SCSI
bus.</para>
<para>And we save the path pointer in the
<structName>softc</structName> structure for future use. After
that we save the value of sim (or we can also discard it on the
exit from <function>xxx_probe()</function> if we wish).</para>
<para>That is all for a minimalistic initialization. To do things
right there is one more issue left. </para>
<para>For a SIM driver there is one particularly interesting
event: when a target device is considered lost. In this case
resetting the SCSI negotiations with this device may be a good
idea. So we register a callback for this event with CAM. The
request is passed to CAM by requesting CAM action on a CAM
control block for this type of request:</para>
<programlisting> struct ccb_setasync csa;
xpt_setup_ccb(&csa.ccb_h, path, /*priority*/5);
csa.ccb_h.func_code = XPT_SASYNC_CB;
csa.event_enable = AC_LOST_DEVICE;
csa.callback = xxx_async;
csa.callback_arg = sim;
xpt_action((union ccb *)&csa);</programlisting>
<para>Now we take a look at the <function>xxx_action()</function>
and <function>xxx_poll()</function> driver entry points.</para>
<para>
<funcSynopsis><funcPrototype>
<funcDef>static void
<function>xxx_action</function>
</funcDef>
<paramdef>
<parameter>struct cam_sim *sim</parameter>,
<parameter>union ccb *ccb</parameter>
</paramdef>
</funcPrototype></funcSynopsis>
</para>
<para>Do some action on request of the CAM subsystem. Sim
describes the SIM for the request, CCB is the request
itself. CCB stands for <quote>CAM Control Block</quote>. It is a union of
many specific instances, each describing arguments for some type
of transactions. All of these instances share the CCB header
where the common part of arguments is stored.</para>
<para>CAM supports the SCSI controllers working in both initiator
(<quote>normal</quote>) mode and target (simulating a SCSI device) mode. Here
we only consider the part relevant to the initiator mode.</para>
<para>There are a few function and macros (in other words,
methods) defined to access the public data in the struct sim:</para>
<itemizedlist>
<listitem><para><function>cam_sim_path(sim)</function> - the
path ID (see above)</para></listitem>
<listitem><para><function>cam_sim_name(sim)</function> - the
name of the sim</para></listitem>
<listitem><para><function>cam_sim_softc(sim)</function> - the
pointer to the softc (driver private data)
structure</para></listitem>
<listitem><para><function> cam_sim_unit(sim)</function> - the
unit number</para></listitem>
<listitem><para><function> cam_sim_bus(sim)</function> - the bus
ID</para></listitem>
</itemizedlist>
<para>To identify the device, <function>xxx_action()</function> can
get the unit number and pointer to its structure softc using
these functions.</para>
<para>The type of request is stored in
<structField>ccb->ccb_h.func_code</structField>. So generally
<function>xxx_action()</function> consists of a big
switch:</para>
<programlisting> struct xxx_softc *softc = (struct xxx_softc *) cam_sim_softc(sim);
struct ccb_hdr *ccb_h = &ccb->ccb_h;
int unit = cam_sim_unit(sim);
int bus = cam_sim_bus(sim);
switch(ccb_h->func_code) {
case ...:
...
default:
ccb_h->status = CAM_REQ_INVALID;
xpt_done(ccb);
break;
}</programlisting>
<para>As can be seen from the default case (if an unknown command
was received) the return code of the command is set into
<structField>ccb->ccb_h.status</structField> and the completed
CCB is returned back to CAM by calling
<function>xpt_done(ccb)</function>. </para>
<para><function>xpt_done()</function> does not have to be called
from <function>xxx_action()</function>: For example an I/O
request may be enqueued inside the SIM driver and/or its SCSI
controller. Then when the device would post an interrupt
signaling that the processing of this request is complete
<function>xpt_done()</function> may be called from the interrupt
handling routine.</para>
<para>Actually, the CCB status is not only assigned as a return
code but a CCB has some status all the time. Before CCB is
passed to the <function>xxx_action()</function> routine it gets
the status CCB_REQ_INPROG meaning that it is in progress. There
are a surprising number of status values defined in
<filename>/sys/cam/cam.h</filename> which should be able to
represent the status of a request in great detail. More
interesting yet, the status is in fact a <quote>bitwise or</quote> of an
enumerated status value (the lower 6 bits) and possible
additional flag-like bits (the upper bits). The enumerated
values will be discussed later in more detail. The summary of
them can be found in the Errors Summary section. The possible
status flags are:</para>
<itemizedlist>
<listitem><para><emphasis>CAM_DEV_QFRZN</emphasis> - if the
SIM driver gets a serious error (for example, the device does
not respond to the selection or breaks the SCSI protocol) when
processing a CCB it should freeze the request queue by calling
<function>xpt_freeze_simq()</function>, return the other
enqueued but not processed yet CCBs for this device back to
the CAM queue, then set this flag for the troublesome CCB and
call <function>xpt_done()</function>. This flag causes the CAM
subsystem to unfreeze the queue after it handles the
error.</para></listitem>
<listitem><para><emphasis>CAM_AUTOSNS_VALID</emphasis> - if
the device returned an error condition and the flag
CAM_DIS_AUTOSENSE is not set in CCB the SIM driver must
execute the REQUEST SENSE command automatically to extract the
sense (extended error information) data from the device. If
this attempt was successful the sense data should be saved in
the CCB and this flag set.</para></listitem>
<listitem><para><emphasis>CAM_RELEASE_SIMQ</emphasis> - like
CAM_DEV_QFRZN but used in case there is some problem (or
resource shortage) with the SCSI controller itself. Then all
the future requests to the controller should be stopped by
<function>xpt_freeze_simq()</function>. The controller queue
will be restarted after the SIM driver overcomes the shortage
and informs CAM by returning some CCB with this flag
set.</para></listitem>
<listitem><para><emphasis>CAM_SIM_QUEUED</emphasis> - when SIM
puts a CCB into its request queue this flag should be set (and
removed when this CCB gets dequeued before being returned back
to CAM). This flag is not used anywhere in the CAM code now,
so its purpose is purely diagnostic.</para></listitem>
</itemizedlist>
<para>The function <function>xxx_action()</function> is not
allowed to sleep, so all the synchronization for resource access
must be done using SIM or device queue freezing. Besides the
aforementioned flags the CAM subsystem provides functions
<function>xpt_release_simq()</function> and
<function>xpt_release_devq()</function> to unfreeze the queues
directly, without passing a CCB to CAM.</para>
<para>The CCB header contains the following fields:</para>
<itemizedlist>
<listitem><para><emphasis>path</emphasis> - path ID for the
request</para></listitem>
<listitem><para><emphasis>target_id</emphasis> - target device
ID for the request</para></listitem>
<listitem><para><emphasis>target_lun</emphasis> - LUN ID of
the target device</para></listitem>
<listitem><para><emphasis>timeout</emphasis> - timeout
interval for this command, in milliseconds</para></listitem>
<listitem><para><emphasis>timeout_ch</emphasis> - a
convenience place for the SIM driver to store the timeout handle
(the CAM subsystem itself does not make any assumptions about
it)</para></listitem>
<listitem><para><emphasis>flags</emphasis> - various bits of
information about the request spriv_ptr0, spriv_ptr1 - fields
reserved for private use by the SIM driver (such as linking to
the SIM queues or SIM private control blocks); actually, they
exist as unions: spriv_ptr0 and spriv_ptr1 have the type (void
*), spriv_field0 and spriv_field1 have the type unsigned long,
sim_priv.entries[0].bytes and sim_priv.entries[1].bytes are byte
arrays of the size consistent with the other incarnations of the
union and sim_priv.bytes is one array, twice
bigger.</para></listitem>
</itemizedlist>
<para>The recommended way of using the SIM private fields of CCB
is to define some meaningful names for them and use these
meaningful names in the driver, like:</para>
<programlisting>#define ccb_some_meaningful_name sim_priv.entries[0].bytes
#define ccb_hcb spriv_ptr1 /* for hardware control block */</programlisting>
<para>The most common initiator mode requests are:</para>
<itemizedlist>
<listitem><para><emphasis>XPT_SCSI_IO</emphasis> - execute an
I/O transaction</para>
<para>The instance <quote>struct ccb_scsiio csio</quote> of the union ccb is
used to transfer the arguments. They are:</para>
<itemizedlist>
<listitem><para><emphasis>cdb_io</emphasis> - pointer to
the SCSI command buffer or the buffer
itself</para></listitem>
<listitem><para><emphasis>cdb_len</emphasis> - SCSI
command length</para></listitem>
<listitem><para><emphasis>data_ptr</emphasis> - pointer to
the data buffer (gets a bit complicated if scatter/gather is
used)</para></listitem>
<listitem><para><emphasis>dxfer_len</emphasis> - length of
the data to transfer</para></listitem>
<listitem><para><emphasis>sglist_cnt</emphasis> - counter
of the scatter/gather segments</para></listitem>
<listitem><para><emphasis>scsi_status</emphasis> - place
to return the SCSI status</para></listitem>
<listitem><para><emphasis>sense_data</emphasis> - buffer
for the SCSI sense information if the command returns an
error (the SIM driver is supposed to run the REQUEST SENSE
command automatically in this case if the CCB flag
CAM_DIS_AUTOSENSE is not set)</para></listitem>
<listitem><para><emphasis>sense_len</emphasis> - the
length of that buffer (if it happens to be higher than size
of sense_data the SIM driver must silently assume the
smaller value) resid, sense_resid - if the transfer of data
or SCSI sense returned an error these are the returned
counters of the residual (not transferred) data. They do not
seem to be especially meaningful, so in a case when they are
difficult to compute (say, counting bytes in the SCSI
controller's FIFO buffer) an approximate value will do as
well. For a successfully completed transfer they must be set
to zero.</para></listitem>
<listitem><para><emphasis>tag_action</emphasis> - the kind
of tag to use:
<itemizedlist>
<listitem><para>CAM_TAG_ACTION_NONE - do not use tags for this
transaction</para></listitem>
<listitem><para>MSG_SIMPLE_Q_TAG, MSG_HEAD_OF_Q_TAG,
MSG_ORDERED_Q_TAG - value equal to the appropriate tag
message (see /sys/cam/scsi/scsi_message.h); this gives only
the tag type, the SIM driver must assign the tag value
itself</para></listitem>
</itemizedlist>
</para></listitem>
</itemizedlist>
<para>The general logic of handling this request is the
following:</para>
<para>The first thing to do is to check for possible races, to
make sure that the command did not get aborted when it was
sitting in the queue:</para>
<programlisting> struct ccb_scsiio *csio = &ccb->csio;
if ((ccb_h->status & CAM_STATUS_MASK) != CAM_REQ_INPROG) {
xpt_done(ccb);
return;
}</programlisting>
<para>Also we check that the device is supported at all by our
controller:</para>
<programlisting> if(ccb_h->target_id > OUR_MAX_SUPPORTED_TARGET_ID
|| cch_h->target_id == OUR_SCSI_CONTROLLERS_OWN_ID) {
ccb_h->status = CAM_TID_INVALID;
xpt_done(ccb);
return;
}
if(ccb_h->target_lun > OUR_MAX_SUPPORTED_LUN) {
ccb_h->status = CAM_LUN_INVALID;
xpt_done(ccb);
return;
}</programlisting>
<para>Then allocate whatever data structures (such as
card-dependent hardware control block) we need to process this
request. If we ca not then freeze the SIM queue and remember
that we have a pending operation, return the CCB back and ask
CAM to re-queue it. Later when the resources become available
the SIM queue must be unfrozen by returning a ccb with the
CAM_SIMQ_RELEASE bit set in its status. Otherwise, if all went
well, link the CCB with the hardware control block (HCB) and
mark it as queued.</para>
<programlisting> struct xxx_hcb *hcb = allocate_hcb(softc, unit, bus);
if(hcb == NULL) {
softc->flags |= RESOURCE_SHORTAGE;
xpt_freeze_simq(sim, /*count*/1);
ccb_h->status = CAM_REQUEUE_REQ;
xpt_done(ccb);
return;
}
hcb->ccb = ccb; ccb_h->ccb_hcb = (void *)hcb;
ccb_h->status |= CAM_SIM_QUEUED;</programlisting>
<para>Extract the target data from CCB into the hardware control
block. Check if we are asked to assign a tag and if yes then
generate an unique tag and build the SCSI tag messages. The
SIM driver is also responsible for negotiations with the
devices to set the maximal mutually supported bus width,
synchronous rate and offset.</para>
<programlisting> hcb->target = ccb_h->target_id; hcb->lun = ccb_h->target_lun;
generate_identify_message(hcb);
if( ccb_h->tag_action != CAM_TAG_ACTION_NONE )
generate_unique_tag_message(hcb, ccb_h->tag_action);
if( !target_negotiated(hcb) )
generate_negotiation_messages(hcb);</programlisting>
<para>Then set up the SCSI command. The command storage may be
specified in the CCB in many interesting ways, specified by
the CCB flags. The command buffer can be contained in CCB or
pointed to, in the latter case the pointer may be physical or
virtual. Since the hardware commonly needs physical address we
always convert the address to the physical one.</para>
<para>A NOT-QUITE RELATED NOTE: Normally this is done by a call
to vtophys(), but for the PCI device (which account for most
of the SCSI controllers now) drivers' portability to the Alpha
architecture the conversion must be done by vtobus() instead
due to special Alpha quirks. [IMHO it would be much better to
have two separate functions, vtop() and ptobus() then vtobus()
would be a simple superposition of them.] In case if a
physical address is requested it is OK to return the CCB with
the status CAM_REQ_INVALID, the current drivers do that. But
it is also possible to compile the Alpha-specific piece of
code, as in this example (there should be a more direct way to
do that, without conditional compilation in the drivers). If
necessary a physical address can be also converted or mapped
back to a virtual address but with big pain, so we do not do
that.</para>
<programlisting> if(ccb_h->flags & CAM_CDB_POINTER) {
/* CDB is a pointer */
if(!(ccb_h->flags & CAM_CDB_PHYS)) {
/* CDB pointer is virtual */
hcb->cmd = vtobus(csio->cdb_io.cdb_ptr);
} else {
/* CDB pointer is physical */
#if defined(__alpha__)
hcb->cmd = csio->cdb_io.cdb_ptr | alpha_XXX_dmamap_or ;
#else
hcb->cmd = csio->cdb_io.cdb_ptr ;
#endif
}
} else {
/* CDB is in the ccb (buffer) */
hcb->cmd = vtobus(csio->cdb_io.cdb_bytes);
}
hcb->cmdlen = csio->cdb_len;</programlisting>
<para>Now it is time to set up the data. Again, the data storage
may be specified in the CCB in many interesting ways,
specified by the CCB flags. First we get the direction of the
data transfer. The simplest case is if there is no data to
transfer:</para>
<programlisting> int dir = (ccb_h->flags & CAM_DIR_MASK);
if (dir == CAM_DIR_NONE)
goto end_data;</programlisting>
<para>Then we check if the data is in one chunk or in a
scatter-gather list, and the addresses are physical or
virtual. The SCSI controller may be able to handle only a
limited number of chunks of limited length. If the request
hits this limitation we return an error. We use a special
function to return the CCB to handle in one place the HCB
resource shortages. The functions to add chunks are
driver-dependent, and here we leave them without detailed
implementation. See description of the SCSI command (CDB)
handling for the details on the address-translation issues.
If some variation is too difficult or impossible to implement
with a particular card it is OK to return the status
CAM_REQ_INVALID. Actually, it seems like the scatter-gather
ability is not used anywhere in the CAM code now. But at least
the case for a single non-scattered virtual buffer must be
implemented, it is actively used by CAM.</para>
<programlisting> int rv;
initialize_hcb_for_data(hcb);
if((!(ccb_h->flags & CAM_SCATTER_VALID)) {
/* single buffer */
if(!(ccb_h->flags & CAM_DATA_PHYS)) {
rv = add_virtual_chunk(hcb, csio->data_ptr, csio->dxfer_len, dir);
}
} else {
rv = add_physical_chunk(hcb, csio->data_ptr, csio->dxfer_len, dir);
}
} else {
int i;
struct bus_dma_segment *segs;
segs = (struct bus_dma_segment *)csio->data_ptr;
if ((ccb_h->flags & CAM_SG_LIST_PHYS) != 0) {
/* The SG list pointer is physical */
rv = setup_hcb_for_physical_sg_list(hcb, segs, csio->sglist_cnt);
} else if (!(ccb_h->flags & CAM_DATA_PHYS)) {
/* SG buffer pointers are virtual */
for (i = 0; i < csio->sglist_cnt; i++) {
rv = add_virtual_chunk(hcb, segs[i].ds_addr,
segs[i].ds_len, dir);
if (rv != CAM_REQ_CMP)
break;
}
} else {
/* SG buffer pointers are physical */
for (i = 0; i < csio->sglist_cnt; i++) {
rv = add_physical_chunk(hcb, segs[i].ds_addr,
segs[i].ds_len, dir);
if (rv != CAM_REQ_CMP)
break;
}
}
}
if(rv != CAM_REQ_CMP) {
/* we expect that add_*_chunk() functions return CAM_REQ_CMP
* if they added a chunk successfully, CAM_REQ_TOO_BIG if
* the request is too big (too many bytes or too many chunks),
* CAM_REQ_INVALID in case of other troubles
*/
free_hcb_and_ccb_done(hcb, ccb, rv);
return;
}
end_data:</programlisting>
<para>If disconnection is disabled for this CCB we pass this
information to the hcb:</para>
<programlisting> if(ccb_h->flags & CAM_DIS_DISCONNECT)
hcb_disable_disconnect(hcb);</programlisting>
<para>If the controller is able to run REQUEST SENSE command all
by itself then the value of the flag CAM_DIS_AUTOSENSE should
also be passed to it, to prevent automatic REQUEST SENSE if the
CAM subsystem does not want it.</para>
<para>The only thing left is to set up the timeout, pass our hcb
to the hardware and return, the rest will be done by the
interrupt handler (or timeout handler).</para>
<programlisting> ccb_h->timeout_ch = timeout(xxx_timeout, (caddr_t) hcb,
(ccb_h->timeout * hz) / 1000); /* convert milliseconds to ticks */
put_hcb_into_hardware_queue(hcb);
return;</programlisting>
<para>And here is a possible implementation of the function
returning CCB:</para>
<programlisting> static void
free_hcb_and_ccb_done(struct xxx_hcb *hcb, union ccb *ccb, u_int32_t status)
{
struct xxx_softc *softc = hcb->softc;
ccb->ccb_h.ccb_hcb = 0;
if(hcb != NULL) {
untimeout(xxx_timeout, (caddr_t) hcb, ccb->ccb_h.timeout_ch);
/* we're about to free a hcb, so the shortage has ended */
if(softc->flags & RESOURCE_SHORTAGE) {
softc->flags &= ~RESOURCE_SHORTAGE;
status |= CAM_RELEASE_SIMQ;
}
free_hcb(hcb); /* also removes hcb from any internal lists */
}
ccb->ccb_h.status = status |
(ccb->ccb_h.status & ~(CAM_STATUS_MASK|CAM_SIM_QUEUED));
xpt_done(ccb);
}</programlisting>
</listitem>
<listitem><para><emphasis>XPT_RESET_DEV</emphasis> - send the SCSI <quote>BUS
DEVICE RESET</quote> message to a device</para>
<para>There is no data transferred in CCB except the header and
the most interesting argument of it is target_id. Depending on
the controller hardware a hardware control block just like for
the XPT_SCSI_IO request may be constructed (see XPT_SCSI_IO
request description) and sent to the controller or the SCSI
controller may be immediately programmed to send this RESET
message to the device or this request may be just not supported
(and return the status CAM_REQ_INVALID). Also on completion of
the request all the disconnected transactions for this target
must be aborted (probably in the interrupt routine).</para>
<para>Also all the current negotiations for the target are lost on
reset, so they might be cleaned too. Or they clearing may be
deferred, because anyway the target would request re-negotiation
on the next transaction.</para></listitem>
<listitem><para><emphasis>XPT_RESET_BUS</emphasis> - send the RESET signal
to the SCSI bus</para>
<para>No arguments are passed in the CCB, the only interesting
argument is the SCSI bus indicated by the struct sim
pointer.</para>
<para>A minimalistic implementation would forget the SCSI
negotiations for all the devices on the bus and return the
status CAM_REQ_CMP.</para>
<para>The proper implementation would in addition actually reset
the SCSI bus (possible also reset the SCSI controller) and mark
all the CCBs being processed, both those in the hardware queue
and those being disconnected, as done with the status
CAM_SCSI_BUS_RESET. Like:</para>
<programlisting> int targ, lun;
struct xxx_hcb *h, *hh;
struct ccb_trans_settings neg;
struct cam_path *path;
/* The SCSI bus reset may take a long time, in this case its completion
* should be checked by interrupt or timeout. But for simplicity
* we assume here that it's really fast.
*/
reset_scsi_bus(softc);
/* drop all enqueued CCBs */
for(h = softc->first_queued_hcb; h != NULL; h = hh) {
hh = h->next;
free_hcb_and_ccb_done(h, h->ccb, CAM_SCSI_BUS_RESET);
}
/* the clean values of negotiations to report */
neg.bus_width = 8;
neg.sync_period = neg.sync_offset = 0;
neg.valid = (CCB_TRANS_BUS_WIDTH_VALID
| CCB_TRANS_SYNC_RATE_VALID | CCB_TRANS_SYNC_OFFSET_VALID);
/* drop all disconnected CCBs and clean negotiations */
for(targ=0; targ <= OUR_MAX_SUPPORTED_TARGET; targ++) {
clean_negotiations(softc, targ);
/* report the event if possible */
if(xpt_create_path(&path, /*periph*/NULL,
cam_sim_path(sim), targ,
CAM_LUN_WILDCARD) == CAM_REQ_CMP) {
xpt_async(AC_TRANSFER_NEG, path, &neg);
xpt_free_path(path);
}
for(lun=0; lun <= OUR_MAX_SUPPORTED_LUN; lun++)
for(h = softc->first_discon_hcb[targ][lun]; h != NULL; h = hh) {
hh=h->next;
free_hcb_and_ccb_done(h, h->ccb, CAM_SCSI_BUS_RESET);
}
}
ccb->ccb_h.status = CAM_REQ_CMP;
xpt_done(ccb);
/* report the event */
xpt_async(AC_BUS_RESET, softc->wpath, NULL);
return;</programlisting>
<para>Implementing the SCSI bus reset as a function may be a good
idea because it would be re-used by the timeout function as a
last resort if the things go wrong.</para></listitem>
<listitem><para><emphasis>XPT_ABORT</emphasis> - abort the specified
CCB</para>
<para>The arguments are transferred in the instance <quote>struct
ccb_abort cab</quote> of the union ccb. The only argument field in it
is:</para>
<para><emphasis>abort_ccb</emphasis> - pointer to the CCB to be
aborted</para>
<para>If the abort is not supported just return the status
CAM_UA_ABORT. This is also the easy way to minimally implement
this call, return CAM_UA_ABORT in any case.</para>
<para>The hard way is to implement this request honestly. First
check that abort applies to a SCSI transaction:</para>
<programlisting> struct ccb *abort_ccb;
abort_ccb = ccb->cab.abort_ccb;
if(abort_ccb->ccb_h.func_code != XPT_SCSI_IO) {
ccb->ccb_h.status = CAM_UA_ABORT;
xpt_done(ccb);
return;
}</programlisting>
<para>Then it is necessary to find this CCB in our queue. This can
be done by walking the list of all our hardware control blocks
in search for one associated with this CCB:</para>
<programlisting> struct xxx_hcb *hcb, *h;
hcb = NULL;
/* We assume that softc->first_hcb is the head of the list of all
* HCBs associated with this bus, including those enqueued for
* processing, being processed by hardware and disconnected ones.
*/
for(h = softc->first_hcb; h != NULL; h = h->next) {
if(h->ccb == abort_ccb) {
hcb = h;
break;
}
}
if(hcb == NULL) {
/* no such CCB in our queue */
ccb->ccb_h.status = CAM_PATH_INVALID;
xpt_done(ccb);
return;
}
hcb=found_hcb;</programlisting>
<para>Now we look at the current processing status of the HCB. It
may be either sitting in the queue waiting to be sent to the
SCSI bus, being transferred right now, or disconnected and
waiting for the result of the command, or actually completed by
hardware but not yet marked as done by software. To make sure
that we do not get in any races with hardware we mark the HCB as
being aborted, so that if this HCB is about to be sent to the
SCSI bus the SCSI controller will see this flag and skip
it.</para>
<programlisting> int hstatus;
/* shown as a function, in case special action is needed to make
* this flag visible to hardware
*/
set_hcb_flags(hcb, HCB_BEING_ABORTED);
abort_again:
hstatus = get_hcb_status(hcb);
switch(hstatus) {
case HCB_SITTING_IN_QUEUE:
remove_hcb_from_hardware_queue(hcb);
/* FALLTHROUGH */
case HCB_COMPLETED:
/* this is an easy case */
free_hcb_and_ccb_done(hcb, abort_ccb, CAM_REQ_ABORTED);
break;</programlisting>
<para>If the CCB is being transferred right now we would like to
signal to the SCSI controller in some hardware-dependent way
that we want to abort the current transfer. The SCSI controller
would set the SCSI ATTENTION signal and when the target responds
to it send an ABORT message. We also reset the timeout to make
sure that the target is not sleeping forever. If the command
would not get aborted in some reasonable time like 10 seconds
the timeout routine would go ahead and reset the whole SCSI bus.
Because the command will be aborted in some reasonable time we
can just return the abort request now as successfully completed,
and mark the aborted CCB as aborted (but not mark it as done
yet).</para>
<programlisting> case HCB_BEING_TRANSFERRED:
untimeout(xxx_timeout, (caddr_t) hcb, abort_ccb->ccb_h.timeout_ch);
abort_ccb->ccb_h.timeout_ch =
timeout(xxx_timeout, (caddr_t) hcb, 10 * hz);
abort_ccb->ccb_h.status = CAM_REQ_ABORTED;
/* ask the controller to abort that HCB, then generate
* an interrupt and stop
*/
if(signal_hardware_to_abort_hcb_and_stop(hcb) < 0) {
/* oops, we missed the race with hardware, this transaction
* got off the bus before we aborted it, try again */
goto abort_again;
}
break;</programlisting>
<para>If the CCB is in the list of disconnected then set it up as
an abort request and re-queue it at the front of hardware
queue. Reset the timeout and report the abort request to be
completed.</para>
<programlisting> case HCB_DISCONNECTED:
untimeout(xxx_timeout, (caddr_t) hcb, abort_ccb->ccb_h.timeout_ch);
abort_ccb->ccb_h.timeout_ch =
timeout(xxx_timeout, (caddr_t) hcb, 10 * hz);
put_abort_message_into_hcb(hcb);
put_hcb_at_the_front_of_hardware_queue(hcb);
break;
}
ccb->ccb_h.status = CAM_REQ_CMP;
xpt_done(ccb);
return;</programlisting>
<para>That is all for the ABORT request, although there is one more
issue. Because the ABORT message cleans all the ongoing
transactions on a LUN we have to mark all the other active
transactions on this LUN as aborted. That should be done in the
interrupt routine, after the transaction gets aborted.</para>
<para>Implementing the CCB abort as a function may be quite a good
idea, this function can be re-used if an I/O transaction times
out. The only difference would be that the timed out transaction
would return the status CAM_CMD_TIMEOUT for the timed out
request. Then the case XPT_ABORT would be small, like
that:</para>
<programlisting> case XPT_ABORT:
struct ccb *abort_ccb;
abort_ccb = ccb->cab.abort_ccb;
if(abort_ccb->ccb_h.func_code != XPT_SCSI_IO) {
ccb->ccb_h.status = CAM_UA_ABORT;
xpt_done(ccb);
return;
}
if(xxx_abort_ccb(abort_ccb, CAM_REQ_ABORTED) < 0)
/* no such CCB in our queue */
ccb->ccb_h.status = CAM_PATH_INVALID;
else
ccb->ccb_h.status = CAM_REQ_CMP;
xpt_done(ccb);
return;</programlisting>
</listitem>
<listitem><para><emphasis>XPT_SET_TRAN_SETTINGS</emphasis> - explicitly
set values of SCSI transfer settings</para>
<para>The arguments are transferred in the instance <quote>struct ccb_trans_setting cts</quote>
of the union ccb:</para>
<itemizedlist>
<listitem><para><emphasis>valid</emphasis> - a bitmask showing
which settings should be updated:</para></listitem>
<listitem><para><emphasis>CCB_TRANS_SYNC_RATE_VALID</emphasis>
- synchronous transfer rate</para></listitem>
<listitem><para><emphasis>CCB_TRANS_SYNC_OFFSET_VALID</emphasis>
- synchronous offset</para></listitem>
<listitem><para><emphasis>CCB_TRANS_BUS_WIDTH_VALID</emphasis>
- bus width</para></listitem>
<listitem><para><emphasis>CCB_TRANS_DISC_VALID</emphasis> -
set enable/disable disconnection</para></listitem>
<listitem><para><emphasis>CCB_TRANS_TQ_VALID</emphasis> - set
enable/disable tagged queuing</para></listitem>
<listitem><para><emphasis>flags</emphasis> - consists of two
parts, binary arguments and identification of
sub-operations. The binary arguments are:</para>
<itemizedlist>
<listitem><para><emphasis>CCB_TRANS_DISC_ENB</emphasis> - enable disconnection</para></listitem>
<listitem><para><emphasis>CCB_TRANS_TAG_ENB</emphasis> -
enable tagged queuing</para></listitem>
</itemizedlist>
</listitem>
<listitem><para>the sub-operations are:</para>
<itemizedlist>
<listitem><para><emphasis>CCB_TRANS_CURRENT_SETTINGS</emphasis>
- change the current negotiations</para></listitem>
<listitem><para><emphasis>CCB_TRANS_USER_SETTINGS</emphasis>
- remember the desired user values sync_period, sync_offset -
self-explanatory, if sync_offset==0 then the asynchronous mode
is requested bus_width - bus width, in bits (not
bytes)</para></listitem>
</itemizedlist>
</listitem>
</itemizedlist>
<para>Two sets of negotiated parameters are supported, the user
settings and the current settings. The user settings are not
really used much in the SIM drivers, this is mostly just a piece
of memory where the upper levels can store (and later recall)
its ideas about the parameters. Setting the user parameters
does not cause re-negotiation of the transfer rates. But when
the SCSI controller does a negotiation it must never set the
values higher than the user parameters, so it is essentially the
top boundary.</para>
<para>The current settings are, as the name says,
current. Changing them means that the parameters must be
re-negotiated on the next transfer. Again, these <quote>new current
settings</quote> are not supposed to be forced on the device, just they
are used as the initial step of negotiations. Also they must be
limited by actual capabilities of the SCSI controller: for
example, if the SCSI controller has 8-bit bus and the request
asks to set 16-bit wide transfers this parameter must be
silently truncated to 8-bit transfers before sending it to the
device.</para>
<para>One caveat is that the bus width and synchronous parameters
are per target while the disconnection and tag enabling
parameters are per lun.</para>
<para>The recommended implementation is to keep 3 sets of
negotiated (bus width and synchronous transfer)
parameters:</para>
<itemizedlist>
<listitem><para><emphasis>user</emphasis> - the user set, as
above</para></listitem>
<listitem><para><emphasis>current</emphasis> - those actually
in effect</para></listitem>
<listitem><para><emphasis>goal</emphasis> - those requested by
setting of the <quote>current</quote> parameters</para></listitem>
</itemizedlist>
<para>The code looks like:</para>
<programlisting> struct ccb_trans_settings *cts;
int targ, lun;
int flags;
cts = &ccb->cts;
targ = ccb_h->target_id;
lun = ccb_h->target_lun;
flags = cts->flags;
if(flags & CCB_TRANS_USER_SETTINGS) {
if(flags & CCB_TRANS_SYNC_RATE_VALID)
softc->user_sync_period[targ] = cts->sync_period;
if(flags & CCB_TRANS_SYNC_OFFSET_VALID)
softc->user_sync_offset[targ] = cts->sync_offset;
if(flags & CCB_TRANS_BUS_WIDTH_VALID)
softc->user_bus_width[targ] = cts->bus_width;
if(flags & CCB_TRANS_DISC_VALID) {
softc->user_tflags[targ][lun] &= ~CCB_TRANS_DISC_ENB;
softc->user_tflags[targ][lun] |= flags & CCB_TRANS_DISC_ENB;
}
if(flags & CCB_TRANS_TQ_VALID) {
softc->user_tflags[targ][lun] &= ~CCB_TRANS_TQ_ENB;
softc->user_tflags[targ][lun] |= flags & CCB_TRANS_TQ_ENB;
}
}
if(flags & CCB_TRANS_CURRENT_SETTINGS) {
if(flags & CCB_TRANS_SYNC_RATE_VALID)
softc->goal_sync_period[targ] =
max(cts->sync_period, OUR_MIN_SUPPORTED_PERIOD);
if(flags & CCB_TRANS_SYNC_OFFSET_VALID)
softc->goal_sync_offset[targ] =
min(cts->sync_offset, OUR_MAX_SUPPORTED_OFFSET);
if(flags & CCB_TRANS_BUS_WIDTH_VALID)
softc->goal_bus_width[targ] = min(cts->bus_width, OUR_BUS_WIDTH);
if(flags & CCB_TRANS_DISC_VALID) {
softc->current_tflags[targ][lun] &= ~CCB_TRANS_DISC_ENB;
softc->current_tflags[targ][lun] |= flags & CCB_TRANS_DISC_ENB;
}
if(flags & CCB_TRANS_TQ_VALID) {
softc->current_tflags[targ][lun] &= ~CCB_TRANS_TQ_ENB;
softc->current_tflags[targ][lun] |= flags & CCB_TRANS_TQ_ENB;
}
}
ccb->ccb_h.status = CAM_REQ_CMP;
xpt_done(ccb);
return;</programlisting>
<para>Then when the next I/O request will be processed it will
check if it has to re-negotiate, for example by calling the
function target_negotiated(hcb). It can be implemented like
this:</para>
<programlisting> int
target_negotiated(struct xxx_hcb *hcb)
{
struct softc *softc = hcb->softc;
int targ = hcb->targ;
if( softc->current_sync_period[targ] != softc->goal_sync_period[targ]
|| softc->current_sync_offset[targ] != softc->goal_sync_offset[targ]
|| softc->current_bus_width[targ] != softc->goal_bus_width[targ] )
return 0; /* FALSE */
else
return 1; /* TRUE */
}</programlisting>
<para>After the values are re-negotiated the resulting values must
be assigned to both current and goal parameters, so for future
I/O transactions the current and goal parameters would be the
same and <function>target_negotiated()</function> would return
TRUE. When the card is initialized (in
<function>xxx_attach()</function>) the current negotiation
values must be initialized to narrow asynchronous mode, the goal
and current values must be initialized to the maximal values
supported by controller.</para></listitem>
<listitem><para><emphasis>XPT_GET_TRAN_SETTINGS</emphasis> - get values of
SCSI transfer settings</para>
<para>This operations is the reverse of
XPT_SET_TRAN_SETTINGS. Fill up the CCB instance <quote>struct
ccb_trans_setting cts</quote> with data as requested by the flags
CCB_TRANS_CURRENT_SETTINGS or CCB_TRANS_USER_SETTINGS (if both
are set then the existing drivers return the current
settings). Set all the bits in the valid field.</para></listitem>
<listitem><para><emphasis>XPT_CALC_GEOMETRY</emphasis> - calculate logical
(BIOS) geometry of the disk</para>
<para>The arguments are transferred in the instance <quote>struct
ccb_calc_geometry ccg</quote> of the union ccb:</para>
<itemizedlist>
<listitem><para><emphasis>block_size</emphasis> - input, block
(A.K.A sector) size in bytes</para></listitem>
<listitem><para><emphasis>volume_size</emphasis> - input,
volume size in bytes</para></listitem>
<listitem><para><emphasis>cylinders</emphasis> - output,
logical cylinders</para></listitem>
<listitem><para><emphasis>heads</emphasis> - output, logical
heads</para></listitem>
<listitem><para><emphasis>secs_per_track</emphasis> - output,
logical sectors per track</para></listitem>
</itemizedlist>
<para>If the returned geometry differs much enough from what the
SCSI controller BIOS thinks and a disk on this SCSI controller
is used as bootable the system may not be able to boot. The
typical calculation example taken from the aic7xxx driver
is:</para>
<programlisting> struct ccb_calc_geometry *ccg;
u_int32_t size_mb;
u_int32_t secs_per_cylinder;
int extended;
ccg = &ccb->ccg;
size_mb = ccg->volume_size
/ ((1024L * 1024L) / ccg->block_size);
extended = check_cards_EEPROM_for_extended_geometry(softc);
if (size_mb > 1024 && extended) {
ccg->heads = 255;
ccg->secs_per_track = 63;
} else {
ccg->heads = 64;
ccg->secs_per_track = 32;
}
secs_per_cylinder = ccg->heads * ccg->secs_per_track;
ccg->cylinders = ccg->volume_size / secs_per_cylinder;
ccb->ccb_h.status = CAM_REQ_CMP;
xpt_done(ccb);
return;</programlisting>
<para>This gives the general idea, the exact calculation depends
on the quirks of the particular BIOS. If BIOS provides no way
set the <quote>extended translation</quote> flag in EEPROM this flag should
normally be assumed equal to 1. Other popular geometries
are:</para>
<programlisting> 128 heads, 63 sectors - Symbios controllers
16 heads, 63 sectors - old controllers</programlisting>
<para>Some system BIOSes and SCSI BIOSes fight with each other
with variable success, for example a combination of Symbios
875/895 SCSI and Phoenix BIOS can give geometry 128/63 after
power up and 255/63 after a hard reset or soft reboot.</para>
</listitem>
<listitem><para><emphasis>XPT_PATH_INQ</emphasis> - path inquiry, in other
words get the SIM driver and SCSI controller (also known as HBA
- Host Bus Adapter) properties</para>
<para>The properties are returned in the instance <quote>struct
ccb_pathinq cpi</quote> of the union ccb:</para>
<itemizedlist>
<listitem><para>version_num - the SIM driver version number, now
all drivers use 1</para></listitem>
<listitem><para>hba_inquiry - bitmask of features supported by
the controller:</para></listitem>
<listitem><para>PI_MDP_ABLE - supports MDP message (something
from SCSI3?)</para></listitem>
<listitem><para>PI_WIDE_32 - supports 32 bit wide
SCSI</para></listitem>
<listitem><para>PI_WIDE_16 - supports 16 bit wide
SCSI</para></listitem>
<listitem><para>PI_SDTR_ABLE - can negotiate synchronous
transfer rate</para></listitem>
<listitem><para>PI_LINKED_CDB - supports linked
commands</para></listitem>
<listitem><para>PI_TAG_ABLE - supports tagged
commands</para></listitem>
<listitem><para>PI_SOFT_RST - supports soft reset alternative
(hard reset and soft reset are mutually exclusive within a
SCSI bus)</para></listitem>
<listitem><para>target_sprt - flags for target mode support, 0
if unsupported</para></listitem>
<listitem><para>hba_misc - miscellaneous controller
features:</para></listitem>
<listitem><para>PIM_SCANHILO - bus scans from high ID to low
ID</para></listitem>
<listitem><para>PIM_NOREMOVE - removable devices not included in
scan</para></listitem>
<listitem><para>PIM_NOINITIATOR - initiator role not
supported</para></listitem>
<listitem><para>PIM_NOBUSRESET - user has disabled initial BUS
RESET</para></listitem>
<listitem><para>hba_eng_cnt - mysterious HBA engine count,
something related to compression, now is always set to
0</para></listitem>
<listitem><para>vuhba_flags - vendor-unique flags, unused
now</para></listitem>
<listitem><para>max_target - maximal supported target ID (7 for
8-bit bus, 15 for 16-bit bus, 127 for Fibre
Channel)</para></listitem>
<listitem><para>max_lun - maximal supported LUN ID (7 for older
SCSI controllers, 63 for newer ones)</para></listitem>
<listitem><para>async_flags - bitmask of installed Async
handler, unused now</para></listitem>
<listitem><para>hpath_id - highest Path ID in the subsystem,
unused now</para></listitem>
<listitem><para>unit_number - the controller unit number,
cam_sim_unit(sim)</para></listitem>
<listitem><para>bus_id - the bus number,
cam_sim_bus(sim)</para></listitem>
<listitem><para>initiator_id - the SCSI ID of the controller
itself</para></listitem>
<listitem><para>base_transfer_speed - nominal transfer speed in
KB/s for asynchronous narrow transfers, equals to 3300 for
SCSI</para></listitem>
<listitem><para>sim_vid - SIM driver's vendor id, a
zero-terminated string of maximal length SIM_IDLEN including
the terminating zero</para></listitem>
<listitem><para>hba_vid - SCSI controller's vendor id, a
zero-terminated string of maximal length HBA_IDLEN including
the terminating zero</para></listitem>
<listitem><para>dev_name - device driver name, a zero-terminated
string of maximal length DEV_IDLEN including the terminating
zero, equal to cam_sim_name(sim)</para></listitem>
</itemizedlist>
<para>The recommended way of setting the string fields is using
strncpy, like:</para>
<programlisting> strncpy(cpi->dev_name, cam_sim_name(sim), DEV_IDLEN);</programlisting>
<para>After setting the values set the status to CAM_REQ_CMP and mark the
CCB as done.</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="scsi-polling">
<title>Polling</title>
<funcSynopsis><funcPrototype>
<funcDef>static void
<function>xxx_poll</function>
</funcDef>
<paramdef>
<parameter>struct cam_sim *sim</parameter>
</paramdef>
</funcPrototype></funcSynopsis>
<para>The poll function is used to simulate the interrupts when
the interrupt subsystem is not functioning (for example, when
the system has crashed and is creating the system dump). The CAM
subsystem sets the proper interrupt level before calling the
poll routine. So all it needs to do is to call the interrupt
routine (or the other way around, the poll routine may be doing
the real action and the interrupt routine would just call the
poll routine). Why bother about a separate function then?
Because of different calling conventions. The
<function>xxx_poll</function> routine gets the struct cam_sim
pointer as its argument when the PCI interrupt routine by common
convention gets pointer to the struct
<structName>xxx_softc</structName> and the ISA interrupt routine
gets just the device unit number. So the poll routine would
normally look as:</para>
<programlisting>static void
xxx_poll(struct cam_sim *sim)
{
xxx_intr((struct xxx_softc *)cam_sim_softc(sim)); /* for PCI device */
}</programlisting>
<para>or</para>
<programlisting>static void
xxx_poll(struct cam_sim *sim)
{
xxx_intr(cam_sim_unit(sim)); /* for ISA device */
}</programlisting>
</sect1>
<sect1 id="scsi-async">
<title>Asynchronous Events</title>
<para>If an asynchronous event callback has been set up then the
callback function should be defined.</para>
<programlisting>static void
ahc_async(void *callback_arg, u_int32_t code, struct cam_path *path, void *arg)</programlisting>
<itemizedlist>
<listitem><para>callback_arg - the value supplied when registering the
callback</para></listitem>
<listitem><para>code - identifies the type of event</para></listitem>
<listitem><para>path - identifies the devices to which the event
applies</para></listitem>
<listitem><para>arg - event-specific argument</para></listitem>
</itemizedlist>
<para>Implementation for a single type of event, AC_LOST_DEVICE,
looks like:</para>
<programlisting> struct xxx_softc *softc;
struct cam_sim *sim;
int targ;
struct ccb_trans_settings neg;
sim = (struct cam_sim *)callback_arg;
softc = (struct xxx_softc *)cam_sim_softc(sim);
switch (code) {
case AC_LOST_DEVICE:
targ = xpt_path_target_id(path);
if(targ <= OUR_MAX_SUPPORTED_TARGET) {
clean_negotiations(softc, targ);
/* send indication to CAM */
neg.bus_width = 8;
neg.sync_period = neg.sync_offset = 0;
neg.valid = (CCB_TRANS_BUS_WIDTH_VALID
| CCB_TRANS_SYNC_RATE_VALID | CCB_TRANS_SYNC_OFFSET_VALID);
xpt_async(AC_TRANSFER_NEG, path, &neg);
}
break;
default:
break;
}</programlisting>
</sect1>
<sect1 id="scsi-interrupts">
<title>Interrupts</title>
<para>The exact type of the interrupt routine depends on the type
of the peripheral bus (PCI, ISA and so on) to which the SCSI
controller is connected.</para>
<para>The interrupt routines of the SIM drivers run at the
interrupt level splcam. So <function>splcam()</function> should
be used in the driver to synchronize activity between the
interrupt routine and the rest of the driver (for a
multiprocessor-aware driver things get yet more interesting but
we ignore this case here). The pseudo-code in this document
happily ignores the problems of synchronization. The real code
must not ignore them. A simple-minded approach is to set
<function>splcam()</function> on the entry to the other routines
and reset it on return thus protecting them by one big critical
section. To make sure that the interrupt level will be always
restored a wrapper function can be defined, like:</para>
<programlisting> static void
xxx_action(struct cam_sim *sim, union ccb *ccb)
{
int s;
s = splcam();
xxx_action1(sim, ccb);
splx(s);
}
static void
xxx_action1(struct cam_sim *sim, union ccb *ccb)
{
... process the request ...
}</programlisting>
<para>This approach is simple and robust but the problem with it
is that interrupts may get blocked for a relatively long time
and this would negatively affect the system's performance. On
the other hand the functions of the <function>spl()</function>
family have rather high overhead, so vast amount of tiny
critical sections may not be good either.</para>
<para>The conditions handled by the interrupt routine and the
details depend very much on the hardware. We consider the set of
<quote>typical</quote> conditions.</para>
<para>First, we check if a SCSI reset was encountered on the bus
(probably caused by another SCSI controller on the same SCSI
bus). If so we drop all the enqueued and disconnected requests,
report the events and re-initialize our SCSI controller. It is
important that during this initialization the controller will not
issue another reset or else two controllers on the same SCSI bus
could ping-pong resets forever. The case of fatal controller
error/hang could be handled in the same place, but it will
probably need also sending RESET signal to the SCSI bus to reset
the status of the connections with the SCSI devices.</para>
<programlisting> int fatal=0;
struct ccb_trans_settings neg;
struct cam_path *path;
if( detected_scsi_reset(softc)
|| (fatal = detected_fatal_controller_error(softc)) ) {
int targ, lun;
struct xxx_hcb *h, *hh;
/* drop all enqueued CCBs */
for(h = softc->first_queued_hcb; h != NULL; h = hh) {
hh = h->next;
free_hcb_and_ccb_done(h, h->ccb, CAM_SCSI_BUS_RESET);
}
/* the clean values of negotiations to report */
neg.bus_width = 8;
neg.sync_period = neg.sync_offset = 0;
neg.valid = (CCB_TRANS_BUS_WIDTH_VALID
| CCB_TRANS_SYNC_RATE_VALID | CCB_TRANS_SYNC_OFFSET_VALID);
/* drop all disconnected CCBs and clean negotiations */
for(targ=0; targ <= OUR_MAX_SUPPORTED_TARGET; targ++) {
clean_negotiations(softc, targ);
/* report the event if possible */
if(xpt_create_path(&path, /*periph*/NULL,
cam_sim_path(sim), targ,
CAM_LUN_WILDCARD) == CAM_REQ_CMP) {
xpt_async(AC_TRANSFER_NEG, path, &neg);
xpt_free_path(path);
}
for(lun=0; lun <= OUR_MAX_SUPPORTED_LUN; lun++)
for(h = softc->first_discon_hcb[targ][lun]; h != NULL; h = hh) {
hh=h->next;
if(fatal)
free_hcb_and_ccb_done(h, h->ccb, CAM_UNREC_HBA_ERROR);
else
free_hcb_and_ccb_done(h, h->ccb, CAM_SCSI_BUS_RESET);
}
}
/* report the event */
xpt_async(AC_BUS_RESET, softc->wpath, NULL);
/* re-initialization may take a lot of time, in such case
* its completion should be signaled by another interrupt or
* checked on timeout - but for simplicity we assume here that
* it's really fast
*/
if(!fatal) {
reinitialize_controller_without_scsi_reset(softc);
} else {
reinitialize_controller_with_scsi_reset(softc);
}
schedule_next_hcb(softc);
return;
}</programlisting>
<para>If interrupt is not caused by a controller-wide condition
then probably something has happened to the current hardware
control block. Depending on the hardware there may be other
non-HCB-related events, we just do not consider them here. Then
we analyze what happened to this HCB:</para>
<programlisting> struct xxx_hcb *hcb, *h, *hh;
int hcb_status, scsi_status;
int ccb_status;
int targ;
int lun_to_freeze;
hcb = get_current_hcb(softc);
if(hcb == NULL) {
/* either stray interrupt or something went very wrong
* or this is something hardware-dependent
*/
handle as necessary;
return;
}
targ = hcb->target;
hcb_status = get_status_of_current_hcb(softc);</programlisting>
<para>First we check if the HCB has completed and if so we check
the returned SCSI status.</para>
<programlisting> if(hcb_status == COMPLETED) {
scsi_status = get_completion_status(hcb);</programlisting>
<para>Then look if this status is related to the REQUEST SENSE
command and if so handle it in a simple way.</para>
<programlisting> if(hcb->flags & DOING_AUTOSENSE) {
if(scsi_status == GOOD) { /* autosense was successful */
hcb->ccb->ccb_h.status |= CAM_AUTOSNS_VALID;
free_hcb_and_ccb_done(hcb, hcb->ccb, CAM_SCSI_STATUS_ERROR);
} else {
autosense_failed:
free_hcb_and_ccb_done(hcb, hcb->ccb, CAM_AUTOSENSE_FAIL);
}
schedule_next_hcb(softc);
return;
}</programlisting>
<para>Else the command itself has completed, pay more attention to
details. If auto-sense is not disabled for this CCB and the
command has failed with sense data then run REQUEST SENSE
command to receive that data.</para>
<programlisting> hcb->ccb->csio.scsi_status = scsi_status;
calculate_residue(hcb);
if( (hcb->ccb->ccb_h.flags & CAM_DIS_AUTOSENSE)==0
&& ( scsi_status == CHECK_CONDITION
|| scsi_status == COMMAND_TERMINATED) ) {
/* start auto-SENSE */
hcb->flags |= DOING_AUTOSENSE;
setup_autosense_command_in_hcb(hcb);
restart_current_hcb(softc);
return;
}
if(scsi_status == GOOD)
free_hcb_and_ccb_done(hcb, hcb->ccb, CAM_REQ_CMP);
else
free_hcb_and_ccb_done(hcb, hcb->ccb, CAM_SCSI_STATUS_ERROR);
schedule_next_hcb(softc);
return;
}</programlisting>
<para>One typical thing would be negotiation events: negotiation
messages received from a SCSI target (in answer to our
negotiation attempt or by target's initiative) or the target is
unable to negotiate (rejects our negotiation messages or does
not answer them).</para>
<programlisting> switch(hcb_status) {
case TARGET_REJECTED_WIDE_NEG:
/* revert to 8-bit bus */
softc->current_bus_width[targ] = softc->goal_bus_width[targ] = 8;
/* report the event */
neg.bus_width = 8;
neg.valid = CCB_TRANS_BUS_WIDTH_VALID;
xpt_async(AC_TRANSFER_NEG, hcb->ccb.ccb_h.path_id, &neg);
continue_current_hcb(softc);
return;
case TARGET_ANSWERED_WIDE_NEG:
{
int wd;
wd = get_target_bus_width_request(softc);
if(wd <= softc->goal_bus_width[targ]) {
/* answer is acceptable */
softc->current_bus_width[targ] =
softc->goal_bus_width[targ] = neg.bus_width = wd;
/* report the event */
neg.valid = CCB_TRANS_BUS_WIDTH_VALID;
xpt_async(AC_TRANSFER_NEG, hcb->ccb.ccb_h.path_id, &neg);
} else {
prepare_reject_message(hcb);
}
}
continue_current_hcb(softc);
return;
case TARGET_REQUESTED_WIDE_NEG:
{
int wd;
wd = get_target_bus_width_request(softc);
wd = min (wd, OUR_BUS_WIDTH);
wd = min (wd, softc->user_bus_width[targ]);
if(wd != softc->current_bus_width[targ]) {
/* the bus width has changed */
softc->current_bus_width[targ] =
softc->goal_bus_width[targ] = neg.bus_width = wd;
/* report the event */
neg.valid = CCB_TRANS_BUS_WIDTH_VALID;
xpt_async(AC_TRANSFER_NEG, hcb->ccb.ccb_h.path_id, &neg);
}
prepare_width_nego_rsponse(hcb, wd);
}
continue_current_hcb(softc);
return;
}</programlisting>
<para>Then we handle any errors that could have happened during
auto-sense in the same simple-minded way as before. Otherwise we
look closer at the details again.</para>
<programlisting> if(hcb->flags & DOING_AUTOSENSE)
goto autosense_failed;
switch(hcb_status) {</programlisting>
<para>The next event we consider is unexpected disconnect. Which
is considered normal after an ABORT or BUS DEVICE RESET message
and abnormal in other cases.</para>
<programlisting> case UNEXPECTED_DISCONNECT:
if(requested_abort(hcb)) {
/* abort affects all commands on that target+LUN, so
* mark all disconnected HCBs on that target+LUN as aborted too
*/
for(h = softc->first_discon_hcb[hcb->target][hcb->lun];
h != NULL; h = hh) {
hh=h->next;
free_hcb_and_ccb_done(h, h->ccb, CAM_REQ_ABORTED);
}
ccb_status = CAM_REQ_ABORTED;
} else if(requested_bus_device_reset(hcb)) {
int lun;
/* reset affects all commands on that target, so
* mark all disconnected HCBs on that target+LUN as reset
*/
for(lun=0; lun <= OUR_MAX_SUPPORTED_LUN; lun++)
for(h = softc->first_discon_hcb[hcb->target][lun];
h != NULL; h = hh) {
hh=h->next;
free_hcb_and_ccb_done(h, h->ccb, CAM_SCSI_BUS_RESET);
}
/* send event */
xpt_async(AC_SENT_BDR, hcb->ccb->ccb_h.path_id, NULL);
/* this was the CAM_RESET_DEV request itself, it's completed */
ccb_status = CAM_REQ_CMP;
} else {
calculate_residue(hcb);
ccb_status = CAM_UNEXP_BUSFREE;
/* request the further code to freeze the queue */
hcb->ccb->ccb_h.status |= CAM_DEV_QFRZN;
lun_to_freeze = hcb->lun;
}
break;</programlisting>
<para>If the target refuses to accept tags we notify CAM about
that and return back all commands for this LUN:</para>
<programlisting> case TAGS_REJECTED:
/* report the event */
neg.flags = 0 & ~CCB_TRANS_TAG_ENB;
neg.valid = CCB_TRANS_TQ_VALID;
xpt_async(AC_TRANSFER_NEG, hcb->ccb.ccb_h.path_id, &neg);
ccb_status = CAM_MSG_REJECT_REC;
/* request the further code to freeze the queue */
hcb->ccb->ccb_h.status |= CAM_DEV_QFRZN;
lun_to_freeze = hcb->lun;
break;</programlisting>
<para>Then we check a number of other conditions, with processing
basically limited to setting the CCB status:</para>
<programlisting> case SELECTION_TIMEOUT:
ccb_status = CAM_SEL_TIMEOUT;
/* request the further code to freeze the queue */
hcb->ccb->ccb_h.status |= CAM_DEV_QFRZN;
lun_to_freeze = CAM_LUN_WILDCARD;
break;
case PARITY_ERROR:
ccb_status = CAM_UNCOR_PARITY;
break;
case DATA_OVERRUN:
case ODD_WIDE_TRANSFER:
ccb_status = CAM_DATA_RUN_ERR;
break;
default:
/* all other errors are handled in a generic way */
ccb_status = CAM_REQ_CMP_ERR;
/* request the further code to freeze the queue */
hcb->ccb->ccb_h.status |= CAM_DEV_QFRZN;
lun_to_freeze = CAM_LUN_WILDCARD;
break;
}</programlisting>
<para>Then we check if the error was serious enough to freeze the
input queue until it gets proceeded and do so if it is:</para>
<programlisting> if(hcb->ccb->ccb_h.status & CAM_DEV_QFRZN) {
/* freeze the queue */
xpt_freeze_devq(ccb->ccb_h.path, /*count*/1);
/* re-queue all commands for this target/LUN back to CAM */
for(h = softc->first_queued_hcb; h != NULL; h = hh) {
hh = h->next;
if(targ == h->targ
&& (lun_to_freeze == CAM_LUN_WILDCARD || lun_to_freeze == h->lun) )
free_hcb_and_ccb_done(h, h->ccb, CAM_REQUEUE_REQ);
}
}
free_hcb_and_ccb_done(hcb, hcb->ccb, ccb_status);
schedule_next_hcb(softc);
return;</programlisting>
<para>This concludes the generic interrupt handling although
specific controllers may require some additions.</para>
</sect1>
<sect1 id="scsi-errors">
<title>Errors Summary</title>
<para>When executing an I/O request many things may go wrong. The
reason of error can be reported in the CCB status with great
detail. Examples of use are spread throughout this document. For
completeness here is the summary of recommended responses for
the typical error conditions:</para>
<itemizedlist>
<listitem><para><emphasis>CAM_RESRC_UNAVAIL</emphasis> - some
resource is temporarily unavailable and the SIM driver cannot
generate an event when it will become available. An example of
this resource would be some intra-controller hardware resource
for which the controller does not generate an interrupt when
it becomes available.</para></listitem>
<listitem><para><emphasis>CAM_UNCOR_PARITY</emphasis> -
unrecovered parity error occurred</para></listitem>
<listitem><para><emphasis>CAM_DATA_RUN_ERR</emphasis> - data
overrun or unexpected data phase (going in other direction
than specified in CAM_DIR_MASK) or odd transfer length for
wide transfer</para></listitem>
<listitem><para><emphasis>CAM_SEL_TIMEOUT</emphasis> - selection
timeout occurred (target does not respond)</para></listitem>
<listitem><para><emphasis>CAM_CMD_TIMEOUT</emphasis> - command
timeout occurred (the timeout function ran)</para></listitem>
<listitem><para><emphasis>CAM_SCSI_STATUS_ERROR</emphasis> - the
device returned error</para></listitem>
<listitem><para><emphasis>CAM_AUTOSENSE_FAIL</emphasis> - the
device returned error and the REQUEST SENSE COMMAND
failed</para></listitem>
<listitem><para><emphasis>CAM_MSG_REJECT_REC</emphasis> - MESSAGE
REJECT message was received</para></listitem>
<listitem><para><emphasis>CAM_SCSI_BUS_RESET</emphasis> - received
SCSI bus reset</para></listitem>
<listitem><para><emphasis>CAM_REQ_CMP_ERR</emphasis> -
<quote>impossible</quote> SCSI phase occurred or something else as weird or
just a generic error if further detail is not
available</para></listitem>
<listitem><para><emphasis>CAM_UNEXP_BUSFREE</emphasis> -
unexpected disconnect occurred</para></listitem>
<listitem><para><emphasis>CAM_BDR_SENT</emphasis> - BUS DEVICE
RESET message was sent to the target</para></listitem>
<listitem><para><emphasis>CAM_UNREC_HBA_ERROR</emphasis> -
unrecoverable Host Bus Adapter Error</para></listitem>
<listitem><para><emphasis>CAM_REQ_TOO_BIG</emphasis> - the request
was too large for this controller</para></listitem>
<listitem><para><emphasis>CAM_REQUEUE_REQ</emphasis> - this
request should be re-queued to preserve transaction ordering.
This typically occurs when the SIM recognizes an error that
should freeze the queue and must place other queued requests
for the target at the sim level back into the XPT
queue. Typical cases of such errors are selection timeouts,
command timeouts and other like conditions. In such cases the
troublesome command returns the status indicating the error,
the and the other commands which have not be sent to the bus
yet get re-queued.</para></listitem>
<listitem><para><emphasis>CAM_LUN_INVALID</emphasis> - the LUN
ID in the request is not supported by the SCSI
controller</para></listitem>
<listitem><para><emphasis>CAM_TID_INVALID</emphasis> - the
target ID in the request is not supported by the SCSI
controller</para></listitem>
</itemizedlist>
</sect1>
<sect1 id="scsi-timeout">
<title>Timeout Handling</title>
<para>When the timeout for an HCB expires that request should be
aborted, just like with an XPT_ABORT request. The only
difference is that the returned status of aborted request should
be CAM_CMD_TIMEOUT instead of CAM_REQ_ABORTED (that is why
implementation of the abort better be done as a function). But
there is one more possible problem: what if the abort request
itself will get stuck? In this case the SCSI bus should be
reset, just like with an XPT_RESET_BUS request (and the idea
about implementing it as a function called from both places
applies here too). Also we should reset the whole SCSI bus if a
device reset request got stuck. So after all the timeout
function would look like:</para>
<programlisting>static void
xxx_timeout(void *arg)
{
struct xxx_hcb *hcb = (struct xxx_hcb *)arg;
struct xxx_softc *softc;
struct ccb_hdr *ccb_h;
softc = hcb->softc;
ccb_h = &hcb->ccb->ccb_h;
if(hcb->flags & HCB_BEING_ABORTED
|| ccb_h->func_code == XPT_RESET_DEV) {
xxx_reset_bus(softc);
} else {
xxx_abort_ccb(hcb->ccb, CAM_CMD_TIMEOUT);
}
}</programlisting>
<para>When we abort a request all the other disconnected requests
to the same target/LUN get aborted too. So there appears a
question, should we return them with status CAM_REQ_ABORTED or
CAM_CMD_TIMEOUT? The current drivers use CAM_CMD_TIMEOUT. This
seems logical because if one request got timed out then probably
something really bad is happening to the device, so if they
would not be disturbed they would time out by themselves.</para>
</sect1>
</chapter>