linux-kernel - Re: [PATCH] drm/msm/dpu: always program dsc active bits

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Date:   Wed, 12 Apr 2023 09:24:10 +0200
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     Abhinav Kumar <quic_abhinavk@...cinc.com>
Cc:     Kuogee Hsieh <quic_khsieh@...cinc.com>, robdclark@...il.com,
        sean@...rly.run, swboyd@...omium.org, dianders@...omium.org,
        vkoul@...nel.org, daniel@...ll.ch, airlied@...il.com,
        agross@...nel.org, dmitry.baryshkov@...aro.org,
        andersson@...nel.org, quic_sbillaka@...cinc.com,
        freedreno@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/msm/dpu: always program dsc active bits

On 2023-04-11 16:45:34, Abhinav Kumar wrote:
[..]
> > Does this flush all DSCs programmed in CTL_DSC_FLUSH as set above?  That
> > is currently still in `if (cfg->dsc)` and never overwritten if all DSCs
> > are disabled, should it be taken out of the `if` to make sure no DSCs
> > are inadvertently flushed, or otherwise cache the "previous mask" to
> > make sure we flush exactly the right DSC blocks?
> > 
> 
> Yes, DSC flush is hierarchical. This is the main DSC flush which will 
> enforce the flush of the DSC's we are trying to flush in the 
> CTL_DSC_FLUSH register.

That's what I was thinking, thanks for confirming.

> So if DSC was active, the CTL_FLUSH will only enforce the flush of the 
> DSC's programmed in CTL_DSC_FLUSH
> 
> If DSC is not active, we still need to flush that as well (that was the 
> missing bit).
> 
> No need to cache previous mask. That programming should be accurate in 
> cfg->dsc already.

This kind of implicit dependency warrants a comment at the very least.

What happens if a device boots without DSC panel connected?  Will
CTL_DSC_FLUSH be zero and not (unnecessarily, I assume) flush any of the
DSC blocks?  Or could this flush uninitialized state to the block?

- Marijn