linux-kernel - Re: [PATCH] drm/msm/dp: remove limitation of link rate at 5.4G to support HBR3

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Message-ID: <CAD=FV=WPde5wVOGCKQYGuGwgCwRebox4FF0MgV_2pPCTsfo_UA@mail.gmail.com>
Date:   Wed, 2 Nov 2022 10:28:22 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Kuogee Hsieh <quic_khsieh@...cinc.com>, robdclark@...il.com,
        sean@...rly.run, swboyd@...omium.org, vkoul@...nel.org,
        daniel@...ll.ch, airlied@...ux.ie, agross@...nel.org,
        quic_abhinavk@...cinc.com, quic_sbillaka@...cinc.com,
        freedreno@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Andersson <andersson@...nel.org>
Subject: Re: [PATCH] drm/msm/dp: remove limitation of link rate at 5.4G to
 support HBR3

Hi,

On Wed, Nov 2, 2022 at 10:23 AM Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> > 1. Someone figures out how to model this with the bridge chain and
> > then we only allow HBR3 if we detect we've got a TCPC that supports
> > it. This seems like the cleanest / best but feels like a long pole.
> > Not only have we been trying to get the TCPC-modeled-as-a-bridge stuff
> > landed for a long time but even when we do it we still don't have a
> > solution for how to communicate the number of lanes and other stuff
> > between the TCPC and the DP controller so we have to enrich the bridge
> > interface.
>
> I think we'd need some OOB interface. For example for DSI interfaces we
> have mipi_dsi_device struct to communicate such OOB data.
>
> Also take a note regarding data-lanes from my previous email.

Right, we can somehow communicate the max link rate through the bridge
chain to the DP controller in an OOB manner that would work.


> > 2. We add in a DT property to the display controller node that says
> > the max link rate for use on this board. This feels like a hack, but
> > maybe it's not too bad. Certainly it would be incredibly simple to
> > implement. Actually... ...one could argue that even if we later model
> > the TCPC as a bridge that this property would still be valid / useful!
> > You could certainly imagine that the SoC supports HBR3 and the TCPC
> > supports HBR3 but that the board routing between the SoC and the TCPC
> > is bad and only supports HBR2. In this case the only way out is
> > essentially a "board constraint" AKA a DT property in the DP
> > controller.
>
> We have been discussing similar topics with Abhinav. Krzysztof suggested
> using link-frequencies property to provide max and min values.

This sounds good to me and seems worth doing even if we eventually do #1.