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References: <20221214-uvc-status-alloc-v3-0-9a67616cc549@chromium.org> <Y5s+kuxCAtS8Eixj@rowland.harvard.edu>
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From:   Ricardo Ribalda <ribalda@chromium.org>
Date:   Fri, 16 Dec 2022 09:55:09 +0100
Message-ID: <CANiDSCudMRATbHU4=hyjiVhwLr6zQubXPzzpYtXCxdMPsZFcuw@mail.gmail.com>
Subject: Re: [PATCH v3 0/2] media: uvcvideo: Code cleanup for dev->status
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Max Staudt <mstaudt@chromium.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Ming Lei <tom.leiming@gmail.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Yunke Cao <yunkec@chromium.org>,
        Christoph Hellwig <hch@lst.de>, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-media@vger.kernel.org
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Hi Alan

On Thu, 15 Dec 2022 at 16:34, Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Thu, Dec 15, 2022 at 11:57:17AM +0100, Ricardo Ribalda wrote:
> > There is no need to make a kzalloc just for 16 bytes. Let's embed the data
> > into the main data structure.
> >
> > Now that we are at it, lets remove all the castings and open coding of
> > offsets for it.
> >
> > [Christoph, do you think dma wise we are violating any non written rules? :) thanks]
>
> There _is_ a rule, and it is not exactly unwritten.  The kerneldoc for
> the transfer_buffer member of struct urb says:
>
>         This buffer must be suitable for DMA; allocate it with
>         kmalloc() or equivalent.
>
> Which in general means that the buffer must not be part of a larger
> structure -- not unless the driver can guarantee that the structure will
> _never_ be accessed while a USB transfer to/from the buffer is taking
> place.
>

Thanks a lot for the clarification. I was mainly looking at the kerneldoc from:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/usb.h#n1687

and I could not see any reference to the DMA requirements.

Mind if I send a patch to add a reference there?


> There are examples all over the USB subsystem where buffers as small as
> one or two bytes get kmalloc'ed in order to obey this rule.  16 bytes is
> certainly big enough that you shouldn't worry about it being allocated
> separately.
>
Yep, we should keep it malloced. Thanks a lot for looking into this :)


> Alan Stern


-- 
Ricardo Ribalda