linux-kernel - Re: [PATCH v3 6/9] drm/bridge: ti-sn65dsi86: Use 18-bit DP if we can

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Date:   Wed, 2 Sep 2020 07:37:46 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Steev Klimaszewski <steev@...i.org>
Cc:     Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        David Airlie <airlied@...ux.ie>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Jeffrey Hugo <jeffrey.l.hugo@...il.com>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Jonas Karlman <jonas@...boo.se>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Rob Clark <robdclark@...omium.org>,
        Rob Clark <robdclark@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        Steev Klimaszewski <steev@...too.org>
Subject: Re: [PATCH v3 6/9] drm/bridge: ti-sn65dsi86: Use 18-bit DP if we can

Hi,

On Tue, Jul 14, 2020 at 8:31 AM Doug Anderson <dianders@...omium.org> wrote:
>
> > Hopefully BOE gets back to you soon, and there's no rush, I'm just an
> > end user who is extremely appreciative of all the work everyone on the
> > list and the kernel in general put in to make my machines usable.
>
> Just FYI that I got confirmation that the panel is truly 6 bpp but it
> will do FRC dithering if given an 8 bpp input.  That means that you
> should be getting just as good picture quality (and possibly more
> tunable) by using the dithering in the display pipeline and leaving
> the panel as 6bpp.  Thus I'm going to assume that's the route we'll go
> down.  If ever we find someone that wants to use this panel on a
> display controller that can't do its own dithering then I guess we'll
> have to figure out what to do then...
>
> In terms of the more optimal pixel clock for saving power, my proposal
> is still being analyzed and I'll report back when I hear more.  I'm
> seeing if BOE can confirm that my proposal will work both for my panel
> (the -n62 variant) and the one you have (the -n61 variant).

To close the loop here: we finally got back an official word that we
shouldn't use my proposed timings that would have allowed us to move
down to a 1.62 GHz pixel clock.  Though they work most of the time,
there are apparently some corner cases where they cause problems /
flickering.  :(  While you could certainly use the timings on your own
system if they happen to work for you, I don't think it'd be a good
idea to switch the default over to them or anything.  I'm told that
hardware makers will take this type of thing into consideration for
future hardware.

-Doug