Re: [RFC PATCH 0/1] Add LTR controls

dikshita
2020-07-14T07:21:56Z

Hi,

A gentle reminder for the review.

Thanks,
Dikshita

On 2020-06-17 00:11, dikshita@codeaurora.org wrote:
> Hi Hans, Nicolas,
>
> Thanks for your comments.
>
> On 2020-06-12 14:41, Hans Verkuil wrote:
>> Hi Dikshita, Nicolas,
>>
>> On 11/06/2020 16:22, Nicolas Dufresne wrote:
>>> Le jeudi 11 juin 2020 à 15:55 +0530, Dikshita Agarwal a écrit :
>>>> LTR (Long Term Reference) frames are the frames that are encoded
>>>> sometime in the past
>>>> and stored in the DPB buffer list to be used as reference to encode
>>>> future frames.
>>>> One usage of LTR encoding is to reduce error propagation for video
>>>> transmission
>>>> in packet lossy networks. For example, encoder may want to specify
>>>> some key frames as
>>>> LTR pictures and use them as reference frames for encoding. With
>>>> extra protection
>>>> selectively on these LTR frames or synchronization with the receiver
>>>> of reception of
>>>> the LTR frames during transmission, decoder can receive reference
>>>> frames more reliably
>>>> than other non-reference frames. As a result, transmission error can
>>>> be effectively
>>>> restricted within certain frames rather than propagated to future
>>>> frames.
>>>>
>>>> We are introducing below V4l2 Controls for this feature
>>>> 1. V4L2_CID_MPEG_VIDEO_LTRCOUNT
>>>> a. This is used to query or configure the number of LTR frames.
>>>> This is a static control and is controlled by the client.
>>>> b. The LTR index varies from 0 to the max LTR-1.
>>>> c. If LTR Count is more than max supported LTR count (max LTR)
>>>> by driver, it will be rejected.
>>>> d. Auto Marking : If LTR count is non zero,
>>>> 1) first LTR count frames would be mark as LTR automatically
>>>> after
>>>> every IDR frame (inclusive).
>>>> 2) For multilayer encoding: first LTR count base layer
>>>> reference frames starting after
>>>> every IDR frame (inclusive) in encoding order would be
>>>> marked as LTR frames by the encoder.
>>>> 3) Auto marking of LTR due to IDR should consider following
>>>> conditions:
>>>> 1. The frame is not already set to be marked as LTR.
>>>> 2. The frame is part of the base layer in the
>>>> hierarchical layer case.
>>>> 3. The number of frames currently marked as LTR is less
>>>> than the maximum LTR frame index plus 1.
>>>> e. Encoder needs to handle explicit Mark/Use command when
>>>> encoder is still doing "auto" marking
>>
>> I don't follow this, quite possibly due to lack of experience with
>> encoders.
>>
>> I kind of would expect to see two modes: either automatic where
>> encoders can
>> mark up to LTR_COUNT frames as long term reference, and userspace just
>> sets
>> LTR_COUNT and doesn't have to do anything else.
>>
>> Or it is manual mode where userspace explicitly marks long term
>> reference
>> frames.
>>
>> From the proposal above it looks like you can mix auto and manual
>> modes.
>>
>> BTW, how do you 'unmark' long term reference frames?
>>
>> This feature is for stateful encoders, right?
>>
>>>
>>> Perhaps we are missing a LONG_TERM_REFERENCE_MODE ? I bet some
>>> encoder
>>> can select by themself long term references and even some encoders
>>> may
>>> not let the user decide.
>>>
>>> (not huge han of LTR acronyme, but that could be fine too, assuming
>>> you
>>> add more _).
>>>
>
> Userspace sets LTR count which signifies the number of LTR frames
> encoder needs to generate or keep.
> The encoder has to build-up its internal buffer reference list (aka
> DBP list or recon buffer list).
> In order to achieve that encoder will fill It's LTR (long term
> references) list and STR (short term references) list
> by auto marking n frames as LTR frames(n is equal to LTR count) based
> on auto-marking dictated by the encoder spec.
> The client then can replace those automatically marked frames with new
> frames using V4L2_CID_MPEG_VIDEO_MARKLTRFRAME and can ask
> encoder to refer the newly marked frame for encoding the next frame
> using V4L2_CID_MPEG_VIDEO_USELTRFRAME.
>
>>>>
>>>> 2. V4L2_CID_MPEG_VIDEO_MARKLTRFRAME :
>>>> a. This signals to mark the current frame as LTR frame. It is a
>>>> dynamic control and also provide the LTR index to be used.
>>>> b. the LTR index provided by this control should never exceed
>>>> the max LTR-1. Else it will be rejected.
>>>
>>> The "current" frame seems a bit loose. Perhaps you wanted to use
>>> buffer
>>> flags ? A bit like what we have to signal TOP/BOTTOM fields in
>>> alternate interlacing.
>>
>> I was thinking the same thing. Using a control for this doesn't seem
>> right.
>>
>
> the client sets this to replace automatically marked frames by the
> encoder with a particular frame.
> this provides an index that ranges from 0 to LTR count-1 and then the
> particular frame will be marked with that index.
> this can be achieved through request by associating this control with
> a specific buffer to make it synchronized.
>
>>>
>>>>
>>>> 3. V4L2_CID_MPEG_VIDEO_USELTRFRAME :
>>>> a. This specifies the LTR frame(s) to be used for encoding the
>>>> current frame. This is a dynamic control.
>>>> b. LTR Use Bitmap : this consists of bits [0, 15]. A total of N
>>>> LSB bits of this field are valid,
>>>> where N is the maximum number of LTRs supported. All the
>>>> other bits are invalid and should be rejected.
>>>> The LSB corresponds to the LTR index 0. Bit N-1 from the LSB
>>>> corresponds to the LTR index max LTR-1.
>>
>> How would userspace know this? Esp. with auto marking since userspace
>> would have
>> to predict how auto marking works (if I understand this correctly).
>>
>
> Client sets LTR count which tells about the number of LTR frames
> automatically marked by the encoder.
> so client can use LTR index (0 to LTR count -1) to ask encoder to
> refer any particular
> frame (marked automatically by driver or marked by client with
> V4L2_CID_MPEG_VIDEO_MARKLTRFRAME) as a reference to encode the next
> frame.
>
>> For which HW encoder is this meant?
>>
> This is primarily meant for H264 and HEVC.
>
> Thanks,
> Dikshita
>
>>>
>>> Note, I haven't captured very well the userspace control flow,
>>> perhaps
>>> this could be enhanced through writing some documentation.
>>>
>>> As per all other generic encoder controls, we need to make sure it
>>> will
>>> be usable and flexible enough for multiple HW blocks, as it can be
>>> tedious to extend later otherwise. It is important that along with
>>> this
>>> RFC you provide some comparisons with with other HW / SW APIs in
>>> order
>>> to help justify the design decisions. I also think there should be
>>> link made V4L2_CID_MPEG_VIDEO_GOP_* , number of B-Frames etc.
>>
>> I agree with Nicolas.
>>
>> Regards,
>>
>> Hans
>>
>>>
>>> regards,
>>> Nicolas
>>>
>>>>
>>>> Dikshita Agarwal (1):
>>>> media: v4l2-ctrls: add control for ltr
>>>>
>>>> drivers/media/v4l2-core/v4l2-ctrls.c | 6 ++++++
>>>> include/uapi/linux/v4l2-controls.h | 4 ++++
>>>> 2 files changed, 10 insertions(+)
>>>>
>>>