On Wed, 20 May 2026, "Nautiyal, Ankit K" <[email protected]> wrote:
> On 5/17/2026 8:17 AM, Suraj Kandpal wrote:
>> This reverts commit 40d2f5820951dee818d05c14677277048bd85f9f.
>>
>> Removing the try_vesa_interface gate caused a backlight regression on
>> panels whose VBT correctly reports INTEL_BACKLIGHT_DISPLAY_DDI and whose
>> PWM path is the actual backlight control, but whose DPCD optimistically
>> advertises DP_EDP_BACKLIGHT_AUX_ENABLE_CAP / _BRIGHTNESS_AUX_SET_CAP.
>> After the commit such panels silently bind to the VESA AUX backlight
>> funcs; AUX writes complete but the panel ignores them, leaving
>> brightness stuck (no-op backlight). Observed on at least KBL and TGL
>> eDP setups.
>>
>> Signed-off-by: Suraj Kandpal <[email protected]>
>
> This will re-open the gitlab issue:
> https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/15679
>
> We might need to add a quirk for the above cases where VBT cannot be
> trusted.
>
> Reviewed-by: Ankit Nautiyal <[email protected]>
I think this is a mess of a thread, with two completely different things
in one. Needs to be sent separately.
BR,
Jani.
>
>
>> ---
>> .../drm/i915/display/intel_dp_aux_backlight.c | 19 ++++++++++++-------
>> 1 file changed, 12 insertions(+), 7 deletions(-)
>>
>> 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 a8d56ebf06a2..7a6c07f6aaeb 100644
>> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
>> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
>> @@ -691,10 +691,9 @@ int intel_dp_aux_init_backlight_funcs(struct
>> intel_connector *connector)
>> struct intel_dp *intel_dp = intel_attached_dp(connector);
>> struct drm_device *dev = connector->base.dev;
>> struct intel_panel *panel = &connector->panel;
>> - bool try_intel_interface = false;
>> + bool try_intel_interface = false, try_vesa_interface = false;
>>
>> - /*
>> - * Check the VBT and user's module parameters to figure out which
>> + /* Check the VBT and user's module parameters to figure out which
>> * interfaces to probe
>> */
>> switch (display->params.enable_dpcd_backlight) {
>> @@ -703,6 +702,7 @@ int intel_dp_aux_init_backlight_funcs(struct
>> intel_connector *connector)
>> case INTEL_DP_AUX_BACKLIGHT_AUTO:
>> switch (panel->vbt.backlight.type) {
>> case INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE:
>> + try_vesa_interface = true;
>> break;
>> case INTEL_BACKLIGHT_DISPLAY_DDI:
>> try_intel_interface = true;
>> @@ -715,12 +715,20 @@ int intel_dp_aux_init_backlight_funcs(struct
>> intel_connector *connector)
>> if (panel->vbt.backlight.type !=
>> INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE)
>> try_intel_interface = true;
>>
>> + try_vesa_interface = true;
>> + break;
>> + case INTEL_DP_AUX_BACKLIGHT_FORCE_VESA:
>> + try_vesa_interface = true;
>> break;
>> case INTEL_DP_AUX_BACKLIGHT_FORCE_INTEL:
>> try_intel_interface = true;
>> break;
>> }
>>
>> + /* For eDP 1.5 and above we are supposed to use VESA interface for
>> brightness control */
>> + if (intel_dp->edp_dpcd[0] >= DP_EDP_15)
>> + try_vesa_interface = true;
>> +
>> /*
>> * Since Intel has their own backlight control interface, the majority
>> of machines out there
>> * using DPCD backlight controls with Intel GPUs will be using this
>> interface as opposed to
>> @@ -733,9 +741,6 @@ int intel_dp_aux_init_backlight_funcs(struct
>> intel_connector *connector)
>> * panel with Intel's OUI - which is also required for us to be able to
>> detect Intel's
>> * backlight interface at all. This means that the only sensible way
>> for us to detect both
>> * interfaces is to probe for Intel's first, and VESA's second.
>> - *
>> - * Also there is a chance some VBTs may advertise false Intel backlight
>> support even if the
>> - * TCON DPCD says otherwise. This means we keep VESA interface as
>> fallback in that case.
>> */
>> if (try_intel_interface && intel_dp->edp_dpcd[0] <= DP_EDP_14b &&
>> intel_dp_aux_supports_hdr_backlight(connector)) {
>> @@ -745,7 +750,7 @@ int intel_dp_aux_init_backlight_funcs(struct
>> intel_connector *connector)
>> return 0;
>> }
>>
>> - if (intel_dp_aux_supports_vesa_backlight(connector)) {
>> + if (try_vesa_interface &&
>> intel_dp_aux_supports_vesa_backlight(connector)) {
>> drm_dbg_kms(dev, "[CONNECTOR:%d:%s] Using VESA eDP backlight
>> controls\n",
>> connector->base.base.id, connector->base.name);
>> panel->backlight.funcs = &intel_dp_vesa_bl_funcs;
--
Jani Nikula, Intel
