[PATCH v2] USB: Improve usb_fill_* documentation

[Ricardo Ribalda <ribalda@chromium.org>]

Make the developer aware of the requirements of transfer buffer.

The buffer must be DMAble, if the developer uses an invalid buffer, data
corruption might happen.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
---
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: Randy Dunlap <rdunlap@infradead.org>
To: Alan Stern <stern@rowland.harvard.edu>
To: Christoph Hellwig <hch@lst.de>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes in v2:
- s/allocatiing/allocating/ Thanks Randy
- Link to v1: https://lore.kernel.org/r/20221220-usb-dmadoc-v1-0-28386d2eb6cd@chromium.org
---
 include/linux/usb.h | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/include/linux/usb.h b/include/linux/usb.h
index 7d5325d47c45..1144ef6e4151 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -1627,13 +1627,20 @@ struct urb {
  * @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
+ * @transfer_buffer: pointer to the transfer 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 might 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 transfer_buffer is embedded
+ * in a bigger structure, there is a risk that the previous and following
+ * fields are left in a corrupted state by the DMA engine, if the platform
+ * is not cache coherent.
  */
 static inline void usb_fill_control_urb(struct urb *urb,
 					struct usb_device *dev,
@@ -1658,13 +1665,20 @@ 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. 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.
+ *
+ * The transfer buffer might 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 transfer_buffer is embedded
+ * in a bigger structure, there is a risk that the previous and following
+ * fields are left in a corrupted state by the DMA engine, if the platform
+ * is not cache coherent.
  */
 static inline void usb_fill_bulk_urb(struct urb *urb,
 				     struct usb_device *dev,
@@ -1687,7 +1701,7 @@ 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. 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 +1711,13 @@ 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.
  *
+ * The transfer buffer might 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 transfer_buffer is embedded
+ * in a bigger structure, there is a risk that the previous and following
+ * fields are left in a corrupted state by the DMA engine, if the platform
+ * is not cache coherent.
+ *
  * 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@chromium.org>