linux-kernel - Re: [RFC PATCH 02/13] dt-bindings: msm/dsi: Document Display Stream Compression (DSC) parameters

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Message-ID: <20210524150815.GH2484@yoga>
Date:   Mon, 24 May 2021 10:08:15 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     Rob Clark <robdclark@...il.com>, linux-arm-msm@...r.kernel.org,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Marek <jonathan@...ek.ca>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Abhinav Kumar <abhinavk@...eaurora.org>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, Rob Herring <robh+dt@...nel.org>,
        devicetree@...r.kernel.org
Subject: Re: [RFC PATCH 02/13] dt-bindings: msm/dsi: Document Display Stream
 Compression (DSC) parameters

On Mon 24 May 02:30 CDT 2021, Vinod Koul wrote:

> On 21-05-21, 09:42, Bjorn Andersson wrote:
> > On Fri 21 May 07:49 CDT 2021, Vinod Koul wrote:
> > 
> > > DSC enables streams to be compressed before we send to panel. This
> > > requires DSC enabled encoder and a panel to be present. So we add this
> > > information in board DTS and find if DSC can be enabled and the
> > > parameters required to configure DSC are added to binding document along
> > > with example
> > > 
> > > Signed-off-by: Vinod Koul <vkoul@...nel.org>
> > > ---
> > >  .../devicetree/bindings/display/msm/dsi.txt       | 15 +++++++++++++++
> > >  1 file changed, 15 insertions(+)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/display/msm/dsi.txt b/Documentation/devicetree/bindings/display/msm/dsi.txt
> > > index b9a64d3ff184..83d2fb92267e 100644
> > > --- a/Documentation/devicetree/bindings/display/msm/dsi.txt
> > > +++ b/Documentation/devicetree/bindings/display/msm/dsi.txt
> > > @@ -48,6 +48,13 @@ Optional properties:
> > >  - pinctrl-n: the "sleep" pinctrl state
> > >  - ports: contains DSI controller input and output ports as children, each
> > >    containing one endpoint subnode.
> > > +- qcom,mdss-dsc-enabled: Display Stream Compression (DSC) is enabled
> > > +- qcom,mdss-slice-height: DSC slice height in pixels
> > > +- qcom,mdss-slice-width: DSC slice width in pixels
> > > +- qcom,mdss-slice-per-pkt: DSC slices per packet
> > > +- qcom,mdss-bit-per-component: DSC bits per component
> > > +- qcom,mdss-bit-per-pixel: DSC bits per pixel
> > > +- qcom,mdss-block-prediction-enable: Block prediction mode of DSC enabled
> > >  
> > >    DSI Endpoint properties:
> > >    - remote-endpoint: For port@0, set to phandle of the connected panel/bridge's
> > > @@ -188,6 +195,14 @@ Example:
> > >  		qcom,master-dsi;
> > >  		qcom,sync-dual-dsi;
> > >  
> > > +		qcom,mdss-dsc-enabled;
> > 
> > To me the activation of DSC seems to be a property of the panel.
> 
> I think there are three parts to the problem
> 1. Panel needs to support it

In the case of DP there's bits to be read in the panel to figure this
out, for DSI panels this seems like a property that the panel (driver)
should know about.

> 2. Host needs to support it

Right, so this needs to be known by the driver. My suggestion is that we
derive it from the compatible or from the HW version.

> 3. Someone needs to decide to use when both the above conditions are
> met.
> 
> There are cases where above 1, 2 will be satisfied, but we might be okay
> without DSC too.. so how to decide when to do DSC :)
> 

Can we describe those cases? E.g. is it because enabling DSC would not
cause a reduction in clock speed that's worth the effort? Or do we only
use DSC for DSI when it allows us to squeeze everything into a single
link?

Regards,
Bjorn

> I feel it is more of a system property. And I also think that these
> parameters here are host configuration and not really for panel...
> 
> > 
> > > +		qcom,mdss-slice-height = <16>;
> > > +		qcom,mdss-slice-width = <540>;
> > > +		qcom,mdss-slice-per-pkt = <1>;
> > > +		qcom,mdss-bit-per-component = <8>;
> > > +		qcom,mdss-bit-per-pixel = <8>;
> > > +		qcom,mdss-block-prediction-enable;
> > 
> > Which of these properties relates to the DSC encoder and what needs to
> > be agreed with the sink? Can't we derive e.g. bpp from the information
> > we have from the attached panel already?
> 
> Let me go back and check on this a bit more
> 
> Thanks
> -- 
> ~Vinod