linux-kernel - Re: [PATCH v5 3/5] drm/dp: Don't read back backlight mode in drm_edp_backlight

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Message-ID: <CAD=FV=VA53LyEA+jDfU6mQ0USwfSBiLK8KfWfc91U9X9f=oN9g@mail.gmail.com>
Date:   Fri, 5 Nov 2021 13:25:08 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Lyude Paul <lyude@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Subject: Re: [PATCH v5 3/5] drm/dp: Don't read back backlight mode in drm_edp_backlight_enable()

Hi,

On Fri, Nov 5, 2021 at 11:34 AM Lyude Paul <lyude@...hat.com> wrote:
>
> As it turns out, apparently some machines will actually leave additional
> backlight functionality like dynamic backlight control on before the OS
> loads. Currently we don't take care to disable unsupported features when
> writing back the backlight mode, which can lead to some rather strange
> looking behavior when adjusting the backlight.
>
> So, let's fix this by just not reading back the current backlight mode on
> initial enable. I don't think there should really be any downsides to this,
> and this will ensure we don't leave any unsupported functionality enabled.
>
> This should fix at least one (but not all) of the issues seen with DPCD
> backlight support on fi-bdw-samus
>
> v5:
> * Just avoid reading back DPCD register - Doug Anderson
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Fixes: 867cf9cd73c3 ("drm/dp: Extract i915's eDP backlight code into DRM helpers")
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 40 ++++++++++-----------------------
>  1 file changed, 12 insertions(+), 28 deletions(-)

You forgot to CC me on this one! ;-)

This looks good to me now, so FWIW:

Reviewed-by: Douglas Anderson <dianders@...omium.org>