[PATCH v3] USB: Improve usb_fill_* documentation

[Ricardo Ribalda]

Document the transfer buffer requirement. That is, the buffer must be
DMAble - otherwise data corruption might occur.

Acked-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
---
USB: Improve usb_fill_* documentation

After trying to "cleanup" the uvc code, I was patiently explained about
the requirements of the urb transfer buffers.

Lets make this explicit, so other developers do not make the same mistake.

To: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
To: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
To: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
To: Christoph Hellwig <hch@xxxxxx>
To: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
To: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: linux-usb@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Changes in v3:
- Improve commit message. Thanks Bagas!
- Improve field description. Thanks Alan!
- Link to v2: https://lore.kernel.org/r/20221220-usb-dmadoc-v2-0-4dd4f198113e@xxxxxxxxxxxx

Changes in v2:
- s/allocatiing/allocating/ Thanks Randy
- Link to v1: https://lore.kernel.org/r/20221220-usb-dmadoc-v1-0-28386d2eb6cd@xxxxxxxxxxxx
---
 include/linux/usb.h | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/include/linux/usb.h b/include/linux/usb.h
index 7d5325d47c45..06cde9ddca97 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -1626,14 +1626,25 @@ struct urb {
  * @urb: pointer to the urb to initialize.
  * @dev: pointer to the struct usb_device for this urb.
  * @pipe: the endpoint pipe
- * @setup_packet: pointer to the setup_packet buffer
- * @transfer_buffer: pointer to the transfer buffer
+ * @setup_packet: pointer to the setup_packet buffer. The buffer must be
+ *	suitable for DMA.
+ * @transfer_buffer: pointer to the transfer buffer. The buffer must be
+ *	suitable for DMA.
  * @buffer_length: length of the transfer buffer
  * @complete_fn: pointer to the usb_complete_t function
  * @context: what to set the urb context to.
  *
  * Initializes a control urb with the proper information needed to submit
  * it to a device.
+ *
+ * The transfer buffer and the setup_packet buffer will most likely be filled
+ * via DMA. The simplest way to get a buffer that can be DMAed to is
+ * allocating it via kmalloc() or equivalent, even for very small buffers.
+ * If the buffers are embedded in a bigger structure, there is a risk that
+ * the buffer itself, the previous field and/or the next field are corrupted
+ * due to cache incoherencies; or slowed down if they are evicted from the
+ * cache.
+ *
  */
 static inline void usb_fill_control_urb(struct urb *urb,
 					struct usb_device *dev,
@@ -1658,13 +1669,17 @@ static inline void usb_fill_control_urb(struct urb *urb,
  * @urb: pointer to the urb to initialize.
  * @dev: pointer to the struct usb_device for this urb.
  * @pipe: the endpoint pipe
- * @transfer_buffer: pointer to the transfer buffer
+ * @transfer_buffer: pointer to the transfer buffer. The buffer must be
+ *	suitable for DMA.
  * @buffer_length: length of the transfer buffer
  * @complete_fn: pointer to the usb_complete_t function
  * @context: what to set the urb context to.
  *
  * Initializes a bulk urb with the proper information needed to submit it
  * to a device.
+ *
+ * Please refer to usb_fill_control_urb() for a description of the
+ * requirements for transfer_buffer.
  */
 static inline void usb_fill_bulk_urb(struct urb *urb,
 				     struct usb_device *dev,
@@ -1687,7 +1702,8 @@ static inline void usb_fill_bulk_urb(struct urb *urb,
  * @urb: pointer to the urb to initialize.
  * @dev: pointer to the struct usb_device for this urb.
  * @pipe: the endpoint pipe
- * @transfer_buffer: pointer to the transfer buffer
+ * @transfer_buffer: pointer to the transfer buffer. The buffer must be
+ *	suitable for DMA.
  * @buffer_length: length of the transfer buffer
  * @complete_fn: pointer to the usb_complete_t function
  * @context: what to set the urb context to.
@@ -1697,6 +1713,9 @@ static inline void usb_fill_bulk_urb(struct urb *urb,
  * Initializes a interrupt urb with the proper information needed to submit
  * it to a device.
  *
+ * Please refer to usb_fill_control_urb() for a description of the
+ * requirements for transfer_buffer.
+ *
  * Note that High Speed and SuperSpeed(+) interrupt endpoints use a logarithmic
  * encoding of the endpoint interval, and express polling intervals in
  * microframes (eight per millisecond) rather than in frames (one per

---
base-commit: b6bb9676f2165d518b35ba3bea5f1fcfc0d969bf
change-id: 20221220-usb-dmadoc-29384acebd48

Best regards,
-- 
Ricardo Ribalda <ribalda@xxxxxxxxxxxx>