Re: [PATCH] media: hevc: fix pictures lists type

[John Cox]

>Le 27/08/2021 à 12:10, John Cox a écrit :
>>> 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.
>
>Ok so keep like it is.
>In this case my patch is enough, right ?

I still think there are better and smaller ways of constructing the
lists Hantro wants (e.g. using an index into the DPB as the L0/L1 lists
do) but this patch is capable of delivering the result you need.

Regards

John Cox

>Benjamin
>
>>
>> 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@xxxxxxxxxxxxx>
>>>>>>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx>
>>>>>>>> ---
>>>>>>>> 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;
>>>>>>>> };
>>>>>>>>