[RFC PATCH v2] usb: urb: add new transfer flag URB_FREE_COHERENT

[Vincent Mailhol <mailhol.vincent@wanadoo.fr>]

When allocating URB memory with kmalloc(), drivers can simply set the
URB_FREE_BUFFER flag in urb::transfer_flags and that way, the memory
will be freed in the background when killing the URB (for example with
usb_kill_anchored_urbs()).

However, there are no equivalent mechanism when allocating DMA memory
(with usb_alloc_coherent()).

This patch adds a new flag: URB_FREE_COHERENT. Setting this flag will
cause the kernel to automatically call usb_free_coherent() on the
transfer buffer when the URB is killed, similarly to how
URB_FREE_BUFFER triggers a call to kfree().

In order to have all the flags in numerical order, URB_DIR_IN is
renumbered from 0x0200 to 0x0400 so that URB_FREE_COHERENT can reuse
value 0x0200.

CC: Alan Stern <stern@rowland.harvard.edu>
CC: Rhett Aultman <rhett.aultman@samsara.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
Hi Rhett Aultman,

I put the code snippet I previously sent into a patch. It is not
tested (this is why I post it as RFC). Please feel free to add this to
your series.

** Changelog **

v1 -> v2:

  * Renumber URB_DIR_IN for 0x0200 to 0x0400 in order to keep all the
    flags in numerical order.
---
 drivers/usb/core/urb.c | 3 +++
 include/linux/usb.h    | 3 ++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/core/urb.c b/drivers/usb/core/urb.c
index 33d62d7e3929..1460fdac0b18 100644
--- a/drivers/usb/core/urb.c
+++ b/drivers/usb/core/urb.c
@@ -22,6 +22,9 @@ static void urb_destroy(struct kref *kref)
 
 	if (urb->transfer_flags & URB_FREE_BUFFER)
 		kfree(urb->transfer_buffer);
+	else if (urb->transfer_flags & URB_FREE_COHERENT)
+		usb_free_coherent(urb->dev, urb->transfer_buffer_length,
+				  urb->transfer_buffer, urb->transfer_dma);
 
 	kfree(urb);
 }
diff --git a/include/linux/usb.h b/include/linux/usb.h
index 60bee864d897..945d68ea1d76 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -1328,9 +1328,10 @@ extern int usb_disabled(void);
 #define URB_NO_INTERRUPT	0x0080	/* HINT: no non-error interrupt
 					 * needed */
 #define URB_FREE_BUFFER		0x0100	/* Free transfer buffer with the URB */
+#define URB_FREE_COHERENT	0x0200  /* Free DMA memory of transfer buffer */
 
 /* The following flags are used internally by usbcore and HCDs */
-#define URB_DIR_IN		0x0200	/* Transfer from device to host */
+#define URB_DIR_IN		0x0400	/* Transfer from device to host */
 #define URB_DIR_OUT		0
 #define URB_DIR_MASK		URB_DIR_IN
 
-- 
2.35.1