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

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Date:   Thu, 16 May 2019 12:40:45 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Tomasz Figa <tfiga@...omium.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>
Cc:     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 5/16/19 11:56 AM, Tomasz Figa wrote:
> 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.

Which drivers allow this today? I think that would be useful information
for the commit log of a v4 of this patch.

Regards,

	Hans

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