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@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: 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
---
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..033ca69b563d 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 allocatiing 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 allocatiing 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 allocatiing 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@xxxxxxxxxxxx>
