Message-ID: <CAAEAJfBmnmgf5PB8YbW5hu73qz7t1O8+nzG3yFTWscAziX-mpQ@mail.gmail.com>
Date:   Fri, 27 Aug 2021 15:46:56 -0300
From:   Ezequiel Garcia <ezequiel@...guardiasur.com.ar>
To:     Benjamin Gaignard <benjamin.gaignard@...labora.com>
Cc:     John Cox <jc@...esim.co.uk>,
        Nicolas Dufresne <nicolas@...fresne.ca>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        linux-media <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Collabora Kernel ML <kernel@...labora.com>,
        Daniel Almeida <daniel.almeida@...labora.com>
Subject: Re: [PATCH] media: hevc: fix pictures lists type

On Fri, 27 Aug 2021 at 12:26, Benjamin Gaignard
<benjamin.gaignard@...labora.com> wrote:
>
>
> Le 27/08/2021 à 14:40, Ezequiel Garcia a écrit :
> > On Fri, 27 Aug 2021 at 09:36, John Cox <jc@...esim.co.uk> wrote:
> >>> 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 ?
> > The problem with the patch is that it breaks existing userspace.
> > Currently, there's no upstreamed userspace so this is not a huge
> > deal.
> >
> > However, it's definitely not a good practice. Even if these are
> > staging controls, I think a proper action item is to start discussing
> > what's missing on the HEVC interface as a whole, so it can be
> > moved to stable.
>
> I do agree I think it could the time to talk about moving the API to stable.
> My plan is to get this patch merge before sending a RFC to move the API.
>

I'd rather not merge this patch, and instead merge all the changes in one go,
so we avoid making further changes to the API.

The patch iself might look good, but I'd rather have the discussion on
the big picture,
and make sure we can all review the interface against the spec.

BTW, this should probably mean we have a plan to extend APIs if we need to.
Daniel Almeida (Cc) has the details for my proposal on that front, if you want
to push it forward.

Thanks,
Ezequiel

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