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    SubjectRe: [PATCH 3/5] drm/i915: Fix DPCD register order in intel_dp_aux_enable_backlight()
    Date
    On Fri, 22 Nov 2019, Lyude Paul <lyude@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 <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)

    --
    Jani Nikula, Intel Open Source Graphics Center

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