On 20/07/2020 16:33, dikshita@xxxxxxxxxxxxxx wrote: > On 2020-07-16 14:25, Hans Verkuil wrote: >> On 16/06/2020 20:41, dikshita@xxxxxxxxxxxxxx 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. >> >> The venus encoder? > yes >> >> Some more questions: >> >> 1) How many LTR frames do h.264 and hevc allow? > According to spec, MAX LTR allowed by H.264 is 15 and HEVC allows max 32 LTR frames. >> 2) Given N LTR frames, is there a ordering of those frames? E.g. >> the LTR frame with index 0 is processed/used differently from >> LTR frame with index 1? Or are they all equal in that it is just a pool >> of LTR frames that the encoder can use as it wishes? > they are different frames marked to be used as LTR and stored in available indices. > Userspace notifies encoder which LTR frame index to use via USE LTR control. One more question: if a frame is marked as a LTR frame, does that mean that userspace can't reuse the buffer containing that frame? I assume that's the case. One thing that I don't like about this API is that it introduces the LTR indices. Would it be possible to instead use buffer indices? (i.e. the v4l2_buffer index field). Then you can use a buffer flag to indicate that a buffer should be an LTR buffer, or (for automarking) the driver can mark a buffer as an LTR buffer by setting this flag before returning the buffer to userspace. You would still need a V4L2_CID_MPEG_VIDEO_USELTRFRAME, but that can be a bitmask using the buffer index. This would avoid introducing a second method of referring to buffers (LTR index), and I think that V4L2_CID_MPEG_VIDEO_MARKLTRFRAME can be dropped. Regards, Hans > > Thanks, > Dikshita >> >> Regards, >> >> Hans >> >>> >>> 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(+) >>>>>> >>>>>
