linux-kernel - [PATCH 1/2] drm/dp: Add drm_edp_backlight_get

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Message-ID: <20250926085401.2808634-1-aaron.ma@canonical.com>
Date: Fri, 26 Sep 2025 16:54:00 +0800
From: Aaron Ma <aaron.ma@...onical.com>
To: dri-devel@...ts.freedesktop.org,
	intel-gfx@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Cc: maarten.lankhorst@...ux.intel.com,
	mripard@...nel.org,
	tzimmermann@...e.de,
	airlied@...il.com,
	simona@...ll.ch,
	jani.nikula@...ux.intel.com,
	rodrigo.vivi@...el.com,
	suraj.kandpal@...el.com,
	imre.deak@...el.com,
	joonas.lahtinen@...ux.intel.com,
	aaron.ma@...onical.com
Subject: [PATCH 1/2] drm/dp: Add drm_edp_backlight_get_level

Implement drm_edp_backlight_get_level() to read the current
backlight brightness level from eDP DPCD registers via AUX channel.

Signed-off-by: Aaron Ma <aaron.ma@...onical.com>
---
 drivers/gpu/drm/display/drm_dp_helper.c | 52 +++++++++++++++++++++++++
 include/drm/display/drm_dp_helper.h     |  1 +
 2 files changed, 53 insertions(+)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index 1ecc3df7e3167..0cfb357ebd9e2 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -3945,6 +3945,58 @@ int drm_dp_pcon_convert_rgb_to_ycbcr(struct drm_dp_aux *aux, u8 color_spc)
 }
 EXPORT_SYMBOL(drm_dp_pcon_convert_rgb_to_ycbcr);
 
+/**
+ * drm_edp_backlight_get_level - Get the backlight level of eDP DPCD via AUX
+ * @aux: The DP aux device
+ * @bl: Backlight capability info from the panel
+ *
+ * Reads the current backlight brightness level from luminance mode
+ * (24-bit value in nits) or DPCD AUX mode(16-bit and 8-bit modes).
+ *
+ * Returns: Current backlight level.
+ */
+u32 drm_edp_backlight_get_level(struct drm_dp_aux *aux, const struct drm_edp_backlight_info *bl)
+{
+	int ret;
+	u8 buf[3] = { 0 };
+	u32 level = 0;
+
+	if (!(bl->aux_set || bl->luminance_set))
+		return 0;
+
+	if (bl->luminance_set) {
+		ret = drm_dp_dpcd_read(aux, DP_EDP_PANEL_TARGET_LUMINANCE_VALUE, buf, sizeof(buf));
+		if (ret < 0) {
+			DRM_DEV_ERROR(aux->drm_dev->dev,
+				      "%s: Failed to read luminance value: %d\n",
+				      aux->name, ret);
+			return 0;
+		}
+		level = (buf[2] << 16 | buf[1] << 8 | buf[0]) / 1000;
+	} else if (bl->lsb_reg_used) {
+		ret = drm_dp_dpcd_read(aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB, buf, 2);
+		if (ret < 0) {
+			DRM_DEV_ERROR(aux->drm_dev->dev,
+				      "%s: Failed to read backlight level: %d\n",
+				      aux->name, ret);
+			return 0;
+		}
+		level = buf[0] << 8 | buf[1];
+	} else {
+		ret = drm_dp_dpcd_read(aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB, buf, 1);
+		if (ret < 0) {
+			DRM_DEV_ERROR(aux->drm_dev->dev,
+				      "%s: Failed to read backlight level: %d\n",
+				      aux->name, ret);
+			return 0;
+		}
+		level = buf[0];
+	}
+
+	return level;
+}
+EXPORT_SYMBOL(drm_edp_backlight_get_level);
+
 /**
  * drm_edp_backlight_set_level() - Set the backlight level of an eDP panel via AUX
  * @aux: The DP AUX channel to use
diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
index 87caa4f1fdb86..0b045a47ae573 100644
--- a/include/drm/display/drm_dp_helper.h
+++ b/include/drm/display/drm_dp_helper.h
@@ -864,6 +864,7 @@ drm_edp_backlight_init(struct drm_dp_aux *aux, struct drm_edp_backlight_info *bl
 		       u32 max_luminance,
 		       u16 driver_pwm_freq_hz, const u8 edp_dpcd[EDP_DISPLAY_CTL_CAP_SIZE],
 		       u32 *current_level, u8 *current_mode, bool need_luminance);
+u32 drm_edp_backlight_get_level(struct drm_dp_aux *aux, const struct drm_edp_backlight_info *bl);
 int drm_edp_backlight_set_level(struct drm_dp_aux *aux, const struct drm_edp_backlight_info *bl,
 				u32 level);
 int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backlight_info *bl,
-- 
2.43.0