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

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Message-ID: <CAJMQK-jFca51-wDCDgb-+mcJpbV2DmX04dxvaG6Xg+p5e2X8kg@mail.gmail.com>
Date:   Mon, 6 Nov 2023 11:39:14 -0800
From:   Hsin-Yi Wang <hsinyi@...omium.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Maxime Ripard <mripard@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        linux-kernel@...r.kernel.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

On Mon, Nov 6, 2023 at 8:21 AM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Mon, Nov 6, 2023 at 12:06 AM Maxime Ripard <mripard@...nel.org> wrote:
> >
> > On Thu, Nov 02, 2023 at 07:33:48AM -0700, Doug Anderson wrote:
> > > 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
> >
> > I disagree. "Flexibility" here just means "the way to shoot itself in
> > the foot without knowing it's aiming at its foot".
> >
> > If a mode is broken, we shouldn't expose it, just like we don't for all
> > modes that require a maximum frequency higher than what the controller
> > can provide on HDMI for example.
>
> In this particular case we aren't saying that modes are broken. There
> are two (somewhat separate) things in Hsin-Yi's series.
>
> The first thing is a quirk for panels with incorrect modes in their
> EDID when using the generic "edp-panel" compatible. In that case we
> now _replace_ the broken mode with a more correct one because, as you
> say, we shouldn't be telling userspace about a broken mode.
>
> The second thing in Hsin-Yi's series is for when we're _not_ using the
> generic "edp-panel". In that case we have a hardcoded mode from the
> "compatible" string but we also have modes from the EDID and that's
> what ${SUBJECT} patch is about. Here we don't truly know that the
> modes in the EDID are broken.
>
>
> > > and also is closer to what we've historically done (though,
> > > historically, we just allowed there to be more than one "preferred"
> > > mode).
> >
> > I have no idea what history you're referring to here
>
> History = historical behavior? As above, I pointed out that the kernel
> has been merging the hardcoded and EDID modes as far back as commit
> 280921de7241 ("drm/panel: Add simple panel support") in 2013.
>
> That being said, the historical behavior has more than one mode marked
> preferred which is bad, so we're changing the behavior anyway. I'm not
> against changing it to just have the hardcoded mode if that's what
> everyone else wants (and it sounds like it is).

I'll change the behavior to: if hard-coded mode presents, don't add
edid mode since it will result in multiple preferred modes.