[RFC PATCH] USB: core: urb: add new transfer flag URB_FREE_COHERENT

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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 calling 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() when the
URB is killed, similarly to how URB_FREE_BUFFER triggers a call to
kfree().

Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
---
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.
---
 drivers/usb/core/urb.c | 3 +++
 include/linux/usb.h    | 1 +
 2 files changed, 4 insertions(+)

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..2200b3785fdb 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -1328,6 +1328,7 @@ 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	0x0400  /* 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 */
-- 
2.35.1




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