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SubjectRe: [PATCH v2] media/doc: Allow sizeimage to be set by v4l clients
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On 5/2/19 2:55 PM, Mauro Carvalho Chehab wrote:
> Em Fri, 12 Apr 2019 18:59:15 +0300
> Stanimir Varbanov <stanimir.varbanov@linaro.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@linaro.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
>

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