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

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Message-ID: <49fd2002-8723-2f00-c972-8d605561b29a@linaro.org>
Date:   Thu, 16 May 2019 18:09:35 +0300
From:   Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:     Hans Verkuil <hverkuil@...all.nl>,
        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

Hi Hans,

On 5/16/19 1:40 PM, Hans Verkuil wrote:
> 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.
> 

I'd say s5p-mfc and mtk-vcodec at least.

-- 
regards,
Stan