linux-kernel - Re: [PATCH v2] media/doc: Allow sizeimage to be set by v4l clients

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Message-ID: <CAAFQd5DBUUAPV0_=thmRBTFqJE+Nd4LZRzZE20rR0D8d7Cjz5g@mail.gmail.com>
Date:   Thu, 16 May 2019 18:56:32 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Stanimir Varbanov <stanimir.varbanov@...aro.org>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] media/doc: Allow sizeimage to be set by v4l clients

On Thu, May 16, 2019 at 5:09 PM Stanimir Varbanov
<stanimir.varbanov@...aro.org> wrote:
>
> Hi Hans,
>
> On 5/14/19 11:54 AM, Hans Verkuil wrote:
> > Hi Stanimir,
> >
> > On 4/12/19 5:59 PM, Stanimir Varbanov wrote:
> >> This changes v4l2_pix_format and v4l2_plane_pix_format sizeimage
> >> field description to allow v4l clients to set bigger image size
> >> in case of variable length compressed data.
> >
> > I've been reconsidering this change. The sizeimage value in the format
> > is the minimum size a buffer should have in order to store the data of
> > an image of the width and height as described in the format.
> >
> > But there is nothing that prevents userspace from calling VIDIOC_CREATEBUFS
> > instead of VIDIOC_REQBUFS to allocate larger buffers.
>
> Sometimes CREATEBUFS cannot be implemented for a particular fw/hw.
>
> CC: Tomasz for his opinion.
>

Thanks Stanimir.

Actually, if called at the same point in time as REQBUFS, CREATE_BUFS
doesn't really differ to much, except that it gives more flexibility
for allocating the buffers and that shouldn't depend on any specific
features of hardware or firmware.

Actually, one could even allocate any buffers any time regardless of
hardware/firmware support, but the ability to use such buffers would
actually depend on such.

Perhaps we should just let the drivers return -EBUSY from CREATE_BUFS
if called at the wrong time?

> >
> > So do we really need this change?
> >

Yes, because this has worked like this all the time, but it was just
not documented. Disallowing this would break quite a bit of existing
userspace.

Best regards,
Tomasz

> > The more I think about this, the more uncomfortable I become with this change.
> >
> > Regards,
> >
> >       Hans
> >
>
> <cut>
>
> --
> regards,
> Stan