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From:   Doug Anderson <dianders@chromium.org>
Date:   Mon, 6 Jun 2022 13:11:47 -0700
Message-ID: <CAD=FV=VKWNeXzDZ=DdXi=U218xghLJAeAQah_uFOjM9WrGZ5sQ@mail.gmail.com>
Subject: Re: [PATCH v4] drm/probe-helper: Default to 640x480 if no EDID on DP
To:     dri-devel <dri-devel@lists.freedesktop.org>,
        Daniel Vetter <daniel@ffwll.ch>
Cc:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Stephen Boyd <swboyd@chromium.org>,
        =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>,
        Sankeerth Billakanti <quic_sbillaka@quicinc.com>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@quicinc.com>,
        "Abhinav Kumar (QUIC)" <quic_abhinavk@quicinc.com>,
        Rob Clark <robdclark@gmail.com>,
        freedreno <freedreno@lists.freedesktop.org>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@quicinc.com>,
        David Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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Hi,

On Wed, Jun 1, 2022 at 11:23 AM Douglas Anderson <dianders@chromium.org> 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. Since we have no true information about the connected
> display, these modes are essentially guesses but better than nothing.
> At the moment, none of the modes returned is marked as preferred, but
> the modes are sorted such that the higher resolution modes are listed
> first.
>
> When userspace sees these modes presented by the kernel it needs to
> figure out which one to pick. At least one userspace, ChromeOS [1]
> seems to use the rules (which seem pretty reasonable):
> 1. Try to pick the first mode marked as preferred.
> 2. Try to pick the mode which matches the first detailed timing
>    descriptor in the EDID.
> 3. If no modes were marked as preferred then pick the first mode.
>
> Unfortunately, userspace's rules combined with what the kernel is
> doing causes us to fail section 4.2.2.6 (EDID Corruption Detection) of
> the DP 1.4a Link CTS. That test case says that, while it's OK to allow
> some implementation-specific fall-back modes if the EDID is bad that
> userspace should _default_ to 640x480.
>
> Let's fix this by marking 640x480 as default for DP in the no-EDID
> case.
>
> NOTES:
> - In the discussion around v3 of this patch [2] there was talk about
>   solving this in userspace and I even implemented a patch that would
>   have solved this for ChromeOS, but then the discussion turned back
>   to solving this in the kernel.
> - Also in the discussion of v3 [2] it was requested to limit this
> 83;40900;0c  change to just DP since folks were worried that it would break some
>   subtle corner case on VGA or HDMI.
>
> [1] https://source.chromium.org/chromium/chromium/src/+/a051f741d0a15caff2251301efe081c30e0f4a96:ui/ozone/platform/drm/common/drm_util.cc;l=488
> [2] https://lore.kernel.org/r/20220513130533.v3.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
> ---
> I put Abhinav's Reviewed-by tag from v2 here since this is nearly the
> same as v2. Hope this is OK.
>
> Changes in v4:
> - Code is back to v2, but limit to just DP.
> - Beefed up the commit message.
>
> Changes in v3:
> - Don't set preferred, just disable the sort.
>
> Changes in v2:
> - Don't modify drm_add_modes_noedid() 'cause that'll break others
> - Set 640x480 as preferred in drm_helper_probe_single_connector_modes()
>
>  drivers/gpu/drm/drm_probe_helper.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)

Pushed to drm-misc-next after cleaning up the turd that I somehow left
in the commit message.

fae7d186403e drm/probe-helper: Default to 640x480 if no EDID on DP

-Doug