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Subject: Re: [3/5] drm/i915: Fix DPCD register order in
 intel_dp_aux_enable_backlight()
To:     Lyude Paul <lyude@redhat.com>, intel-gfx@lists.freedesktop.org
Cc:     David Airlie <airlied@linux.ie>, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org
References: <20191122231616.2574-4-lyude@redhat.com>
From:   Perr Yuan <pyuan@redhat.com>
Message-ID: <c1f93ab5-0eae-5c07-4ad4-abab81e0b596@redhat.com>
Date:   Mon, 23 Dec 2019 02:19:00 -0500
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On 11/22/19 6:16 PM, Lyude Paul wrote:
> For eDP panels, it appears it's expected that so long as the panel is in
> DPCD control mode that the brightness value is never set to 0. Instead,
> if the desired effect is to set the panel's backlight to 0 we're
> expected to simply turn off the backlight through the
> DP_EDP_DISPLAY_CONTROL_REGISTER.
> 
> We already do the latter correctly in intel_dp_aux_disable_backlight().
> But, we make the mistake of writing the DPCD registers in the wrong
> order when enabling the backlight in intel_dp_aux_enable_backlight()
> since we currently enable the backlight through
> DP_EDP_DISPLAY_CONTROL_REGISTER before writing the brightness level. On
> the X1 Extreme 2nd Generation, this appears to have the potential of
> confusing the panel in such a way that further attempts to set the
> brightness don't actually change the backlight as expected and leave it
> off. Presumably, this happens because the incorrect register writing
> order briefly leaves the panel with DPCD mode enabled and a 0 brightness
> level set.
> 
> So, reverse the order we write the DPCD registers when enabling the
> panel backlight so that we write the brightness value first, and enable
> the backlight second. This fix appears to be the final bit needed to get
> the backlight on the ThinkPad X1 Extreme 2nd Generation's AMOLED screen
> working.
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Reviewed-by: Jani Nikula <jani.nikula@intel.com>
> ---
>   drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> index 0bf8772bc7bb..87b59db9ffe3 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> @@ -205,8 +205,9 @@ static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_st
>   		}
>   	}
>   
> +	intel_dp_aux_set_backlight(conn_state,
> +				   connector->panel.backlight.level);
>   	set_aux_backlight_enable(intel_dp, true);
> -	intel_dp_aux_set_backlight(conn_state, connector->panel.backlight.level);
>   }
>   
>   static void intel_dp_aux_disable_backlight(const struct drm_connector_state *old_conn_state)
> 

Tested-by:Perry Yuan <pyuan@redhat.com>