linux-kernel - Re: [Freedreno] [PATCH v3 06/10] drm/msm/dsi: Migrate to drm_dsc_compute_rc

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Message-ID: <20221013093646.c65mbjc6oekd7gha@SoMainline.org>
Date:   Thu, 13 Oct 2022 11:36:46 +0200
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     Abhinav Kumar <quic_abhinavk@...cinc.com>
Cc:     phone-devel@...r.kernel.org, Vinod Koul <vkoul@...nel.org>,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        David Airlie <airlied@...il.com>,
        linux-arm-msm@...r.kernel.org,
        Vladimir Lypak <vladimir.lypak@...il.com>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        dri-devel@...ts.freedesktop.org,
        Douglas Anderson <dianders@...omium.org>,
        Rob Clark <robdclark@...il.com>,
        Martin Botka <martin.botka@...ainline.org>,
        ~postmarketos/upstreaming@...ts.sr.ht,
        Daniel Vetter <daniel@...ll.ch>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        freedreno@...ts.freedesktop.org, Sean Paul <sean@...rly.run>,
        linux-kernel@...r.kernel.org
Subject: Re: [Freedreno] [PATCH v3 06/10] drm/msm/dsi: Migrate to
 drm_dsc_compute_rc_parameters()

On 2022-10-12 16:03:06, Abhinav Kumar wrote:
> 
> 
> On 10/9/2022 11:50 AM, Marijn Suijten wrote:
> > As per the FIXME this code is entirely duplicate with what is already
> > provided inside drm_dsc_compute_rc_parameters(), and it is yet unknown
> > why this comment was put in place instead of resolved from the get-go.
> > Not only does it save on duplication, it would have also spared certain
> > issues.
> > 
> > For example, this code from downstream assumed dsc->bits_per_pixel to
> > contain an integer value, whereas the upstream drm_dsc_config struct has
> > it with 4 fractional bits.  drm_dsc_compute_rc_parameters() already
> > accounts for this feat, and the sole remaining use of
> > dsc->bits_per_pixel inside dsi_populate_dsc_params() will be addressed
> > in a separate patch.
> > 
> 
> This is a nice cleanup! Thanks for doing this. I would actually like to 
> move towards the drm_dsc_compute_rc_parameters() API.
> 
> But I would like to hold back this change till Vinod clarifies because 
> Vinod had mentioned that with drm_dsc_compute_rc_parameters() he was 
> seeing a mismatch in the computation of two values.
> 
> slice_bpg_offset and the final_offset.

Unsurprisingly so because final_offset, and slice_bpg_offset through
initial_offset depend directly on bits_per_pixel.  The main takeaway of
this series is that Vinod was interpreting this field as integer instead
of containing 4 fractional bits.  If he updates his the panel driver [1]
to set bits_per_pixel = 8 << 4 instead of just 8 to account for this,
the values should check out once again.

[1]: https://git.linaro.org/people/vinod.koul/kernel.git/commit/?h=topic/pixel3_5.18-rc1&id=1d7d98ad564f1ec69e7525e07418918d90f247a1

Once Vinod (or someone else in the posession of a Pixel 3) confirms
this, I can respin this series and more explicitly explain why the FIXME
was put in place, instead of being resolved outright?

- Marijn

> 
> The difference between the upstream drm_dsc_compute_rc_parameters() and 
> dsi_populate_dsc_params() causing this was not clear to me from his 
> explanation earlier.
> 
> So this was left as a to-do item.
> 
> I would like this to be re-tested on pixel3 and check if this works for 
> vinod. If not, i think its the right time to debug why and not delay 
> this more.
> 
> Thanks
> 
> Abhinav