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From:   Doug Anderson <dianders@chromium.org>
Date:   Wed, 17 Aug 2022 16:45:29 -0700
Message-ID: <CAD=FV=W9sK2dKMnDZmW_ipLGZJFrrvNHz6zHsXVaSCD_u4JpiA@mail.gmail.com>
Subject: Re: [RFC PATCH] drm/edid: Make 144 Hz not preferred on Sharp LQ140M1JW46
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Abhinav Kumar <quic_abhinavk@quicinc.com>,
        Rob Clark <robdclark@gmail.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        freedreno <freedreno@lists.freedesktop.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Sankeerth Billakanti <quic_sbillaka@quicinc.com>
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Precedence: bulk

Hi,

On Sun, Aug 14, 2022 at 11:46 PM Maxime Ripard <maxime@cerno.tech> wrote:
>
> On Fri, Jul 29, 2022 at 12:57:40PM -0700, Doug Anderson wrote:
> > Hi,
> >
> > On Fri, Jul 29, 2022 at 9:41 AM Maxime Ripard <maxime@cerno.tech> wrote:
> > >
> > > On Fri, Jul 29, 2022 at 07:50:20AM -0700, Doug Anderson wrote:
> > > > On Fri, Jul 29, 2022 at 12:51 AM Maxime Ripard <maxime@cerno.tech> wrote:
> > > > >
> > > > > On Thu, Jul 28, 2022 at 02:18:38PM -0700, Doug Anderson wrote:
> > > > > > Hi,
> > > > > >
> > > > > > On Thu, Jul 28, 2022 at 10:34 AM Abhinav Kumar
> > > > > > <quic_abhinavk@quicinc.com> wrote:
> > > > > > >
> > > > > > > Hi Rob and Doug
> > > > > > >
> > > > > > > On 7/22/2022 10:36 AM, Rob Clark wrote:
> > > > > > > > On Fri, Jul 22, 2022 at 9:48 AM Doug Anderson <dianders@chromium.org> wrote:
> > > > > > > >>
> > > > > > > >> Hi,
> > > > > > > >>
> > > > > > > >> On Fri, Jul 22, 2022 at 9:37 AM Abhinav Kumar <quic_abhinavk@quicinc.com> wrote:
> > > > > > > >>>
> > > > > > > >>> + sankeerth
> > > > > > > >>>
> > > > > > > >>> Hi Doug
> > > > > > > >>>
> > > > > > > >>> On 7/21/2022 3:23 PM, Douglas Anderson wrote:
> > > > > > > >>>> The Sharp LQ140M1JW46 panel is on the Qualcomm sc7280 CRD reference
> > > > > > > >>>> board. This panel supports 144 Hz and 60 Hz. In the EDID, the 144 Hz
> > > > > > > >>>> mode is listed first and thus is marked preferred. The EDID decode I
> > > > > > > >>>> ran says:
> > > > > > > >>>>
> > > > > > > >>>>     First detailed timing includes the native pixel format and preferred
> > > > > > > >>>>     refresh rate.
> > > > > > > >>>>
> > > > > > > >>>>     ...
> > > > > > > >>>>
> > > > > > > >>>>     Detailed Timing Descriptors:
> > > > > > > >>>>       DTD 1:  1920x1080  143.981 Hz  16:9   166.587 kHz  346.500 MHz
> > > > > > > >>>>                    Hfront   48 Hsync  32 Hback  80 Hpol N
> > > > > > > >>>>                    Vfront    3 Vsync   5 Vback  69 Vpol N
> > > > > > > >>>>       DTD 2:  1920x1080   59.990 Hz  16:9    69.409 kHz  144.370 MHz
> > > > > > > >>>>                    Hfront   48 Hsync  32 Hback  80 Hpol N
> > > > > > > >>>>                    Vfront    3 Vsync   5 Vback  69 Vpol N
> > > > > > > >>>>
> > > > > > > >>>> I'm proposing here that the above is actually a bug and that the 60 Hz
> > > > > > > >>>> mode really should be considered preferred by Linux.
> > > > > > >
> > > > > > > Its a bit tricky to say that this is a bug but I think we can certainly
> > > > > > > add here that for an internal display we would have ideally had the
> > > > > > > lower resolution first to indicate it as default.
> > > > > >
> > > > > > Yeah, it gets into the vagueness of the EDID spec in general. As far
> > > > > > as I can find it's really up to the monitor to decide by what means it
> > > > > > chooses the "preferred" refresh rate if the monitor can support many.
> > > > > > Some displays may decide that the normal rate is "preferred" and some
> > > > > > may decide that the high refresh rate is "preferred". Neither display
> > > > > > is "wrong" per say, but it's nice to have some consistency here and to
> > > > > > make it so that otherwise "dumb" userspace will get something
> > > > > > reasonable by default. I'll change it to say:
> > > > > >
> > > > > > While the EDID spec appears to allow a display to use any criteria for
> > > > > > picking which refresh mode is "preferred" or "optimal", that vagueness
> > > > > > is a bit annoying. From Linux's point of view let's choose the 60 Hz
> > > > > > one as the default.
> > > > >
> > > > > And if we start making that decision, it should be for all panels with a
> > > > > similar constraint, so most likely handled by the core, and the new
> > > > > policy properly documented.
> > > > >
> > > > > Doing that just for a single panel is weird.
> > > >
> > > > Yeah, though having a "general policy" in the core can be problematic.
> > > >
> > > > In general I think panel EDIDs are only trustworthy as far as you can
> > > > throw them. They are notorious for having wrong and incorrect
> > > > information, which is why the EDID quirk list exists to begin with.
> > > > Trying to change how we're going to interpret all EDIDs, even all
> > > > EDIDs for eDP panels, seems like it will break someone somewhere.
> > > > Maybe there are EDIDs out there that were only ever validated at the
> > > > higher refresh rate and they don't work / flicker / cause digitizer
> > > > noise at the lower refresh rate. Heck, we've seen eDP panel vendors
> > > > that can't even get their checksum correct, so I'm not sure I want to
> > > > make a global assertion that all panels validated their "secondary"
> > > > display mode.
> > > >
> > > > In this particular case, we have validated that this particular Sharp
> > > > panel works fine at the lower refresh rate.
> > > >
> > > > I would also note that, as far as I understand it, ODMs actually can
> > > > request different EDIDs from the panel vendors. In the past we have
> > > > been able to get panel vendors to change EDIDs. Thus for most panels
> > > > I'd expect that we would discover this early, change the EDID default,
> > > > and be done with it. The case here is a little unusual in that by the
> > > > time we got involved and started digging into this panel too many were
> > > > created and nobody wants to throw away those old panels. This is why
> > > > I'm treating it as a quirk/bug. Really: we should have updated the
> > > > EDID of the panel but we're unable to in this case.
> > >
> > > You raise some good points, but most of the discussion around that patch
> > > were mostly around performances, power consumption and so on.
> > >
> > > This is very much a policy decision, and if there is some panel where
> > > the EDID reports 60Hz but is broken, then that panel should be the
> > > exception to the policy
> > >
> > > But doing it for a single panel is just odd
> >
> > OK, fair enough. I'll abandon this patch at least as far as mainline
> > is concerned, then.
>
> That wasn't really my point though :)
>
> If you think that this change is needed, then we should totally discuss
> it and I'm not opposed to it.
>
> What I don't really like about this patch is that it's about a single
> panel: if we're doing it we should do it for all the panels.
>
> Where we do it can also be discussed, but we should remain consistent
> there.

I was never massively confident about it, which is why I added the
"RFC" tag to begin with. ;-) In general I suspect that either change
will make people upset. In other words, if we programmatically always
try to put the "high refresh rate" first for all displays then people
will be upset and if we programmatically always try to put the "60 Hz
rate" first then people will be upset. Unless someone wants to stand
up and say that one side or the other is wrong, I think we're going to
simply leave this up to the whim of individual panels. Someone could
stand up and demand that it go one way or the other, but I certainly
don't have that clout.

The spec, as far as I can tell, says that it's up to the panel vendor
to use whatever means they want to decide on the "preferred" refresh
rate. Thus, as far as the spec is concerned this decision is made on
an individual panel basis. ;-) This was really the justification for
why my patch was just on one panel.

In any case, as I said I'm OK w/ dropping this. We'll find other ways
to work around the issue.

-Doug