linux-kernel - Re: [PATCH 3/3] drm/panel-edp: Choose correct preferred mode

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Date:   Thu, 2 Nov 2023 07:33:48 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Hsin-Yi Wang <hsinyi@...omium.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        linux-kernel@...r.kernel.org, Maxime Ripard <mripard@...nel.org>,
        dri-devel@...ts.freedesktop.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH 3/3] drm/panel-edp: Choose correct preferred mode

Hi,

On Wed, Nov 1, 2023 at 11:31 PM Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> On Wed, 1 Nov 2023 at 23:26, Hsin-Yi Wang <hsinyi@...omium.org> wrote:
> >
> > If a non generic edp-panel is under aux-bus, the mode read from edid would
> > still be selected as preferred and results in multiple preferred modes,
> > which is ambiguous.
> >
> > If a hard-coded mode is present, unset the preferred bit of the modes read
> > from edid.
>
> Can we skip the EDID completely if the hardcoded override is present?

Yeah, I wondered about that too. The blending of the hardcoded with
the EDID predates my involvement with the driver. You can see even as
of commit 280921de7241 ("drm/panel: Add simple panel support") that
the driver would start with the EDID modes (if it had them) and then
go onto add the hardcoded modes. At least for eDP panels, though,
nobody (or almost nobody?) actually provided panel-simple a DDC bus at
the same time it was given a hardcoded panel.

I guess I could go either way, but I have a slight bias to adding the
extra modes and just making it clear to userspace that none of them
are "preferred". That seems like it would give userspace the most
flexibility and also is closer to what we've historically done
(though, historically, we just allowed there to be more than one
"preferred" mode).

One thing we definitely want to do, though, is to still expose the
EDID to userspace even if we're using a hardcoded mode. I believe
that, at least on ChromeOS, there are some tools that look at the EDID
directly for some reason or another.

> > Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> > ---
> >  drivers/gpu/drm/drm_modes.c       | 16 ++++++++++++++++
> >  drivers/gpu/drm/panel/panel-edp.c |  7 +++++--
> >  include/drm/drm_modes.h           |  1 +
> >  3 files changed, 22 insertions(+), 2 deletions(-)
>
> Anyway, this should be split into two patches. One for drm_modes.c,
> another one for the panel driver.

Yeah, that's probably a good idea.

-Doug