drm/i915/backlight: Check if VESA backlight is possible

Check if BACKLIGHT_BRIGHTNESS_AUX_SET_CAPABLE bit is
set then EDP_PWMGEN_BIT_COUNT_CAP_MIN and EDP_PWMGEN_BIT_COUNT_CAP_MAX
follow the eDP 1.4b Section 10.3. Which states min should
be >= 1 and max should be >= min. Some legacy panels
do not follow this properly. They set the
BACKLIGHT_BRIGHTNESS_AUX_SET_CAPABLE bit while not correctly
populating the min and max fields leading to a 0 max value.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/7514
Fixes: 40d2f5820951 ("drm/i915/backlight: Remove try_vesa_interface")
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Pranay Samala <pranay.samala@intel.com>
Link: https://patch.msgid.link/20260316031850.81794-1-suraj.kandpal@intel.com

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 a7b186d0e3c4740f029d748672dcc82f9c7c37e6..d0c76632a946a3b29df990d32aa3e5b27efb0b7f 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -609,6 +609,34 @@ static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector,
        return 0;
 }
 
+static bool
+check_if_vesa_backlight_possible(struct intel_dp *intel_dp)
+{
+       int ret;
+       u8 bit_min, bit_max;
+
+       if (!(intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP))
+               return true;
+
+       ret = drm_dp_dpcd_read_byte(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &bit_min);
+       if (ret < 0)
+               return false;
+
+       bit_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+       if (bit_min < 1)
+               return false;
+
+       ret = drm_dp_dpcd_read_byte(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &bit_max);
+       if (ret < 0)
+               return false;
+
+       bit_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+       if (bit_max < bit_min)
+               return false;
+
+       return true;
+}
+
 static bool
 intel_dp_aux_supports_vesa_backlight(struct intel_connector *connector)
 {
@@ -625,12 +653,14 @@ intel_dp_aux_supports_vesa_backlight(struct intel_connector *connector)
                return true;
        }
 
-       if (drm_edp_backlight_supported(intel_dp->edp_dpcd)) {
+       if (drm_edp_backlight_supported(intel_dp->edp_dpcd) &&
+           check_if_vesa_backlight_possible(intel_dp)) {
                drm_dbg_kms(display->drm,
                            "[CONNECTOR:%d:%s] AUX Backlight Control Supported!\n",
                            connector->base.base.id, connector->base.name);
                return true;
        }
+
        return false;
 }