[RFC RESEND 0/3] vp9 v4l2 stateless uapi

Hans Verkuil hverkuil at xs4all.nl
Mon Apr 26 07:38:34 UTC 2021

Hi Andrzej,

Thank you for working on this!

On 21/04/2021 12:00, Andrzej Pietrasiewicz wrote:
> Dear All,
> This is an RFC on stateless uapi for vp9 decoding with v4l2. This work is based on https://lkml.org/lkml/2020/11/2/1043, but has been substantially reworked. The important change is that the v4l2 control used to pass boolean decoder probabilities has been made unidirectional, and is now called V4L2_CID_STATELESS_VP9_COMPRESSED_HDR_PROBS.
> In the previous proposal, to queue a frame the userspace must fully dequeue the previous one, which effectively results in a forced lockstep behavior and defeats vb2's capability to enqueue multiple buffers. Such a design was a consequence of backward probability updates being performed by the kernel driver (which has direct access to appropriate counter values) but forward probability updates being coupled with compressed header parsing performed by the userspace.
> In vp9 the boolean decoder used to decode the bitstream needs certain parameters to work. Those are probabilities, which change with each frame. After each frame is decoded it is known how many times a given symbol occured in the frame, so the probabilities can be adapted. This process is known as backward probabilities update. A next frame header can also contain information which modifies probabilities resulting from backward update. The said modification is called forward probabilities update. The data for backward update is generated by the decoder hardware, while the data for forward update is prepared by reading the compressed frame header. The natural place to parse something is userspace, while the natural place to access hardware-provided counters is the kernel. Such responsibilties assignment was used in the original work.
> To overcome the lockstep, we moved forward probability updates to the kernel, while leaving parsing them in userspace. This way the v4l2 control which is used to pass the probs becomes unidirectional (user->kernel) and the userspace can keep parsing and enqueueing succeeding frames.
> If a particular driver parses the compressed header and does backward probability updates on its own then V4L2_CID_STATELESS_VP9_COMPRESSED_HDR_PROBS does not need to be used.
> This series adds vp9 uapi in proper locations, which means it is a proper, "official" uapi, as opposed to staging uapi which was proposed in the above mentioned lkml thread.

Why? I rather liked the way that the other codec APIs started life in a private header
(like include/media/vp8-ctrls.h) and were given time to mature before moving them to
the uAPI. Is there a reason why you think that VP9 doesn't need that?

> The series adds vp9 support to rkvdec driver.
> Rebased onto media_tree.
> I kindly ask for your comments.
> - potentially fine-tune the uAPI (add/remove fields, move between structs)
> - write another driver (intended g2 @ iMX8)
> - verify the added documentation
> Regards,
> Andrzej
> Andrzej Pietrasiewicz (1):
>   media: uapi: Add VP9 stateless decoder controls
> Boris Brezillon (1):
>   media: rkvdec: Add the VP9 backend
> Ezequiel Garcia (1):
>   media: rkvdec: Fix .buf_prepare

Isn't this just a bug fix? Should it be part of this series at all?

If it is just a bug fix, then please post it separately and let me know if it is
a fix that should go to 5.13 (i.e. the current mainline) or if 5.14 is fine.



>  .../userspace-api/media/v4l/biblio.rst        |   10 +
>  .../media/v4l/ext-ctrls-codec-stateless.rst   |  523 +++
>  .../media/v4l/pixfmt-compressed.rst           |   15 +
>  .../media/v4l/vidioc-g-ext-ctrls.rst          |    8 +
>  .../media/v4l/vidioc-queryctrl.rst            |   12 +
>  .../media/videodev2.h.rst.exceptions          |    2 +
>  drivers/media/v4l2-core/v4l2-ctrls.c          |  244 ++
>  drivers/media/v4l2-core/v4l2-ioctl.c          |    1 +
>  drivers/staging/media/rkvdec/Makefile         |    2 +-
>  drivers/staging/media/rkvdec/rkvdec-vp9.c     | 2846 +++++++++++++++++
>  drivers/staging/media/rkvdec/rkvdec.c         |   62 +-
>  drivers/staging/media/rkvdec/rkvdec.h         |    6 +
>  include/media/v4l2-ctrls.h                    |    4 +
>  include/uapi/linux/v4l2-controls.h            |  455 +++
>  include/uapi/linux/videodev2.h                |    6 +
>  15 files changed, 4190 insertions(+), 6 deletions(-)
>  create mode 100644 drivers/staging/media/rkvdec/rkvdec-vp9.c

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