linux-kernel - Re: [PATCH v2] drm/msm/dpu: Fall back to a single DSC encoder (1:1:1) on small SoCs

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Message-ID: <kzmxf6r5kiyh2pr6hdzlwi3gwrlh3g257zlrpk4p5s66rhofio@qm2lbenuc2gw>
Date: Wed, 22 Jan 2025 20:44:03 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Marijn Suijten <marijn.suijten@...ainline.org>
Cc: Rob Clark <robdclark@...il.com>, 
	Abhinav Kumar <quic_abhinavk@...cinc.com>, Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, ~postmarketos/upstreaming@...ts.sr.ht, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, Konrad Dybcio <konrad.dybcio@....qualcomm.com>, 
	Martin Botka <martin.botka@...ainline.org>, Jami Kettunen <jami.kettunen@...ainline.org>, 
	linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org, Luca Weiss <luca.weiss@...rphone.com>
Subject: Re: [PATCH v2] drm/msm/dpu: Fall back to a single DSC encoder
 (1:1:1) on small SoCs

On Wed, Jan 22, 2025 at 05:23:44PM +0100, Marijn Suijten wrote:
> Some SoCs such as SC7280 (used in the Fairphone 5) have only a single
> DSC "hard slice" encoder.  The current hardcoded use of 2:2:1 topology
> (2 LM and 2 DSC for a single interface) make it impossible to use
> Display Stream Compression panels with mainline, which is exactly what's
> installed on the Fairphone 5.
> 
> By loosening the hardcoded `num_dsc = 2` to fall back to `num_dsc =
> 1` when the catalog only contains one entry, we can trivially support
> this phone and unblock further panel enablement on mainline.  A few
> more supporting changes in this patch ensure hardcoded constants of 2
> DSC encoders are replaced to count or read back the actual number of
> DSC hardware blocks that are enabled for the given virtual encoder.
> Likewise DSC_MODE_SPLIT_PANEL can no longer be unconditionally enabled.
> 
> Cc: Luca Weiss <luca.weiss@...rphone.com>
> Signed-off-by: Marijn Suijten <marijn.suijten@...ainline.org>
> ---
> Note that this doesn't paint the full picture in case there are SoCs
> with more DSC hardware blocks, but when multiple virtual encoders
> have already allocated most of them.  My initial plan was to code
> ahead for dynamically tracking and reallocating these blocks in RM, if
> some virtual encoder could potentially be using too many DSC encoders
> which, while "power optimal", may not be able to support the number of
> requested displays/interfaces.  Such a solution would automatically
> ensure DSCmerge is *not* used when there are not enough hardware blocks
> available in the first place.
> ---
> Changes in v2:
> - Fairphone is one word, lowercase the P from phone (Luca);
> - Skip unnecessary if (dpu_enc->dsc) check and always count the number
>   of non-NULL dpu_enc->hw_dsc[i] instead (Dmitry);
> - Revert irrelevant whitespace cleanup in DSC comment, even if
>   worthless enough to send separately (Dmitry);
> - Drop dsc_common_mode=0 initialization and move existing 0-assignment
>   from a random place in the function down to where this variable is
>   actually being updated (Dmitry);
> - Link to v1: https://lore.kernel.org/r/20250121-dpu-111-topology-v1-1-d01987205c53@somainline.org
> ---
>  drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c | 47 +++++++++++++++--------------
>  1 file changed, 25 insertions(+), 22 deletions(-)
> 

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

-- 
With best wishes
Dmitry