USB: serial: make bulk_out_size a lower limit

Drivers are allowed to override the default bulk-out buffer size
(endpoint maximum packet size) in order to increase throughput, but it
does not make much sense to allow buffers smaller than the default.

Note that this is already how bulk_in_size is defined.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/usb/serial/usb-serial.c

@@ -923,9 +923,8 @@ static int usb_serial_probe(struct usb_interface *interface,
 		port = serial->port[i];
 		if (kfifo_alloc(&port->write_fifo, PAGE_SIZE, GFP_KERNEL))
 			goto probe_error;
-		buffer_size = serial->type->bulk_out_size;
-		if (!buffer_size)
-			buffer_size = usb_endpoint_maxp(endpoint);
+		buffer_size = max_t(int, serial->type->bulk_out_size,
+						usb_endpoint_maxp(endpoint));
 		port->bulk_out_size = buffer_size;
 		port->bulk_out_endpointAddress = endpoint->bEndpointAddress;
 

include/linux/usb/serial.h

@@ -190,7 +190,8 @@ static inline void usb_set_serial_data(struct usb_serial *serial, void *data)
  * @num_ports: the number of different ports this device will have.
  * @bulk_in_size: minimum number of bytes to allocate for bulk-in buffer
  *	(0 = end-point size)
- * @bulk_out_size: bytes to allocate for bulk-out buffer (0 = end-point size)
+ * @bulk_out_size: minimum number of bytes to allocate for bulk-out buffer
+ *	(0 = end-point size)
  * @calc_num_ports: pointer to a function to determine how many ports this
  *	device has dynamically.  It will be called after the probe()
  *	callback is called, but before attach()