serial: add zero-copy dma i/o api

arch/at91sam3x8e/serial.c

@@ -84,31 +84,42 @@ void arch_serial_exit(struct serial_device *dev)
 
 ssize_t arch_serial_write(struct serial_device *dev, const void *buf, size_t len)
 {
-	struct dmabuf *dmabuf = NULL;
+	int ret;
+	struct dmabuf *dmabuf = dmabuf_create(&dev->device, len);
+	if (dmabuf == NULL)
+		return -ENOMEM;
+
+	memcpy(dmabuf->data, buf, len);
+	ret = serial_write_dma(dev, dmabuf);
+	dmabuf_put(dmabuf);
+	return ret;
+}
+
+ssize_t serial_write_dma(struct serial_device *dev, struct dmabuf *buf)
+{
+	uint16_t len;
 	struct arch_serial_device *arch_dev = to_arch_serial_device(dev);
 
+	dmabuf_get(buf);
+
 	if (arch_dev->tx_next != NULL)
 		return -EBUSY;
 
-	if (len >= (1 << 16)) /* DMA uses 16-bit counters */
+	if (buf->len >= 0xffff)
 		len = 0xffff;
-
-	dmabuf = dmabuf_create(&dev->device, len);
-	if (dmabuf == NULL)
-		return -ENOMEM;
-
-	memcpy(&dmabuf->data[0], buf, len);
+	else
+		len = (uint16_t)buf->len;
 
 	if (arch_dev->tx_current == NULL) {
-		arch_dev->tx_current = dmabuf;
-		REG_UART_PDC_TPR = (uint32_t)&dmabuf->data[0];
-		REG_UART_PDC_TCR = (uint16_t)dmabuf->len;
+		arch_dev->tx_current = buf;
+		REG_UART_PDC_TPR = (uint32_t)buf->data;
+		REG_UART_PDC_TCR = len;
 		/* we weren't transmitting, so the interrupt was masked */
 		REG_UART_IER = REG_UART_IER_ENDTX_MASK;
 	} else {
-		arch_dev->tx_next = dmabuf;
-		REG_UART_PDC_TNPR = (uint32_t)&dmabuf->data[0];
-		REG_UART_PDC_TNCR = (uint16_t)dmabuf->len;
+		arch_dev->tx_next = buf;
+		REG_UART_PDC_TNPR = (uint32_t)buf->data;
+		REG_UART_PDC_TNCR = len;
 	}
 
 	return (ssize_t)len;

include/arch/serial.h

@@ -5,6 +5,7 @@
 
 #include <arch/arch_include.h>
 
+#include <ardix/dma.h>
 #include <ardix/serial.h>
 
 int arch_serial_init(struct serial_device *dev);
@@ -21,6 +22,18 @@ void arch_serial_exit(struct serial_device *dev);
  */
 ssize_t arch_serial_write(struct serial_device *dev, const void *buf, size_t len);
 
+/**
+ * Directly enqueue a DMA buffer to a serial device, resulting in a zero-copy
+ * write.  This will increment the buffer's refcount and decrement it again when
+ * it has been written out completely, so the caller is responsible for calling
+ * `dmabuf_put` as well in order to prevent a memory leak.
+ *
+ * @param dev: serial device to write to
+ * @param buf: raw DMA buffer to append
+ * @returns actual bytes that will be written, or a negative error code
+ */
+ssize_t serial_write_dma(struct serial_device *dev, struct dmabuf *buf);
+
 #include ARCH_INCLUDE(serial.h)
 
 /*

kernel/fs/write.c

@@ -1,6 +1,9 @@
 /* SPDX-License-Identifier: GPL-3.0-or-later */
 /* See the end of this file for copyright, license, and warranty information. */
 
+#include <arch/serial.h>
+
+#include <ardix/dma.h>
 #include <ardix/malloc.h>
 #include <ardix/serial.h>
 #include <ardix/syscall.h>
@@ -13,20 +16,19 @@
 ssize_t sys_write(int fd, __user const void *buf, size_t len)
 {
 	ssize_t ret;
-	void *copy;
+	struct dmabuf *dma;
 
 	if (fd != 1) /* we only support stdout (serial console) right now */
 		return -EBADF;
 
-	copy = malloc(len);
-	if (copy == NULL)
+	dma = dmabuf_create(&serial_default_device->device, len);
+	if (dma == NULL)
 		return -ENOMEM;
-	ret = (ssize_t)copy_from_user(copy, buf, len);
 
+	copy_from_user(dma->data, buf, len);
 	/* TODO: reschedule if blocking */
-	ret = serial_write(serial_default_device, copy, (size_t)ret);
-
-	free(copy);
+	ret = serial_write_dma(serial_default_device, dma);
+	dmabuf_put(dma);
 
 	return ret;
 }