On Fri, 22 Nov 2019, Lyude Paul <ly...@redhat.com> 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 <ly...@redhat.com>

Reviewed-by: Jani Nikula <jani.nik...@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)

-- 
Jani Nikula, Intel Open Source Graphics Center
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