> Subject: Re: [PATCH v3 1/8] drm/i915/backlight: Use default/max brightness for
> VESA AUX backlight init
>
> On Tue, 24 Feb 2026, Suraj Kandpal <[email protected]> wrote:
> > If the brightness fetched from VBT/previous state is 0 on backlight
> > initialization, then set the brightness to a default/max value.
> > Whenever the minimum brightness is reported as 0 there are chances we
> > end up with blank screen. This confuses the user into thinking the
> > display is acting weird. This occurs in eDP 1.5 when we are using
> > PANEL_LUMINANCE_OVERRIDE mode to mainpulate brightness via luminance
> > values.
> >
> > Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/15671
> > Signed-off-by: Suraj Kandpal <[email protected]>
> > Reviewed-by: Arun R Murthy <[email protected]>
> > ---
> > v1 -> v2:
> > - Let users set brightness to 0, make it so that it's just not done by
> > default (Arun)
> >
> > v2 -> v3:
> > -Update commit header and message (Arun)
> >
> > drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > 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 eb05ef4bd9f6..c40ce310ad97 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > @@ -564,6 +564,8 @@ static int intel_dp_aux_vesa_setup_backlight(struct
> intel_connector *connector,
> > }
> > panel->backlight.level =
> intel_dp_aux_vesa_get_backlight(connector, 0);
> > panel->backlight.enabled = panel->backlight.level != 0;
> > + if (!panel->backlight.level)
> > + panel->backlight.level = panel->backlight.max;
>
> How does this help when .enabled is still based on level != 0 above?
>
Well we keep the backlight.enabled as false if we read a 0 back from the DPCD
or the current level state is 0.
This is to maintain the policy that if during setup we get 0 as backlight value
eDP backlight is currently disabled
(which means __intel_backlight_enable needs be called). We then change the
current level to max so that when
backlight enable is called after setup from intel_backlight_update, we enable
backlight with max level so that we
do not end up with a blank screen. This is also where we set backlight.enabled
= true.
This is to tackle different eDP behavior where, some preserve the last
brightness value programmed in them (in that case
users want the same brightness to continue) while others don't and just 0 it
out instead of having some default value (in that
case we keep backlight.enabled = false later to be made true during the
__intel_backlight_enable call).
We face these scenarios in some compositors during the pass key phase where the
compositor is still totally not doing everything and does not send
us any explicit brightness value to set thinking eDP would have some basic
default value of it's own . We end up getting a 0 from DPCD and we enable and
set the backlight enable with 0 value which anyways later causes us to call
backlight disable.
In this case during authentication in some compositors like Fedora there are
cases where we do not get a explicitly backlight value till the user has to
blindly enter their
Passkey, after which the compositor sends us some sane value which we then
program.
Regards,
Suraj Kandpal
> > drm_dbg_kms(display->drm,
> > "[CONNECTOR:%d:%s] AUX VESA Nits backlight level
> is controlled through DPCD\n",
> > connector->base.base.id, connector->base.name);
> @@ -573,6
> > +575,8 @@ static int intel_dp_aux_vesa_setup_backlight(struct
> intel_connector *connector,
> > if (current_mode ==
> DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
> > panel->backlight.level = current_level;
> > panel->backlight.enabled = panel->backlight.level != 0;
> > + if (!panel->backlight.level)
> > + panel->backlight.level = panel->backlight.max;
>
> Ditto.
>
> > } else {
> > panel->backlight.level = panel->backlight.max;
> > panel->backlight.enabled = false;
>
> --
> Jani Nikula, Intel
