linux-kernel - Re: [RFC PATCH] drm/edid: drm_add_modes_noedid() should set lowest resolution as preferred

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Date:   Tue, 26 Apr 2022 13:52:52 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Abhinav Kumar <quic_abhinavk@...cinc.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...il.com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@...cinc.com>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] drm/edid: drm_add_modes_noedid() should set lowest
 resolution as preferred

Hi,

On Tue, Apr 26, 2022 at 1:46 PM Abhinav Kumar <quic_abhinavk@...cinc.com> wrote:
>
> On 4/26/2022 1:21 PM, Douglas Anderson wrote:
> > If we're unable to read the EDID for a display because it's corrupt /
> > bogus / invalid then we'll add a set of standard modes for the
> > display. When userspace looks at these modes it doesn't really have a
> > good concept for which mode to pick and it'll likely pick the highest
> > resolution one by default. That's probably not ideal because the modes
> > were purely guesses on the part of the Linux kernel.
> >
> > Let's instead set 640x480 as the "preferred" mode when we have no EDID.
> >
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
>
> drm_dmt_modes array is sorted but you are also relying on this check to
> eliminate the non-60fps modes
>
> 5611            if (drm_mode_vrefresh(ptr) > 61)
> 5612                    continue;
>
> I am not sure why we filter out the modes > 61 vrefresh.
>
> If that check will remain this is okay.
>
> If its not, its not reliable that the first mode will be 640x480@60

I suspect that the check will remain. I guess I could try to do
something fancier if people want, but I'd be interested in _what_
fancier thing I should do if so. Do we want the rule to remain that we
always prefer 640x480, or do we want to prefer the lowest resolution?
...do we want to prefer 60 Hz or the lowest refresh rate? Do we do
this only for DP (which explicitly calls out 640x480 @60Hz as the best
failsafe) or for everything?

For now, the way it's coded up seems reasonable (to me). It's the
lowest resolution _and_ it's 640x480 just because of the current
values of the table. I suspect that extra lower resolution failsafe
modes won't be added, but we can always change the rules here if/when
they are.

-Doug