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

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Date:   Thu, 2 May 2019 15:16:54 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>
Cc:     linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] media/doc: Allow sizeimage to be set by v4l clients

On 5/2/19 2:55 PM, Mauro Carvalho Chehab wrote:
> Em Fri, 12 Apr 2019 18:59:15 +0300
> Stanimir Varbanov <stanimir.varbanov@...aro.org> escreveu:
> 
>> 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 | 13 ++++++++++++-
>>  Documentation/media/uapi/v4l/pixfmt-v4l2.rst        | 11 ++++++++++-
>>  2 files changed, 22 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst b/Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst
>> index 5688c816e334..005428a8121e 100644
>> --- a/Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst
>> +++ b/Documentation/media/uapi/v4l/pixfmt-v4l2-mplane.rst
>> @@ -31,7 +31,18 @@ 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 number of bytes required by the
>> +	codec to support the worst-case compression scenario.
>> +
>> +	For uncompressed images the driver will set the value. For
>> +	variable length compressed data clients are allowed to set
>> +	the sizeimage field, but the driver may ignore it and set the
>> +	value itself, or it may modify the provided value based on
>> +	alignment requirements or minimum/maximum size requirements.
>> +	If the client wants to leave this to the driver, then it should
>> +	set sizeimage to 0.
>>      * - __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..0f7771151db9 100644
>> --- a/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
>> +++ b/Documentation/media/uapi/v4l/pixfmt-v4l2.rst
>> @@ -89,7 +89,16 @@ 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.
>> +	number of bytes required by the codec to support the worst-case
>> +	compression scenario.
>> +
>> +	For uncompressed images the driver will set the value. For
>> +	variable length compressed data clients are allowed to set
>> +	the sizeimage field, but the driver may ignore it and set the
>> +	value itself, or it may modify the provided value based on
>> +	alignment requirements or minimum/maximum size requirements.
>> +	If the client wants to leave this to the driver, then it should
>> +	set sizeimage to 0.
> 
> It is very confusing to understand what you meant by the above paragraph,
> as you inverted the sentence order and forgot a comma.
> 
> I would, instead, write the phrases using the direct order, and break
> into two paragraphs, e. g., changing the above to:
> 
> 	"The driver will set the value for uncompressed images.
> 
> 	Clients are allowed to set the sizeimage field for variable length
> 	compressed data, but the driver may ignore it and set the
> 	value itself, or it may modify the provided value based on
> 	alignment requirements or minimum/maximum size requirements.
> 	If the client wants to leave this to the driver, then it should
> 	set sizeimage to 0."
> 
> That makes it a lot easier to read, hopefully preventing mistakes from
> app and driver developers when reading about sizeimage.
> 
> Yet, I'm not too comfortable on letting this too generic. I mean,
> how an app writer would know what formats are "variable length
> compressed data", specially since libv4l may actually change that.

It's actually quite clearly defined: compressed formats set the
V4L2_FMT_FLAG_COMPRESSED flag in VIDIOC_ENUMFMT.

Also bytesperline will be 0 for compressed formats.

Regards,

	Hans

> 
> Thanks,
> Mauro
>