linux-kernel - Re: [PATCH] drm/i915/display/lspcon: Increase LSPCON mode settle timeout

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Date:   Thu, 6 Oct 2022 15:59:22 -0700
From:   Pablo Ceballos <pceballos@...gle.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Maxime Ripard <maxime@...no.tech>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915/display/lspcon: Increase LSPCON mode settle timeout

On Tue, Oct 4, 2022 at 10:38 AM Jani Nikula <jani.nikula@...ux.intel.com> wrote:
> Got any bug report with more info, or any other details to back this up?
> This is kind of thin. What's the 800 ms based on?

This issue affected several different CometLake-based Chrome OS device
designs. The details of the original report are in the Google partner
issue tracker (issue # 178169843), but I believe this requires a
Google partner account to access:
https://partnerissuetracker.corp.google.com/issues/178169843

The summary is that we were seeing these "*ERROR* LSPCON mode hasn't
settled" messages in the kernel logs followed by the display not
working at all. We increased the timeout to 500ms while investigation
continued and this reduced the number of occurrences of this issue:
https://chromium.googlesource.com/chromiumos/third_party/kernel/+/7b2899fc1a6f9409e8075b3153baaf02c4d1fc75

The problem continued to occur on about 2% of devices even after
increasing the timeout to 500ms. The investigation continued in issue
# 188035814, with engineers from Parade and Intel involved.
Ultimately, the recommendation from Intel engineers was to increase
the timeout further:
https://partnerissuetracker.corp.google.com/issues/188035814

The timeout was then increased to 1000ms:
https://chromium.googlesource.com/chromiumos/third_party/kernel/+/a16cfc2062e768c8e5ad8fa09b8ca127aa1ead9a

I recently ran 100 reboot trials on one device and found that the
median time for the LSPCON mode to settle was 440ms and the max was
444ms. But we know from the original reports that even after we set
the timeout to 500ms the issue continued to happen on some small
percentage of devices. So this is why I picked the larger value of
800ms.