On Wed, Dec 21, 2022 at 11:15:14AM +0100, Ricardo Ribalda wrote: > 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. No, no! The setup_packet buffer will never be filled via DMA -- it is _read_ via DMA. The transfer buffer may be filled via DMA; it depends on the direction of the transfer. For OUT transfers, the buffer is read via DMA; for IN transfers it is 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. _This_ right here is the kerneldoc for usb_fill_control_urb()! You should have written "Please refer to the kerneldoc for struct urb". > */ > 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. Similar to the above. > + * > * 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 Why didn't you also make the same change to usb_fill_int_urb()? Alan Stern > > --- > base-commit: b6bb9676f2165d518b35ba3bea5f1fcfc0d969bf > change-id: 20221220-usb-dmadoc-29384acebd48 > > Best regards, > -- > Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
