linux-kernel - Re: [PATCH v3] drm/probe-helper: Make 640x480 first if no EDID

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Date:   Mon, 23 May 2022 17:59:02 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Stéphane Marchesin <marcheu@...omium.org>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@...cinc.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Rob Clark <robdclark@...il.com>,
        Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        "Abhinav Kumar (QUIC)" <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Stephen Boyd <swboyd@...omium.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v3] drm/probe-helper: Make 640x480 first if no EDID

Hi,

On Fri, May 20, 2022 at 5:01 PM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Mon, May 16, 2022 at 3:28 AM Thomas Zimmermann <tzimmermann@...e.de> wrote:
> >
> > Hi Douglas,
> >
> > I understand that you're trying to tell userspace that the modelist has
> > been made up, but it's not something that should be done via fragile
> > heuristics IMHO.
> >
> > I looked at the Chromium source code that you linked, but I cannot say
> > whether it's doing the correct thing. It all depends on what your
> > program needs.
> >
> > In that function, you could also search for 'DRM_MODE_TYPE_USERDEF'.
> > It's the mode that the user specified on the kernel command line. If
> > Chromium's automatic mode selection fails, you'd give your users direct
> > control over it.
>
> That doesn't really work for Chrome OS. Certainly a kernel hacker
> could do this, but it's not something I could imagine us exposing to
> an average user of a Chromebook.
>
>
> > When there's no flagged mode or if
> > /sys/class/drm/card<...>/status contains "unconnected", you can assume
> > that the modelist is artificial and try the modes in an appropriate order.
>
> So "no flagged" means that nothing is marked as preferred, correct?
>
> ...so I guess what you're suggesting is that the order that the kernel
> is presenting the modes to userspace is not ABI. If there are no
> preferred modes then userspace shouldn't necessarily assume that the
> first mode returned is the best mode. Instead it should assume that if
> there is no preferred mode then the mode list is made up and it should
> make its own decisions about the best mode to start with. If this is
> the ABI from the kernel then plausibly I could convince people to
> change userspace to pick 640x480 first in this case.
>
> > If we really want the kernel to give additional guarantees, we should
> > have a broader discussion about this topic IMHO.
>
> Sure. I've added Stéphane Marchesin to this thread in case he wants to
> chime in about anything.
>
> Overall, my take on the matter:
>
> * Mostly I got involved because, apparently, a DP compliance test was
> failing. The compliance test was upset that when it presented us with
> no EDID that we didn't default to 640x480. There was a push to make a
> fix for this in the Qualcomm specific driver but that didn't sit right
> with me.
>
> * On all devices I'm currently working with (laptops), the DP is a
> secondary display. If a user was trying to plug in a display with a
> bad EDID and the max mode (1024x768) didn't work, they could just use
> the primary display to choose a different resolution. It seems
> unlikely a user would truly be upset and would probably be happy they
> could get their broken display to work at all. Even if this is a
> primary display, I believe there are documented key combos to change
> the resolution of the primary display even if you can't see anything.
>
> * That all being said, defaulting to 640x480 when there's no EDID made
> sense to me, especially since it's actually defined in the DP spec. So
> I'm trying to do the right thing and solve this corner case. That
> being said, if it's truly controversial I can just drop it.
>
>
> So I guess my plan will be to give Stéphane a little while in case he
> wants to chime in. If not then I guess I'll try a Chrome patch...
> ...and if that doesn't work, I'll just drop it.

OK, this userspace code seems to work:

https://crrev.com/c/3662501 - ozone/drm: Try 640x480 before picking
the first mode if no EDID

...so we'll see how review of that goes. :-)