linux-kernel - Re: [RFC PATCH 1/3] dt-bindings: display: simple: Add the panel on sc7180-trogdor-pompom

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Message-ID: <CAD=FV=VDKQCMtxd2NMRefxQm5uzDS6_rUVP5YsTsDEYX+jSipw@mail.gmail.com>
Date:   Mon, 29 Mar 2021 09:07:07 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Rob Clark <robdclark@...il.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Rob Clark <robdclark@...omium.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Nicolas Boichat <drinkcat@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Andy Gross <agross@...nel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Steev Klimaszewski <steev@...i.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/3] dt-bindings: display: simple: Add the panel on sc7180-trogdor-pompom

Hi,

On Fri, Mar 26, 2021 at 5:38 AM Thierry Reding <thierry.reding@...il.com> wrote:
>
> The point remains that unless we describe exactly which panel we're
> dealing with, we ultimately have no way of properly quirking anything if
> we ever have to.

Just to clarify here: with my initial proposal we actually could still
quirk things if we had to. If the quirk needed to be applied before
power on we'd just have to apply the quirk to the whole board (which
we'd have to do anyway). After the panel was powered on then we could
read the EDID and apply a quirk based on what the EDID tells us,
right?

> Also, once we allow this kind of wildcard we can
> suddenly get into a situation where people might want to reuse this on
> something that's not at all a google-pompom board because the same
> particular power sequence happens to work on on some other board.

That's a legit concern. Of course, people could already do that with
existing panels right? One would also hope that if they reused this
they also used the "more specific to least specific" rule, so someone
could reuse (without any problems) with:

compatible = "some-other-company,some-other-board-panel", "google,pompom-panel"

That doesn't seem like it would be terrible.

> Similarly I can imagine a situation where we could now have the same
> panel supported by multiple different wildcard compatible strings. How
> is that supposed to be any cleaner than what we have now?

I'm tempted to call this (same panel supported by multiple different
compatible strings) a feature, actually. Specifically:

Even if the exact same hardware is shipped with more than one board,
it may have a different EDID programmed into it. From what I've seen
the timings used on a panel may need to be adjusted based on the SoC
used (and what clock rates it can provide / features of the underlying
display driver), EMI concerns, and power consumption concerns. Once a
different EDID is programmed in it then it sorta becomes a "different"
panel, right? I think sometimes (?) panel vendors assign a slightly
different model number per board, but I'm not sure.

-Doug