Message-ID: <r4ehigheq602qijsnjd8govhl4f1dpnr35@4ax.com>
Date:   Fri, 27 Aug 2021 11:10:17 +0100
From:   John Cox <jc@...esim.co.uk>
To:     Benjamin Gaignard <benjamin.gaignard@...labora.com>
Cc:     Nicolas Dufresne <nicolas@...fresne.ca>, mchehab@...nel.org,
        hverkuil-cisco@...all.nl, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH] media: hevc: fix pictures lists type

>Le 26/08/2021 à 18:09, Nicolas Dufresne a écrit :
>> Le lundi 23 août 2021 à 12:35 +0100, John Cox a écrit :
>>> Hi
>>>
>>>> Le 23/08/2021 à 11:50, John Cox a écrit :
>>>>>> The lists embedded Picture Order Count values which are s32 so their type
>>>>>> most be s32 and not u8.
>>>>> I'm not convinced that you can't calculate all of those lists from the
>>>>> info already contained in the DPB array so this is probably redundant
>>>>> info though I grant that having the list pre-calced might make your life
>>>>> easier, and the userland side will have calculated the lists to
>>>>> calculate other required things so it isn't much extra work for it.
>>>> Yes the userland have already compute these lists and the number of items
>>>> in each of them.
>>>> Build them in the kernel would means to also compute the values of NumPocStCurrBefore,
>>>> NumPocStCurrAfter, NumPocLtCurr, NumPocStCurrAfter, NumPocStCurrBefore and NumPocLtCurr
>>>> and that requires information (NumNegativePics, NumPositivePics...) not provided to the kernel.
>>>> Since it have to be done in userland anyway, I'm reluctant to modify the API to redo in the kernel.
>>> Well, fair enough, I'm not going to argue
>>>
>>>>> Even if you do need the lists wouldn't it be a better idea to have them
>>>>> as indices into the DPB (you can't have a frame in any of those lists
>>>>> that isn't in the DPB) which already contains POCs then it will still
>>>>> fit into u8 and be smaller?
>>>> Hantro HW works with indexes but I think it is more simple to send PoC rather than indexes.
>>> I'd disagree but as I don't use the info I'm not concerned. Though I
>>> think I should point out that when Hantro converts the POCs to indicies
>>> it compares the now s32 POC in these lists with the u16 POC in the DPB
>>> so you might need to fix that too; by std (8.3.1) no POC diff can be
>>> outside s16 so you can mask & compare or use u16 POCs in the lists or
>>> s32 in the DPB.
>> Fun fact, my interpretation with the API when I drafted GStreamer support was
>> that it was DPB indexes:
>>
>> https://gitlab.freedesktop.org/ndufresne/gst-plugins-bad/-/blob/hevc_wip/sys/v4l2codecs/gstv4l2codech265dec.c#L850
>>
>> It felt quite natural to be, since this is also how we pass references for l0/l1
>> (unused by hantro I guess).
>>
>> Looking at old rkvdec code as a refresher:
>>
>>    for (j = 0; j < run->num_slices; j++) {
>>                  sl_params = &run->slices_params[j];
>>                  dpb = sl_params->dpb;
>>
>>                  hw_ps = &priv_tbl->rps[j];
>>                  memset(hw_ps, 0, sizeof(*hw_ps));
>>
>>                  for (i = 0; i <= sl_params->num_ref_idx_l0_active_minus1; i++) {
>>                          WRITE_RPS(!!(dpb[sl_params->ref_idx_l0[i]].rps == V4L2_HEVC_DPB_ENTRY_RPS_LT_CURR),
>>                                    REF_PIC_LONG_TERM_L0(i));
>>                          WRITE_RPS(sl_params->ref_idx_l0[i], REF_PIC_IDX_L0(i));
>>                  }
>>
>>                  for (i = 0; i <= sl_params->num_ref_idx_l1_active_minus1; i++) {
>>                          WRITE_RPS(!!(dpb[sl_params->ref_idx_l1[i]].rps == V4L2_HEVC_DPB_ENTRY_RPS_LT_CURR),
>>                                    REF_PIC_LONG_TERM_L1(i));
>>                          WRITE_RPS(sl_params->ref_idx_l1[i], REF_PIC_IDX_L1(i));
>>                  }
>>
>>
>> This is code is clearly unsafe, but now I remember that dpb_entry has a flag
>> "rps". So we know from the DPB in which of the list the reference lives, if any.
>> In the case of RKVDEC the HW only cares to know if this is long term or not.
>>
>> So without looking at the spec, is that dpb represention enough to reconstruct
>> these array ? If we pass these array, shall we keep the rps flag ? I think a
>> little step back and cleanup will be needed. I doubt there is a single answer,
>> perhaps list what others do (VA, DXVA, NVDEC, Khronos, etc) and we can
>> collectively decide were we want V4L2 to sit ?
>
>I have done some tests with Hantro driver and look at the spec, the order of the PoC
>in the reference lists matters. You can deducted the order for DPB rps flags.
>I would suggest to remove rps flags to avoid information duplication.

I want the DPB rps member for long term reference marking.  I don't care
about before / after, but LTR can't be deduced from PoC and if you are
going to keep the member you might as well keep before / after.

John Cox

>Benjamin
>
>>
>>> Regards
>>>
>>> John Cox
>>>
>>>> Benjamin
>>>>
>>>>> Full disclosure: Pi decode doesn't use this info at all so I'm only
>>>>> arguing from a theoretical point of view - I think it is only relevant
>>>>> if your h/w is parsing the reference list setups.
>>>>>
>>>>> Regards
>>>>>
>>>>> John Cox
>>>>>
>>>>>> Reported-by: John Cox <jc@...esim.co.uk>
>>>>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
>>>>>> ---
>>>>>> Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst | 6 +++---
>>>>>> include/media/hevc-ctrls.h                                | 6 +++---
>>>>>> 2 files changed, 6 insertions(+), 6 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>>>>> index 976d34445a24..db9859ddc8b2 100644
>>>>>> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>>>>> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>>>>> @@ -3323,15 +3323,15 @@ enum v4l2_mpeg_video_hevc_size_of_length_field -
>>>>>>       * - __u8
>>>>>>         - ``num_poc_lt_curr``
>>>>>>         - The number of reference pictures in the long-term set.
>>>>>> -    * - __u8
>>>>>> +    * - __s32
>>>>>>         - ``poc_st_curr_before[V4L2_HEVC_DPB_ENTRIES_NUM_MAX]``
>>>>>>         - PocStCurrBefore as described in section 8.3.2 "Decoding process for reference
>>>>>>           picture set.
>>>>>> -    * - __u8
>>>>>> +    * - __s32
>>>>>>         - ``poc_st_curr_after[V4L2_HEVC_DPB_ENTRIES_NUM_MAX]``
>>>>>>         - PocStCurrAfter as described in section 8.3.2 "Decoding process for reference
>>>>>>           picture set.
>>>>>> -    * - __u8
>>>>>> +    * - __s32
>>>>>>         - ``poc_lt_curr[V4L2_HEVC_DPB_ENTRIES_NUM_MAX]``
>>>>>>         - PocLtCurr as described in section 8.3.2 "Decoding process for reference
>>>>>>           picture set.
>>>>>> diff --git a/include/media/hevc-ctrls.h b/include/media/hevc-ctrls.h
>>>>>> index 781371bff2ad..04cd62e77f25 100644
>>>>>> --- a/include/media/hevc-ctrls.h
>>>>>> +++ b/include/media/hevc-ctrls.h
>>>>>> @@ -219,9 +219,9 @@ struct v4l2_ctrl_hevc_decode_params {
>>>>>> 	__u8	num_poc_st_curr_before;
>>>>>> 	__u8	num_poc_st_curr_after;
>>>>>> 	__u8	num_poc_lt_curr;
>>>>>> -	__u8	poc_st_curr_before[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
>>>>>> -	__u8	poc_st_curr_after[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
>>>>>> -	__u8	poc_lt_curr[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
>>>>>> +	__s32	poc_st_curr_before[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
>>>>>> +	__s32	poc_st_curr_after[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
>>>>>> +	__s32	poc_lt_curr[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
>>>>>> 	__u64	flags;
>>>>>> };
>>>>>>
>>

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