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Date:   Wed, 21 Dec 2022 15:26:01 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Ricardo Ribalda <ribalda@chromium.org>
Cc:     Bagas Sanjaya <bagasdotme@gmail.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Christoph Hellwig <hch@lst.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] USB: Improve usb_fill_* documentation
Message-ID: <Y6Nr2aE1PX4cvQIT@rowland.harvard.edu>
References: <20221220-usb-dmadoc-v3-0-6009f4d27631@chromium.org>
 <Y6NCNLq3V4IqHp9E@rowland.harvard.edu>
 <CANiDSCvRBX-g17ThOb=kcQ=JFmxoeicBXzg=3fqy3cARKhXBDw@mail.gmail.com>
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On Wed, Dec 21, 2022 at 08:34:42PM +0100, Ricardo Ribalda wrote:
> Hi Alan
> 
> On Wed, 21 Dec 2022 at 18:28, Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > 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@infradead.org>
> > > 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: Bagas Sanjaya <bagasdotme@gmail.com>
> > > 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 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@chromium.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, 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.
> 
> I tried to have a short sentence :), but you are right, let's make it
> explicit, at the end of the day that is the whole purpose of this
> patch :)
> 
> I have written now: will most likely be filled or read via DMA.

Okay.

> > > 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".
> >
> 
> Please try applying the patch. I believe it is on the right location.
> diff does not know about kerneldoc and this is why it copies the
> previous header (struct urb), instead of usb_fill_control_urb.
> 
> This is why it seems that all the messages are shifted one location up.

You're right; sorry for jumping to unwarranted conclusions.

> I would also prefer to keep the: refer to usb_fill_control_urb, as the
> functions are more indexed than the structures: Eg:
> https://manpages.debian.org/testing/linux-manual-4.8/usb_fill_control_urb.9.en.html
> 
> What about adding a reference to struct urb at usb_fill_control_urb() ?

That's a good idea.

Alan Stern