ACK/CMNT: [SRU][R][PATCH 1/1] Revert "drm/i915/backlight: Remove try_vesa_interface"

[Alice C. Munduruca]

On Mon Jul 27, 2026 at 11:21 AM EDT, Krystian Kaniewski via kernel-team wrote:
> From: Suraj Kandpal <suraj.kandpal at intel.com>
>
BugLink: https://bugs.launchpad.net/bugs/2161309

> 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 <suraj.kandpal at intel.com>
> Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal at intel.com>
> Link: https://patch.msgid.link/20260517024709.1016121-1-suraj.kandpal@intel.com
> (cherry picked from commit f30fddb4402313aa5301a74d721638d343395269)
> Signed-off-by: Krystian Kaniewski <krystian.kaniewski at canonical.com>
> ---
>  .../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;

Hi Krystian,

A buglink is needed for each patch as well. You can use
ubuntu-check-commit [1] to check your commits for the
proper format before sending.

Otherwise your submission looks good :)

[1]: https://kernel.ubuntu.com/forgejo/actions/ubuntu-check-commit

Acked-by: Alice C. Munduruca <alice.munduruca at canonical.com>