Date:   Mon, 15 Aug 2022 08:45:43 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Rob Clark <robdclark@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Sankeerth Billakanti <quic_sbillaka@...cinc.com>
Subject: Re: [RFC PATCH] drm/edid: Make 144 Hz not preferred on Sharp
 LQ140M1JW46

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@...no.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@...no.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@...cinc.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@...omium.org> wrote:
> > > > > > >>
> > > > > > >> Hi,
> > > > > > >>
> > > > > > >> On Fri, Jul 22, 2022 at 9:37 AM Abhinav Kumar <quic_abhinavk@...cinc.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.

Maxime

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