Re: [RFC PATCH 1/2] drm/msm/dpu: add dsc helper functions

[Dmitry Baryshkov]

On Mon, 27 Feb 2023 at 01:49, Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> wrote:
>
>
>
> On 2/26/2023 5:09 AM, Dmitry Baryshkov wrote:
> > On 26/02/2023 02:47, Abhinav Kumar wrote:
> >> Hi Dmitry
> >>
> >> On 2/25/2023 7:23 AM, Dmitry Baryshkov wrote:
> >>> On 25/02/2023 02:36, Abhinav Kumar wrote:
> >>>>
> >>>>
> >>>> On 2/24/2023 3:53 PM, Dmitry Baryshkov wrote:
> >>>>> On Sat, 25 Feb 2023 at 00:26, Abhinav Kumar
> >>>>> <quic_abhinavk@xxxxxxxxxxx> wrote:
> >>>>>> On 2/24/2023 1:36 PM, Dmitry Baryshkov wrote:
> >>>>>>> 24 февраля 2023 г. 23:23:03 GMT+02:00, Abhinav Kumar
> >>>>>>> <quic_abhinavk@xxxxxxxxxxx> пишет:
> >>>>>>>> On 2/24/2023 1:13 PM, Dmitry Baryshkov wrote:
> >>>>>>>>> On 24/02/2023 21:40, Kuogee Hsieh wrote:
> >>>>>>>>>> Add DSC helper functions based on DSC configuration profiles
> >>>>>>>>>> to produce
> >>>>>>>>>> DSC related runtime parameters through both table look up and
> >>>>>>>>>> runtime
> >>>>>>>>>> calculation to support DSC on DPU.
> >>>>>>>>>>
> >>>>>>>>>> There are 6 different DSC configuration profiles are supported
> >>>>>>>>>> currently.
> >>>>>>>>>> DSC configuration profiles are differiented by 5 keys, DSC
> >>>>>>>>>> version (V1.1),
> >>>>>>>>>> chroma (444/422/420), colorspace (RGB/YUV), bpc(8/10),
> >>>>>>>>>> bpp (6/7/7.5/8/9/10/12/15) and SCR (0/1).
> >>>>>>>>>>
> >>>>>>>>>> Only DSC version V1.1 added and V1.2 will be added later.
> >>>>>>>>>
> >>>>>>>>> These helpers should go to
> >>>>>>>>> drivers/gpu/drm/display/drm_dsc_helper.c
> >>>>>>>>> Also please check that they can be used for i915 or for amdgpu
> >>>>>>>>> (ideally for both of them).
> >>>>>>>>>
> >>>>>>>>
> >>>>>>>> No, it cannot. So each DSC encoder parameter is calculated based
> >>>>>>>> on the HW core which is being used.
> >>>>>>>>
> >>>>>>>> They all get packed to the same DSC structure which is the
> >>>>>>>> struct drm_dsc_config but the way the parameters are computed is
> >>>>>>>> specific to the HW.
> >>>>>>>>
> >>>>>>>> This DPU file helper still uses the drm_dsc_helper's
> >>>>>>>> drm_dsc_compute_rc_parameters() like all other vendors do but
> >>>>>>>> the parameters themselves are very HW specific and belong to
> >>>>>>>> each vendor's dir.
> >>>>>>>>
> >>>>>>>> This is not unique to MSM.
> >>>>>>>>
> >>>>>>>> Lets take a few other examples:
> >>>>>>>>
> >>>>>>>> AMD:
> >>>>>>>> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/amd/display/dc/dml/dsc/rc_calc_fpu.c#L165
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> i915:
> >>>>>>>> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/i915/display/intel_vdsc.c#L379
> >>>>>>>>
> >>>>>>>
> >>>>>>> I checked several values here. Intel driver defines more bpc/bpp
> >>>>>>> combinations, but the ones which are defined in intel_vdsc and in
> >>>>>>> this patch seem to match. If there are major differences there,
> >>>>>>> please point me to the exact case.
> >>>>>>>
> >>>>>>> I remember that AMD driver might have different values.
> >>>>>>>
> >>>>>>
> >>>>>> Some values in the rc_params table do match. But the
> >>>>>> rc_buf_thresh[] doesnt.
> >>>>>
> >>>>> Because later they do:
> >>>>>
> >>>>> vdsc_cfg->rc_buf_thresh[i] = rc_buf_thresh[i] >> 6;
> >>>>>
> >>>>>>
> >>>>>> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/i915/display/intel_vdsc.c#L40
> >>>>>>
> >>>>>>
> >>>>>> Vs
> >>>>>>
> >>>>>> +static u16 dpu_dsc_rc_buf_thresh[DSC_NUM_BUF_RANGES - 1] = {
> >>>>>> +               0x0e, 0x1c, 0x2a, 0x38, 0x46, 0x54,
> >>>>>> +               0x62, 0x69, 0x70, 0x77, 0x79, 0x7b, 0x7d, 0x7e
> >>>>>> +};
> >>>>>
> >>>>> I'd prefer to have 896, 1792, etc. here, as those values come from the
> >>>>> standard. As it's done in the Intel driver.
> >>>>>
> >>>>
> >>>> Got it, thanks
> >>>>
> >>>>>> I dont know the AMD calculation very well to say that moving this
> >>>>>> to the
> >>>>>> helper is going to help.
> >>>>>
> >>>>> Those calculations correspond (more or less) at the first glance to
> >>>>> what intel does for their newer generations. I think that's not our
> >>>>> problem for now.
> >>>>>
> >>>>
> >>>> Well, we have to figure out if each value matches and if each of
> >>>> them come from the spec for us and i915 and from which section. So
> >>>> it is unfortunately our problem.
> >>>
> >>> Otherwise it will have to be handled by Marijn, me or anybody else
> >>> wanting to hack up the DSC code. Or by anybody adding DSC support to
> >>> the next platform and having to figure out the difference between
> >>> i915, msm and their platform.
> >>>
> >>
> >> Yes, I wonder why the same doubt didn't arise when the other vendors
> >> added their support both from other maintainers and others.
> >>
> >> Which makes me think that like I wrote in my previous response, these
> >> are "recommended" values in the spec but its not mandatory.
> >
> > I think, it is because there were no other drivers to compare. In other
> > words, for a first driver it is pretty logical to have everything
> > handled on its own. As soon as we start getting other implementations of
> > a feature, it becomes logical to think if the code can be generalized.
> > This is what we see we with the HDCP series or with the code being moved
> > to DP helpers.
> >
>
> We were not the second, MSM was/is the third to add support for DSC afer
> i915 and AMD. Thats what made me think why whoever was the second didnt
> end up generalizing. Was it just missed out or was it intentionally left
> in the vendor driver.

I didn't count AMD here, since it calculates some of the params rather
than using the fixed ones from the model.

>
> >>
> >> Moving this to the drm_dsc_helper is generalizing the tables and not
> >> giving room for the vendors to customize even if they want to (which
> >> the spec does allow).
> >
> > That depends on the API you select. For example, in
> > intel_dsc_compute_params() I see customization being applied to
> > rc_buf_thresh in 6bpp case. I'd leave that to the i915 driver.
> >
>
> Thanks for going through the i915 to figure out that the 6bpp is handled
> in a customized way. So what you are saying is let the helper first fill
> up the recommended values of the spec, whatever is changed from that let
> the vendor driver override that.
>
> Thats where the case-by-case handling comes.
>
> Why not we do this way? Like you mentioned lets move these tables to the
> drm_dsc_helper and let MSM driver first use those.
>
> Then in a separate patchset if i915 and AMD would like to move to that,
> let them handle it for their respective drivers instead of MSM going
> through whats customized for each calculation and doing it.
>
> I am hesitant to take up that effort.

Writing a tool to convert model's rc_Nbpc_Mbpp_foo.cfg into C
languages structures used by Intel code took 15-20 minutes. Plugging
generated structures took another 5 minutes. I will send the patches
later today or tomorrow, as I find a time slot to clean them. Thank
you for spending more time on arguing than it took me to generate &
verify the data.

>
> If the recommended values work for the vendor, they can clean it up and
> move to the drm_dsc_helper themselves and preserving their
> customizations rather than one vendor doing it for all of them.
>
> > In case the driver needs to perform customization of the params, nothing
> > stops it drop applying after filling all the RC params in the
> > drm_dsc_config struct via the generic helper.
> >
> >
> >> So if this has any merit and if you or Marijn would like to take it
> >> up, go for it. We would do the same thing as either of you would have
> >> to in terms of figuring out the difference between msm and the i915 code.
> >>
> >> This is not a generic API we are trying to put in a helper, these are
> >> hard-coded tables so there is a difference between looking at these Vs
> >> looking at some common code which can move to the core.
> >>
> >>>>
> >>>>>>
> >>>>>> Also, i think its too risky to change other drivers to use
> >>>>>> whatever math
> >>>>>> we put in the drm_dsc_helper to compute thr RC params because
> >>>>>> their code
> >>>>>> might be computing and using this tables differently.
> >>>>>>
> >>>>>> Its too much ownership for MSM developers to move this to
> >>>>>> drm_dsc_helper
> >>>>>> and own that as it might cause breakage of basic DSC even if some
> >>>>>> values
> >>>>>> are repeated.
> >>>>>
> >>>>> It's time to stop thinking about ownership and start thinking about
> >>>>> shared code. We already have two instances of DSC tables. I don't
> >>>>> think having a third instance, which is a subset of an existing
> >>>>> dataset, would be beneficial to anybody.
> >>>>> AMD has complicated code which supports half-bit bpp and calculates
> >>>>> some of the parameters. But sharing data with the i915 driver is
> >>>>> straightforward.
> >>>>>
> >>>>
> >>>> Sorry, but I would like to get an ack from i915 folks if this is going
> >>>> to be useful to them if we move this to helper because we have to
> >>>> look at every table. Not just one.
> >>>
> >>> Added i915 maintainers to the CC list for them to be able to answer.
> >>>
> >>
> >> Thanks, lets wait to hear from them about where finally these tables
> >> should go but thats can be taken up as a separate effort too.
> >>
> >>>>
> >>>> Also, this is just 1.1, we will add more tables for 1.2. So we will
> >>>> have to end up changing both 1.1 and 1.2 tables as they are
> >>>> different for QC.
> >>>
> >>> I haven't heard back from Kuogee about the possible causes of using
> >>> rc/qp values from 1.2 even for 1.1 panels. Maybe you can comment on
> >>> that? In other words, can we always stick to the values from 1.2
> >>> standard? What will be the drawback?
> >>>
> >>> Otherwise, we'd have to have two different sets of values, like you
> >>> do in your vendor driver.
> >>>
> >>
> >> I have responded to this in the other email.
> >>
> >> All this being said, even if the rc tables move the drm_dsc_helper
> >> either now or later on, we will still need MSM specific calculations
> >> for many of the other encoder parameters (which are again either
> >> hard-coded or calculated). Please refer to the
> >> sde_dsc_populate_dsc_config() downstream. And yes, you will not find
> >> those in the DP spec directly.
> >>
> >> So we will still need a dsc helper for MSM calculations to be common
> >> for DSI / DP irrespective of where the tables go.
> >>
> >> So, lets finalize that first.
> >
> > I went on and trimmed sde_dsc_populate_dsc_config() to remove
> > duplication with the drm_dsc_compute_rc_parameters() (which we already
> > use for the MSM DSI DSC).
> >
> > Not much is left:
> >
> > dsc->first_line_bpg_offset set via the switch
> >
> > dsc->line_buf_depth = bpc + 1;
> > dsc->mux_word_size = bpc > 10 ? DSC_MUX_WORD_SIZE_12_BPC:
> >          DSC_MUX_WORD_SIZE_8_10_BPC;
> >
> > if ((dsc->dsc_version_minor == 0x2) && (dsc->native_420))
> >      dsc->nsl_bpg_offset = (2048 *
> >               (DIV_ROUND_UP(dsc->second_line_bpg_offset,
> >                                  (dsc->slice_height - 1))));
> >
> > dsc->initial_scale_value = 8 * dsc->rc_model_size /
> >                          (dsc->rc_model_size - dsc->initial_offset);
> >
> >
> > mux_word_size comes from the standard (must)
> > initial_scale_value calculation is recommended, but not required
> > nsl_bpg_offset follows the standard (must), also see below (*).
> >
> > first_line_bpg_offset calculation differs between three drivers. The
> > standard also provides a recommended formulas. I think we can leave it
> > as is for now.
> >
> > I think, that mux_word_size and nsl_bpg_offset calculation should be
> > moved to drm_dsc_compute_rc_parameters(), while leaving
> > initial_scale_value in place (in the driver code).
> >
> > * I think nsl_bpg_offset is slightly incorrectly calculated. Standard
> > demands that it is set to 'second_line_bpg_offset / (slice_height - 1),
> > rounded up to 16 fraction bits', while SDE driver code sets it to the
> > value rounded up to the next integer (having 16 fraction bits
> > representation).
> >
> > In my opinion correct calculation should be:
> > dsc->nsl_bpg_offset = DIV_ROUND_UP(2048 * dsc->second_line_bpg_offset,
> >                                  (dsc->slice_height - 1));
> >
> > Could you please check, which one is correct according to the standard?
> >
> >
>
> Sure, i will check about nsl_bpg_offset. But sorry if I was not more
> clear about this but sde_dsc_populate_dsc_config() is only one example
> which from your analysis can be moved to the drm_dsc_helper() but not
> the initial line calculation _dce_dsc_initial_line_calc(),
> _dce_dsc_ich_reset_override_needed() , _dce_dsc_setup_helper().

The initial_line is already calculated in dpu_encoder.c. As for the
_dce_dsc_ich_reset_override_needed(), I don't think we support partial
updates in the upstream driver.

>
> All of these are again common between DSI and DP.
>
> So in addition to thinking about what can be moved to the drm_dsc_helper
> also think about what is specific to MSM but common to DSI and DP modules.
>
> That was the bigger picture I was trying to convey.



-- 
With best wishes
Dmitry