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SubjectRe: [RFC] METADATA design using V4l2 Request API
On 28/05/2020 12:48, wrote:
> Hi Hans,
> Thanks for the review.
> On 2020-05-26 16:27, Hans Verkuil wrote:
>> Hi Dikshita,
>> My apologies for the delay, this was (mostly) due to various vacation
>> days.
>> On 08/05/2020 08:21, Dikshita Agarwal wrote:
>>> There are many commercialized video use cases which needs metadata
>>> info
>>> to be circulated between v4l2 client and v4l2 driver.
>>> METADATA has following requirements associated:
>>> •Metadata is an optional info available for a buffer. It is not
>>> mandatorily for every buffer.
>>> For ex. consider metadata ROI (Region Of Interest). ROI is specified
>>> by clients to indicate
>>> the region where enhanced quality is desired. This metadata is given
>>> as an input information
>>> to encoder output plane. Client may or may not specify the ROI for a
>>> frame during encode as
>>> an input metadata. Also if the client has not provided ROI metadata
>>> for a given frame,
>>> it would be incorrect to take the metadata from previous frame. If
>>> the data and
>>> metadata is asynchronous, it would be difficult for hardware to
>>> decide if it
>>> needs to wait for metadata buffer or not before processing the input
>>> frame for encoding.
>>> •Synchronize the buffer requirement across both the video node/session
>>> (incase metadata is being processed as a separate v4l2 video
>>> node/session).
>>> This is to avoid buffer starvation.
>>> •Associate the metadata buffer with the data buffer without adding any
>>> pipeline delay
>>> in waiting for each other. This is applicable both at the hardware
>>> side (the processing end)
>>> and client side (the receiving end).
>>> •Low latency usecases like WFD/split rendering/game streaming/IMS have
>>> sub-50ms e2e latency
>>> requirements, and it is not practical to stall the pipeline due to
>>> inherent framework latencies.
>>> High performance usecase like high-frame rate playback/record can
>>> lead to frame loss during any pipeline latency.
>>> To address all above requirements, we used v4l2 Request API as
>>> interlace.
>>> As an experiment, We have introduced new control
>>> to contain the METADATA info. Exact controls can be finalized once the
>>> interface is discussed.
>>> For setting metadata from userspace to kernel, let say on encode
>>> output plane,
>>> following code sequence was followed
>>> 1. Video driver is registering for media device and creating a media
>>> node in /dev
>>> 2. Request fd is allocated by calling MEDIA_IOC_REQUEST_ALLOC IOCTL on
>>> media fd.
>>> 3. METADATA configuration is being applied on request fd using
>>> and the same request fd is added to buf structure structure before
>>> calling VIDIOC_QBUF on video fd.
>>> 4. The same request is queued through MEDIA_REQUEST_IOC_QUEUE IOCTL to
>>> driver then, as a result
>>> to which METADATA control will be applied to buffer through S_CTRL.
>>> 5. Once control is applied and request is completed,
>>> to re-initialize the request.
>> This is fine and should work well. It's what the Request API is for,
>> so no problems here.
>>> We could achieve the same on capture plane as well by removing few
>>> checks present currently
>>> in v4l2 core which restrict the implementation to only output plane.
>> Why do you need the Request API for the capture side in a
>> memory-to-memory driver? It is not
>> clear from this patch series what the use-case is. There are reasons
>> why this is currently
>> not allowed. So I need to know more about this.
>> Regards,
>> Hans
> we need this for use cases like HDR10+ where metadata info is part of
> the bitstream.
> To handle such frame specific data, support for request api on capture
> plane would be needed.

That's for the decoder, right? So the decoder extracts the HDR10+ metadata
and fills in a control with the metadata?

If that's the case, then it matches a similar request I got from mediatek.
What is needed is support for 'read-only' requests: i.e. the driver can
associate controls with a capture buffer and return that to userspace. But
it is not possible to *set* controls in userspace when queuing the request.

If you think about it you'll see that setting controls in userspace for
a capture queue request makes no sense, but having the driver add set
read-only controls when the request is finished is fine and makes sense.

Implementing this shouldn't be a big job: you'd need a new V4L2_BUF_CAP_SUPPORTS_RO_REQUESTS
capability, a corresponding new flag in struct vb2_queue, a new ro_requests flag in
struct v4l2_ctrl_handler, and try_set_ext_ctrls() should check that flag and refuse to
try/set any controls if it is true.

Finally, the v4l2_m2m_qbuf() function should be updated to just refuse the case where both
capture and output queue set V4L2_BUF_CAP_SUPPORTS_REQUESTS.

And the documentation needs to be updated.

I've added Yunfei Dong to the CC list, perhaps mediatek did some work on
this already.



> Thanks,
> Dikshita
>>> We profiled below data with this implementation :
>>> 1. Total time taken ( round trip ) for setting up control data on
>>> video driver
>>> with VIDIOC_S_EXT_CTRLS, QBUF and Queue Request: 737us
>>> 2. Time taken for first QBUF on Output plane to reach driver with
>>> REQUEST API enabled (One way): 723us
>>> 3. Time taken for first QBUF on Output plane to reach driver without
>>> REQUEST API (One way) : 250us
>>> 4. Time taken by each IOCTL to complete ( round trip ) with REQUEST
>>> API enabled :
>>> a. VIDIOC_S_EXT_CTRLS : 201us
>>> b. VIDIOC_QBUF : 92us
>>> Kindly share your feedback/comments on the design/call sequence.
>>> Also as we experimented and enabled the metadata on capture plane as
>>> well, please comment if any issue in
>>> allowing the metadata exchange on capture plane as well.
>>> Reference for client side implementation can be found at [1].
>>> Thanks,
>>> Dikshita
>>> [1]
>>> Dikshita Agarwal (3):
>>> Register for media device
>>> - Initialize and register for media device
>>> - define venus_m2m_media_ops
>>> - Implement APIs to register/unregister media controller.
>>> Enable Request API for output buffers
>>> - Add dependency on MEDIA_CONTROLLER_REQUEST_API in Kconfig.
>>> - Initialize vb2 ops buf_out_validate and buf_request_complete.
>>> - Add support for custom Metadata control
>>> - Implemeted/Integrated APIs for Request setup/complete.
>>> Enable Request API for Capture Buffers
>>> drivers/media/common/videobuf2/videobuf2-v4l2.c | 4 +-
>>> drivers/media/platform/Kconfig | 2 +-
>>> drivers/media/platform/qcom/venus/core.h | 36 ++++
>>> drivers/media/platform/qcom/venus/helpers.c | 247
>>> +++++++++++++++++++++++-
>>> drivers/media/platform/qcom/venus/helpers.h | 15 ++
>>> drivers/media/platform/qcom/venus/venc.c | 63 +++++-
>>> drivers/media/platform/qcom/venus/venc_ctrls.c | 61 +++++-
>>> drivers/media/v4l2-core/v4l2-ctrls.c | 10 +
>>> drivers/media/v4l2-core/v4l2-mem2mem.c | 17 +-
>>> include/media/v4l2-ctrls.h | 1 +
>>> include/media/venus-ctrls.h | 22 +++
>>> 11 files changed, 465 insertions(+), 13 deletions(-)
>>> create mode 100644 include/media/venus-ctrls.h

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