Add base stuff for preemptive scheduler

4 years ago · c19e131f20
parent d5c958012b
commit c19e131f20
13 changed files with 700 additions and 1 deletions
--- a/1
+++ b/1
@ -76,6 +76,7 @@ include arch/Makefile

 include init/Makefile
 include lib/Makefile
+include kernel/Makefile

 ARDIX_OBJS = $(ARDIX_SOURCES:.c=.o)
 ARDIX_ASM_OBJS = $(ARDIX_ASM_SOURCES:.S=.o)
--- a/arch/at91sam3x8e/Makefile
+++ b/arch/at91sam3x8e/Makefile
@ -26,10 +26,15 @@
 ARDIX_ARCH_PWD = $(PWD)/arch/at91sam3x8e

 ARDIX_SOURCES += \
+	$(ARDIX_ARCH_PWD)/isr_sched.c \
 	$(ARDIX_ARCH_PWD)/startup.c

+ARDIX_ASM_SOURCES += \
+	$(ARDIX_ARCH_PWD)/isr_pend_sv.S
+
 CFLAGS += \
 	-DARCH_AT91SAM3X8E

 LDFLAGS += \
+	-T$(ARDIX_ARCH_PWD)/config.ld \
 	-T$(ARDIX_ARCH_PWD)/flash.ld
--- a/arch/at91sam3x8e/config.ld
+++ b/arch/at91sam3x8e/config.ld
@ -0,0 +1,32 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are permitted
+ * provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+ *    conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+ * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/*
+ * stack size is 32K (uses the entire sram1 bank)
+ * TODO: modify flash.ld to actually align this properly
+ */
+STACK_SIZE = 0x8000;
--- a/arch/at91sam3x8e/isr_pend_sv.S
+++ b/arch/at91sam3x8e/isr_pend_sv.S
@ -0,0 +1,98 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are permitted
+ * provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+ *    conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+ * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+.syntax unified
+.thumb
+
+.text
+
+.extern sched_process_switch
+
+.thumb_func
+.global isr_pend_sv
+.type isr_pend_sv, %function
+
+/* void isr_pend_sv(void); */
+isr_pend_sv:
+	/*
+	 * There seems to be a limitation in Thumb which prohibits popping to
+	 * any register > r7.  Hence, we need to split the PUSHes and POPs into
+	 * multiple operations and MOV register values around.
+	 */
+
+	/*
+	 * Some registers have already been saved by hardware at this point,
+	 * we only need to take care of r4-r11 and lr (the latter of which is
+	 * required because lr is overwritten when entering the isr).
+	 * The stuff we push onto the stack manually looks about like this:
+	 *
+	 * >>> stack grow direction >>>
+	 * r7 r6 r5 r4 lr r11 r10 r9 r8
+	 */
+
+	push	{r4-r7}			/* r4-r7 */
+
+	mov	r3,	r8
+	mov	r4,	r9
+	mov	r5,	r10
+	mov	r6,	r11
+	mov	r7,	lr
+	push	{r3-r7}			/* r8-r11, lr */
+
+	/*
+	 * Now that our stack is completely saved, we can proceed to call the
+	 * Kernel's scheduler.  This updates `_current_process` to the process
+	 * we want to execute next.
+	 */
+	/* TODO: Implement banked stack pointer */
+	mov	r0,	sp
+	bl	sched_process_switch	/* sp = sched_process_switch(sp); */
+	mov	sp,	r0
+
+	/*
+	 * The new stack pointer contains the state of the new process, so we
+	 * load it into our registers using the same procedure as above,
+	 * just in reverse order.
+	 */
+
+	pop	{r3-r7}			/* r8-r11, lr */
+	mov	lr,	r7
+	mov	r11,	r6
+	mov	r10,	r5
+	mov	r9,	r4
+	mov	r8,	r3
+
+	pop	{r4-r7}			/* r4-r7 */
+
+	/*
+	 * ... and now get the fuck outta here, hoping I never need to touch
+	 * this again, even though deep inside I know I will return here at some
+	 * point because something broke (or I feel the need to micro-optimize).
+	 */
+	bx	lr
+
+.size isr_pend_sv, .-isr_pend_sv
--- a/arch/at91sam3x8e/isr_sched.c
+++ b/arch/at91sam3x8e/isr_sched.c
@ -0,0 +1,48 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are permitted
+ * provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+ *    conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+ * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <arch/at91sam3x8e/hardware.h>
+#include <arch/at91sam3x8e/interrupt.h>
+#include <ardix/sched.h>
+#include <toolchain.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
+void isr_sys_tick(void)
+{
+	/*
+	 * fire a PendSV interrupt and do the actual context switching there
+	 * because it is faster that way (according to the docs, at least)
+	 */
+	SCB_ICSR |= SCB_PENDSVSET_MASK;
+}
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif /* __cplusplus */
--- a/include/arch/at91sam3x8e/hardware.h
+++ b/include/arch/at91sam3x8e/hardware.h
@ -0,0 +1,198 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are permitted
+ * provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+ *    conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+ * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <ardix/types.h>
+
+/** CPU clock frequency in Hertz */
+#define F_CPU 84000000UL
+
+/** Pointer size in bytes */
+#define PTRSIZE 4
+
+/* The following three structs are basically just for reference */
+
+/**
+ * A software snapshot of all registers that have not been saved by automated
+ * hardware routines on ISR entry.  Required for scheduling / context switching.
+ */
+struct reg_sw_snapshot {
+	uint32_t r4;
+	uint32_t r5;
+	uint32_t r6;
+	uint32_t r7;
+	uint32_t r8;
+	uint32_t r9;
+	uint32_t r10;
+	uint32_t r11;
+	/*
+	 * lr is saved by hardware, but we need to store it twice
+	 * because the ISR entry overwrites it
+	 */
+	uint32_t lr;	/* alias r14 */
+};
+
+/**
+ * All registers that are automatically saved by hardware routines when entering
+ * an ISR, in the correct order.
+ */
+struct reg_hw_snapshot {
+	uint32_t r0;
+	uint32_t r1;
+	uint32_t r2;
+	uint32_t r3;
+	uint32_t r12;
+	uint32_t lr;	/* alias r14 */
+	uint32_t pc;	/* alias r15 */
+	uint32_t psr;
+};
+
+/** Combination of `struct reg_sw_snapshot` and `struct reg_hw_snapshot`. */
+struct reg_snapshot {
+	struct reg_sw_snapshot sw;
+	struct reg_hw_snapshot hw;
+};
+
+/*
+ * Real-time Timer (RTT)
+ */
+
+/** Real-time Timer Mode Register */
+#define REG_RTT_MR		(*(uint32_t *)0x400E1A30U)
+/** Real-time Timer Restart bitmask (for `RTT_MR`) */
+#define REG_RTTRST_MASK		((uint32_t)1 << 18)
+/** Real-time Timer Increment Interrupt Enable bitmask (for `REG_RTT_MR`) */
+#define REG_RTTINCIEN_MASK	((uint32_t)1 << 17)
+/** Alarm Interrupt Enable bitmask (for `REG_RTT_MR`) */
+#define REG_ALMIEN_MASK		((uint32_t)1 << 16)
+/** Real-time Timer Prescaler Value */
+#define REG_RTPRES		(*(uint16_t *)0x400E1A30U)
+
+/** Real-time Timer Alarm Register */
+#define REG_RTT_AR		(*(uint32_t *)0x400E1A34U)
+/** Real-time Timer Alarm Value */
+#define REG_ALMV		REG_RTT_AR
+
+/** Real-time timer Value Register */
+#define REG_RTT_VR		(*(uint32_t *)0x400E1A38U)
+/** Current Real-time Value */
+#define REG_CRTV		REG_RTT_VR
+
+/** Real-time Timer Status Register */
+#define REG_RTT_SR		(*(const uint32_t *)0x400E1A3CU)
+/** Real-time Timer Increment bitmask (for `REG_RTT_SR`) */
+#define REG_RTTINC_MASK		((uint32_t)1 << 1)
+/** Real-time Timer Alarm Status bitmask (for `REG_RTT_SR`) */
+#define REG_ALMS_MASK		((uint32_t)1)
+
+/*
+ * SysTick
+ */
+
+/** SysTick Control and Status Register */
+#define REG_SYSTICK_CTRL			(*(uint32_t *)0xE000E010U)
+/** SysTick CTRL COUNTFLAG bitmask (for `REG_SYSTICK_CTRL`) */
+#define REG_SYSTICK_CTRL_COUNTFLAG_MASK		((uint32_t)1 << 16)
+/** SysTick CTRL CLKSOURCE bitmask (for `REG_SYSTICK_CTRL`) */
+#define REG_SYSTICK_CTRL_CLKSOURCE_MASK		((uint32_t)1 << 2)
+/** SysTick CTRL exception request enable bitmask (for `REG_SYSTICK_CTRL`) */
+#define REG_SYSTICK_CTRL_TICKINT_MASK		((uint32_t)1 << 1)
+/** SysTick CTRL enable bitmask (for `REG_SYSTICK_CTRL`) */
+#define REG_SYSTICK_CTRL_ENABLE_MASK		((uint32_t)1)
+
+/** SysTick Reload Value Register */
+#define REG_SYSTICK_LOAD		(*(uint32_t *)0xE000E014U)
+#define REG_SYSTICK_LOAD_RELOAD_MASK	((uint32_t)0x00FFFFFFU)
+
+/** SysTick Current Value Register */
+#define REG_SYSTICK_VAL			(*(uint32_t *)0xE000E018U)
+/** SysTick Current Value bitmask (for `REG_SYSTICK_VAL`) */
+#define REG_SYSTICK_VAL_CURRENT_MASK	((uint32_t)0x00FFFFFFU)
+
+/** SysTick Calibration Value Register */
+#define REG_SYSTICK_CALIB		(*(uint32_t *)0xE000E01CU)
+#define REG_SYSTICK_CALIB_NOREF_MASK	((uint32_t)1 << 31)
+#define REG_SYSTICK_CALIB_SKEW_MASK	((uint32_t)1 << 30)
+#define REG_SYSTICK_CALIB_TENMS_MASK	((uint32_t)0x00FFFFFFU)
+
+/*
+ * Real-time Clock (RTC)
+ */
+
+/** RTC Control Register */
+#define REG_RTC_CR		(*(uint32_t *)0x400E1A60U)
+
+/*
+ * Universal Asynchronous Receiver Transceiver (UART)
+ */
+
+/** UART Control Register */
+#define REG_UART_CR		(*(uint32_t *)0x400E0800U)
+/** UART Control Register Reset Status Bits bitmask (for `REG_UART_CR`) */
+#define REG_RSTSTA_MASK		((uint32_t)1 << 8)
+/** UART Control Register Transmitter Disable bitmaask (for `REG_UART_CR`) */
+#define REG_TXDIS_MASK		((uint32_t)1 << 7)
+/** UART Control Register Transmitter Enable bitmaask (for `REG_UART_CR`) */
+#define REG_TXEN_MASK		((uint32_t)1 << 6)
+/** UART Control Register Receiver Disable bitmask (for `REG_UART_CR`) */
+#define REG_RXDIS_MASK		((uint32_t)1 << 5)
+/** UART Control Register Receiver Enable bitmask (for `REG_UART_CR`) */
+#define REG_RXEN_MASK		((uint32_t)1 << 4)
+/** UART Control Register Reset Transmitter bitmask (for `REG_UART_CR`) */
+#define REG_RSTTX_MASK		((uint32_t)1 << 3)
+/** UART Control Register Reset Receiver bitmask (for `REG_UART_CR`) */
+#define REG_RSTRX_MASK		((uint32_t)1 << 2)
+
+/** UART Mode Register */
+#define REG_UART_MR		(*(uint32_t *)0x400E0804U)
+
+/*
+ * Nested Vectored Interrupt Controller
+ */
+
+/** System Control Block: Interrupt Control and State Register */
+#define SCB_ICSR		(*(uint32_t *)0xE000ED04U)
+/** ICSR PendSV set-pending bit bitmask (for `SCB_ICSR`) */
+#define SCB_PENDSVSET_MASK	((uint32_t)1 << 28)
+/** ICSR PendSV clear-pending bit bitmask (for `SCB_ICSR`) */
+#define SCB_PENDSVCLR_MASK	((uint32_t)1 << 27)
+/** ICSR SysTick exception set-pending bit bitmask (for `SCB_ICSR`) */
+#define SCB_PENDSTSET_MASK	((uint32_t)1 << 26)
+/** ICSR SysTick exception clear-pending bit bitmask (for `SCB_ICSR`) */
+#define SCB_PENDSTCLR_MASK	((uint32_t)1 << 25)
+/** ICSR Interrupt pending flag, excluding Faults bitmask (for `SCB_ICSR`) */
+#define SCB_ISRPENDING_MASK	((uint32_t)1 << 22)
+/**
+ * ICSR bitmask for highest priority pending & enabled exception
+ * (for `SCB_ICSR`).  The value is shifted 12 to the left.
+ */
+#define SCB_VECTPENDING_MASK	((uint32_t)0b1111111111u << 12)
+/** ICSR bitmask for whether there are preempted active exceptions (for `SCB_ICSR`) */
+#define SCB_RETTOBASE_MASK	((uint32_t)1 << 11)
+/** ICSR active exception number bitmask (for `SCB_ICSR`) */
+#define SCB_VECTACTIVE_MASK	((uint32_t)0b111111111u)
--- a/include/arch/at91sam3x8e/interrupt.h
+++ b/include/arch/at91sam3x8e/interrupt.h
@ -44,7 +44,7 @@ void isr_svc(void);
 /** Debug handler (reserved) */
 void isr_debug_mon(void);
 /** Pending SV interrupt handler */
-void isr_pend_sv(void);
+extern void isr_pend_sv(void);
 /** SysTick interrupt handler */
 void isr_sys_tick(void);

--- a/include/arch/hardware.h
+++ b/include/arch/hardware.h
@ -0,0 +1,36 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are permitted
+ * provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+ *    conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+ * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#if defined(ARCH_ATMEGA328P)
+#error "ATmega328p is not implemented (yet?)"
+#elif defined(ARCH_AT91SAM3X8E)
+#include <arch/at91sam3x8e/hardware.h>
+#else
+#error "Unsupported architecture"
+#endif
--- a/include/ardix/sched.h
+++ b/include/ardix/sched.h
@ -0,0 +1,128 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are permitted
+ * provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+ *    conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+ * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <ardix/types.h>
+#include <arch/hardware.h>
+
+#ifndef SCHED_MAXPROC
+/** The maximum number of processes. */
+#define SCHED_MAXPROC 16
+#endif /* SCHED_MAXPROC */
+
+#if SCHED_MAXPROC > 64
+#warning "SCHED_MAXPROC is > 64, this could have a significant impact on performance"
+#endif /* SCHED_MAXPROC > 64 */
+
+enum proc_state {
+	/** Process is dead / ddoesn't exist */
+	PROC_DEAD,
+	/** Process is ready for execution or currently running. */
+	PROC_READY,
+	/** Process is waiting for its next time share. */
+	PROC_QUEUE,
+	/** Process is sleeping, `sleep_until` specifies when to wake it up. */
+	PROC_SLEEP,
+};
+
+/** Stores an entire process image. */
+struct process {
+	/** Next process in the (circular) list. */
+	struct process *next;
+	/** Stack pointer. */
+	void *sp;
+	/** Bottom of the stack (i.e. the highest address). */
+	void *stack_bottom;
+	/** If `state` is `PROC_SLEEP`, the last execution time. */
+	unsigned long int lastexec;
+	/** If `state` is `PROC_SLEEP`, the amount of us to sleep in total. */
+	unsigned long int sleep_usecs;
+	/** Process ID. */
+	pid_t pid;
+	/** Process state. */
+	enum proc_state state;
+};
+
+/** The currently executing process. */
+extern struct process *_current_process;
+
+/**
+ * Initialize the scheduler subsystem.
+ * This sets up a hardware interrupt timer (SysTick for Cortex-M3).
+ */
+int sched_init(void);
+
+/**
+ * Switch to the next process (atomic / interrupt handler context only).
+ * Must be called directly from within an interrupt routine.
+ * Takes care of the following stuff:
+ *
+ * - Selecting a new process to run, or putting the CPU to sleep
+ * - Updating the `_current_process` pointer
+ * - Setting the `state` member of both the old and new process to the
+ *   appropriate value
+ *
+ * To avoid the horrors of inline assembly, the stack pointer is modified in
+ * actual assembler files rather than here.
+ *
+ * @param curr_sp: The stack pointer of the current process.
+ * @returns The stack pointer of the new process.
+ */
+void *sched_process_switch(void *curr_sp);
+
+/**
+ * Create a new process.
+ *
+ * @param exec: The process executor.
+ * @returns A pointer to the new process, or `NULL` if something went wrong.
+ *
+ * TODO: make something like errno to tell what *exactly* went wrong
+ */
+struct process *sched_process_create(void (*exec)(void));
+
+/**
+ * Suspend the current process for the specified amount of milliseconds.
+ * Note that there are slight deviations from this time interval because of the
+ * round-robin scheduling algorithm.
+ * If the sleep time is required to be exactly accurate, use `atomic_udelay()`.
+ * Note, however, that this will block *all* other processes, even including
+ * I/O, for the entire time period.
+ *
+ * @param msecs: The amount of milliseconds to (approximately) sleep for.
+ */
+void sleep(unsigned long int msecs);
+
+/**
+ * Block the entire CPU from execution for the specified amount of microseconds.
+ * Note that this will temporarily disable the scheduler, meaning that *nothing*
+ * (not even I/O) will be executed.  The only reason you would ever want to use
+ * this is for mission-critical, very short (<= 100 us) periods of time.
+ *
+ * @param usecs: The amount of microseconds to halt the CPU for.
+ */
+void atomic_udelay(unsigned long int usecs);
--- a/include/ardix/types.h
+++ b/include/ardix/types.h
@ -0,0 +1,42 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are permitted
+ * provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+ *    conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+ * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <stdint.h>
+
+#ifndef __SIZE_TYPE__
+#define __SIZE_TYPE__ unsigned long int
+#endif /* __SIZE_TYPE__ */
+
+/** Unsigned size specifier. */
+typedef __SIZE_TYPE__		size_t;
+/** Signed size specifier (negative sizes mean error codes). */
+typedef signed long int		ssize_t;
+
+/** Process identifier. */
+typedef int8_t			pid_t;
--- a/include/toolchain.h
+++ b/include/toolchain.h
@ -39,6 +39,11 @@
 #define __always_inline inline __attribute__((always_inline))
 #endif /* __always_inline */

+#ifndef __naked
+/** Function attribute for disabling register saving. */
+#define __naked __attribute__((naked))
+#endif
+
 #ifndef __weak
 /**
 * Add the `weak` attribute to a symbol.
--- a/kernel/Makefile
+++ b/kernel/Makefile
@ -0,0 +1,29 @@
+#
+# Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+# 1. Redistributions of source code must retain the above copyright notice, this list of
+#    conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials
+#    provided with the distribution.
+# 3. Neither the name of the copyright holder nor the names of its contributors may be
+#    used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+# WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+# WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#
+
+ARDIX_KERNEL_PWD = $(PWD)/kernel
+
+ARDIX_SOURCES += \
+	$(ARDIX_KERNEL_PWD)/sched.c
--- a/kernel/sched.c
+++ b/kernel/sched.c
@ -0,0 +1,77 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2020 Felix Kopp <sandtler@sandtler.club>
+ *
+ * Redistribution and use in source and binary forms, with or without modification, are permitted
+ * provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
+ *    conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software without specific prior
+ *    written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
+ * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <ardix/clock.h>
+#include <ardix/sched.h>
+#include <ardix/types.h>
+#include <stddef.h>
+
+extern uint32_t _sstack;
+extern uint32_t _estack;
+
+struct process *_current_process;
+
+static struct process procs[SCHED_MAXPROC + 1];
+
+int sched_init(void)
+{
+	int i;
+
+	_current_process = &procs[0];
+	_current_process->next = _current_process;
+	_current_process->sp = &_sstack;
+	_current_process->stack_bottom = &_estack;
+	_current_process->pid = 0;
+	_current_process->state = PROC_READY;
+
+	for (i = 1; i < SCHED_MAXPROC + 1; i++) {
+		procs[i].state = PROC_DEAD;
+		procs[i].pid = -1;
+	}
+
+	/* TODO: initialize SysTick */
+
+	return 0;
+}
+
+void *sched_process_switch(void *curr_sp)
+{
+	struct process *nextproc = _current_process;
+	_current_process->sp = curr_sp;
+
+	while (true) {
+		nextproc = nextproc->next;
+		if (nextproc->state == PROC_QUEUE) {
+			_current_process->state = PROC_QUEUE;
+			nextproc->state = PROC_READY;
+			_current_process = nextproc;
+			break;
+		}
+	}
+
+	return _current_process->sp;
+}