linux-kernel - Re: [PATCH] drm/dp: For drm_panel_dp_aux_backlight(), init backlight as disabled

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Date:   Thu, 15 Jul 2021 08:06:09 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Rajeev Nandan <rajeevny@...eaurora.org>,
        Lyude Paul <lyude@...hat.com>,
        Robert Foss <robert.foss@...aro.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.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>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/dp: For drm_panel_dp_aux_backlight(), init backlight
 as disabled

Hi,

On Wed, Jul 14, 2021 at 10:17 AM Douglas Anderson <dianders@...omium.org> wrote:
>
> Even after the DP AUX backlight on my board worked OK after applying
> the patch ("drm/panel-simple: Power the panel when probing DP AUX
> backlight") [1], I still noticed some strange timeouts being reported
> by ti_sn_aux_transfer(). Digging, I realized the problem was this:
> * Even though `enabled` in `struct dp_aux_backlight` was false, the
>   base backlight structure (`base` in that structure) thought that the
>   backlight was powered on.
> * If userspace wrote to sysfs in this state then we'd try to enable
>   the backlight.
> * Unfortunatley, enabling the backlight didn't work because the panel
>   itself wasn't powered.
>
> We can only use the backlight if the panel is on and the panel is not
> officially on when we probe (it's temporarily just on enough for us to
> talk to it).
>
> The important thing we want here is to get `BL_CORE_FBBLANK` set since
> userspace can't mess with that. This will keep us disabled until
> drm_panel enables us, which means that the panel is enabled
> first. Ideally we'd just set this in our `props` before calling
> devm_backlight_device_register() but the comments in the header file
> are pretty explicit that we're not supposed to much with the `state`
> ourselves. Because of this, there may be a small window where the
> backlight device is registered and someone could try to tweak with the
> backlight. This isn't likely to happen and even if it did, I don't
> believe this causes any huge problem.
>
> [1] https://lore.kernel.org/lkml/20210714093334.1.Idb41f87e5abae4aee0705db7458b0097fc50e7ab@changeid/
>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
>
>  drivers/gpu/drm/drm_dp_helper.c | 2 ++
>  1 file changed, 2 insertions(+)

Pushed with Lyude's review to drm-misc-next:

17a1837d07be drm/dp: For drm_panel_dp_aux_backlight(), init backlight
as disabled