linux-kernel - Re: [RFC PATCH 03/10] drm/panel: otm8009a: Don't double check prepared/enabled

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Message-ID: <CAD=FV=WtN_Ag5JsmZGb2arPr8RjhkaBPb14j+a2-+FB-aeTeXw@mail.gmail.com>
Date:   Wed, 13 Sep 2023 11:20:47 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     dri-devel@...ts.freedesktop.org, Maxime Ripard <mripard@...nel.org>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Sam Ravnborg <sam@...nborg.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 03/10] drm/panel: otm8009a: Don't double check prepared/enabled

Hi,

On Fri, Aug 4, 2023 at 2:07 PM Douglas Anderson <dianders@...omium.org> wrote:
>
> As talked about in commit d2aacaf07395 ("drm/panel: Check for already
> prepared/enabled in drm_panel"), we want to remove needless code from
> panel drivers that was storing and double-checking the
> prepared/enabled state. Even if someone was relying on the
> double-check before, that double-check is now in the core and not
> needed in individual drivers.
>
> For the "otm8009a" driver we fully remove the storing of the "enabled"
> state and we remove the double-checking, but we still keep the storing
> of the "prepared" state since the backlight code in the driver checks
> it. This backlight code may not be perfectly safe since there doesn't
> appear to be sufficient synchronization between the backlight driver
> (which userspace can call into directly) and the code that's
> unpreparing the panel. However, this lack of safety is not new and can
> be addressed in a future patch.
>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
> From quick inspection, I think the right way to handle the backlight
> properly is:
> 1. Start calling backlight_get_brightness() instead of directly
>    getting "bd->props.brightness" and bd->props.power. This should
>    return 0 for a disabled (or blanked or powered off) backlight.
> 2. Cache the backlight level in "struct otm8009a"
> 3. If the backlight isn't changing compared to the cached value, make
>    otm8009a_backlight_update_status() a no-op.
> 4. Remove the caching of the "prepared" value.
>
> That should work and always be safe because we always enable/disable
> the backlight in the panel's enable() and disable() functions. The
> backlight core has proper locking in this case. A disabled backlight
> will always return a level of 0 which will always make the backlight's
> update_status a no-op when the panel is disabled and keep us from
> trying to talk to the panel when it's off. Userspace can't directly
> cause a backlight to be enabled/disabled, it can only affect the other
> blanking modes.

Note: I'm not planning to take on the cleanup of making the backlight
of this driver work better. Ideally someone who uses / maintains the
affected hardware could give it a shot.


>  .../gpu/drm/panel/panel-orisetech-otm8009a.c    | 17 -----------------
>  1 file changed, 17 deletions(-)


In response to the cover letter [1], I proposed landing patches #1-#3
directly from here while we resolve the issues talked about in
response to patch #4 [2]. I didn't hear any complaints, so I took
Linus W's review tag from the cover letter and pushed this to
drm-misc-next.

1e0465eb16a4 drm/panel: otm8009a: Don't double check prepared/enabled

[1] https://lore.kernel.org/r/CAD=FV=UFuUsrrZmkL8_RL5WLvkJryDwRSAy_PWTa-hX_p0dF+Q@mail.gmail.com
[2] https://lore.kernel.org/r/20230804140605.RFC.4.I930069a32baab6faf46d6b234f89613b5cec0f14@changeid/