[SRU][R][PATCH 10/13] drm/i915/backlight: Check if VESA backlight is possible

[Werner Sembach]

From: Suraj Kandpal <suraj.kandpal at intel.com>

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 at intel.com>
Reviewed-by: Pranay Samala <pranay.samala at intel.com>
Link: https://patch.msgid.link/20260316031850.81794-1-suraj.kandpal@intel.com
Signed-off-by: Werner Sembach <wse at tuxedocomputers.com>
---
 .../drm/i915/display/intel_dp_aux_backlight.c | 32 ++++++++++++++++++-
 1 file changed, 31 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 8d3f114a458b1..4d6ef1dca4a79 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -600,6 +600,34 @@ 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)
 {
@@ -616,12 +644,14 @@ 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;
 }
 
-- 
2.43.0