USB: serial: generalise write buffer preparation

Generalise write buffer preparation.

This allows for drivers to manipulate (e.g. add headers) to bulk out
data before it is sent.

This adds a new function pointer to usb_serial_driver:

int (*prepare_write_buffer)(struct usb_serial_port *port,
		void **dest, size_t size, const void *src, size_t count);

The function is generic and can be used with either kfifo-based or
multi-urb writes:

If *dest is NULL the implementation should allocate dest.
If src is NULL the implementation should use the port write fifo.

If not set, a generic implementation is used which simply uses memcpy or
kfifo_out.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/generic.c

@@ -167,12 +167,35 @@ void usb_serial_generic_close(struct usb_serial_port *port)
 }
 EXPORT_SYMBOL_GPL(usb_serial_generic_close);
 
+int usb_serial_generic_prepare_write_buffer(struct usb_serial_port *port,
+		void **dest, size_t size, const void *src, size_t count)
+{
+	if (!*dest) {
+		size = count;
+		*dest = kmalloc(count, GFP_ATOMIC);
+		if (!*dest) {
+			dev_err(&port->dev, "%s - could not allocate buffer\n",
+					__func__);
+			return -ENOMEM;
+		}
+	}
+	if (src) {
+		count = size;
+		memcpy(*dest, src, size);
+	} else {
+		count = kfifo_out_locked(&port->write_fifo, *dest, size,
+								&port->lock);
+	}
+	return count;
+}
+EXPORT_SYMBOL_GPL(usb_serial_generic_prepare_write_buffer);
+
 static int usb_serial_multi_urb_write(struct tty_struct *tty,
 	struct usb_serial_port *port, const unsigned char *buf, int count)
 {
 	unsigned long flags;
 	struct urb *urb;
-	unsigned char *buffer;
+	void *buffer;
 	int status;
 
 	spin_lock_irqsave(&port->lock, flags);
@@ -191,16 +214,14 @@ static int usb_serial_multi_urb_write(struct tty_struct *tty,
 		goto err_urb;
 	}
 
+	buffer = NULL;
 	count = min_t(int, count, PAGE_SIZE);
-	buffer = kmalloc(count, GFP_ATOMIC);
-	if (!buffer) {
-		dev_err(&port->dev, "%s - could not allocate buffer\n",
-				__func__);
-		status = -ENOMEM;
+	count = port->serial->type->prepare_write_buffer(port, &buffer, 0,
+								buf, count);
+	if (count < 0) {
+		status = count;
 		goto err_buf;
 	}
-
-	memcpy(buffer, buf, count);
 	usb_serial_debug_data(debug, &port->dev, __func__, count, buffer);
 	usb_fill_bulk_urb(urb, port->serial->dev,
 			usb_sndbulkpipe(port->serial->dev,
@@ -242,7 +263,6 @@ err_urb:
  */
 static int usb_serial_generic_write_start(struct usb_serial_port *port)
 {
-	unsigned char *data;
 	int result;
 	int count;
 	unsigned long flags;
@@ -255,10 +275,11 @@ static int usb_serial_generic_write_start(struct usb_serial_port *port)
 	port->write_urb_busy = 1;
 	spin_unlock_irqrestore(&port->lock, flags);
 
-	data = port->write_urb->transfer_buffer;
-	count = kfifo_out_locked(&port->write_fifo, data, port->bulk_out_size, &port->lock);
-	usb_serial_debug_data(debug, &port->dev, __func__, count, data);
-
+	count = port->serial->type->prepare_write_buffer(port,
+					&port->write_urb->transfer_buffer,
+					port->bulk_out_size, NULL, 0);
+	usb_serial_debug_data(debug, &port->dev, __func__,
+				count, port->write_urb->transfer_buffer);
 	port->write_urb->transfer_buffer_length = count;
 
 	/* send the data out the bulk port */

drivers/usb/serial/usb-serial.c

@@ -1299,6 +1299,7 @@ static void fixup_generic(struct usb_serial_driver *device)
 	set_to_generic_if_null(device, disconnect);
 	set_to_generic_if_null(device, release);
 	set_to_generic_if_null(device, process_read_urb);
+	set_to_generic_if_null(device, prepare_write_buffer);
 }
 
 int usb_serial_register(struct usb_serial_driver *driver)

include/linux/usb/serial.h

@@ -277,6 +277,9 @@ struct usb_serial_driver {
 	void (*write_bulk_callback)(struct urb *urb);
 	/* Called by the generic read bulk callback */
 	void (*process_read_urb)(struct urb *urb);
+	/* Called by the generic write implementation */
+	int (*prepare_write_buffer)(struct usb_serial_port *port,
+		void **dest, size_t size, const void *src, size_t count);
 };
 #define to_usb_serial_driver(d) \
 	container_of(d, struct usb_serial_driver, driver)
@@ -329,6 +332,8 @@ extern void usb_serial_generic_deregister(void);
 extern int usb_serial_generic_submit_read_urb(struct usb_serial_port *port,
 						 gfp_t mem_flags);
 extern void usb_serial_generic_process_read_urb(struct urb *urb);
+extern int usb_serial_generic_prepare_write_buffer(struct usb_serial_port *port,
+		void **dest, size_t size, const void *src, size_t count);
 extern int usb_serial_handle_sysrq_char(struct tty_struct *tty,
 					struct usb_serial_port *port,
 					unsigned int ch);