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

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Date:   Thu, 14 Mar 2019 14:11:04 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        linux-media@...r.kernel.org
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Vikash Garodia <vgarodia@...eaurora.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>,
        Malathi Gottam <mgottam@...eaurora.org>
Subject: Re: [RFC PATCH] media/doc: Allow sizeimage to be set by v4l clients

On 1/21/19 11:48 AM, Stanimir Varbanov wrote:
> Hi Hans,
> 
> On 1/18/19 11:13 AM, Hans Verkuil wrote:
>> On 1/16/19 1:37 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.
>>>
>>> Signed-off-by: Stanimir Varbanov <stanimir.varbanov@...aro.org>
>>> ---
>>>  Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst | 5 ++++-
>>>  Documentation/media/uapi/v4l/pixfmt-v4l2.rst        | 3 ++-
>>>  2 files changed, 6 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst b/Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst
>>> index 7f82dad9013a..dbe0b74e9ba4 100644
>>> --- a/Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst
>>> +++ b/Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst
>>> @@ -30,7 +30,10 @@ describing all planes of that format.
>>>  
>>>      * - __u32
>>>        - ``sizeimage``
>>> -      - Maximum size in bytes required for image data in this plane.
>>> +      - Maximum size in bytes required for image data in this plane,
>>> +        set by the driver. When the image consists of variable length
>>> +        compressed data this is the maximum number of bytes required
>>> +        to hold an image, and it is allowed to be set by the client.
>>>      * - __u32
>>>        - ``bytesperline``
>>>        - Distance in bytes between the leftmost pixels in two adjacent
>>> diff --git a/Documentation/media/uapi/v4l/pixfmt-v4l2.rst b/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
>>> index 71eebfc6d853..54b6d2b67bd7 100644
>>> --- a/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
>>> +++ b/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
>>> @@ -89,7 +89,8 @@ Single-planar format structure
>>>        - Size in bytes of the buffer to hold a complete image, set by the
>>>  	driver. Usually this is ``bytesperline`` times ``height``. When
>>>  	the image consists of variable length compressed data this is the
>>> -	maximum number of bytes required to hold an image.
>>> +	maximum number of bytes required to hold an image, and it is
>>> +	allowed to be set by the client.
>>>      * - __u32
>>>        - ``colorspace``
>>>        - Image colorspace, from enum :c:type:`v4l2_colorspace`.
>>>
>>
>> Hmm. "maximum number of bytes required to hold an image": that's not actually true
>> for bitstream formats like MPEG. It's just the size of the buffer used to store the
>> bitstream, i.e. one buffer may actually contain multiple compressed images, or a
>> compressed image is split over multiple buffers.
>>
> 
> Do you want me to change something in the current documentation, i.e.
> the quoted above?

Hmm, it looks like this discussion stalled (i.e. I forgot to reply).

How about this:

"When the image consists of variable length compressed data this is the
number of bytes required by the encoder to support the worst-case
compression scenario. Clients are allowed to set this field. However,
drivers may ignore the value or modify it."

Regards,

	Hans

> 
>> Only for MJPEG is this statement true since each buffer will contain a single
>> compressed JPEG image.
>>
>> Regards,
>>
>> 	Hans
>>
>