[PATCH v2] drm/dp: fallback to minimum when PWM bit count is zero

[Christopher Obbard]

According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
if DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
the sink is required to use the MIN value as the effective bit count.

Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
valid non-zero MIN and MAX capability values. This patch updates the logic
to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
backlight brightness values.

This improves compatibility with panels like the Samsung ATNA40YK20 used
on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
bit count of 0 but declares an 11-bit PWM capability range.

Co-developed-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
Signed-off-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
Signed-off-by: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
---
Changes in v2:
- Split backlight brightness patch from T14s OLED enablement series.
- Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
- Rework commit message to reference eDP spec.
- Rebase on drm-misc-next.
- Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@xxxxxxxxxx/
---
 drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
 1 file changed, 33 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -4083,7 +4083,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 {
 	int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
 	int ret;
-	u8 pn, pn_min, pn_max;
+	u8 pn, pn_min, pn_max, bl_caps;
 
 	if (!bl->aux_set)
 		return 0;
@@ -4094,8 +4094,39 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 			    aux->name, ret);
 		return -ENODEV;
 	}
-
 	pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
+	if (ret != 1) {
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
+			    aux->name, ret);
+		return 0;
+	}
+	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
+	if (ret != 1) {
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
+			    aux->name, ret);
+		return 0;
+	}
+	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, &bl_caps);
+	if (ret != 1) {
+		bl_caps = 0;
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight adjustment cap: %d\n",
+			aux->name, ret);
+	}
+
+	/*
+	 * Some eDP panels report brightness byte count support, but the byte count
+	 * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead.
+	 */
+	if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
+	    && pn_min)
+		pn = pn_min;
+
 	bl->max = (1 << pn) - 1;
 	if (!driver_pwm_freq_hz)
 		return 0;
@@ -4122,21 +4153,6 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 	 * - FxP is within 25% of desired value.
 	 *   Note: 25% is arbitrary value and may need some tweak.
 	 */
-	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
-	if (ret != 1) {
-		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
-			    aux->name, ret);
-		return 0;
-	}
-	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
-	if (ret != 1) {
-		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
-			    aux->name, ret);
-		return 0;
-	}
-	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-
 	/* Ensure frequency is within 25% of desired value */
 	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
 	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);

---
base-commit: ee20c69c789b6cb2179a535cf440d72b98f4a134
change-id: 20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-4020865b6580

Best regards,
-- 
Christopher Obbard <christopher.obbard@xxxxxxxxxx>