drm/panel: visionox-rm69299: Add backlight support

The shift6mq's variant supports controlling the backlight via DSI
commands. Use that if a max_brightness is set in the device specific
data.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250910-shift6mq-panel-v3-3-a7729911afb9@sigxcpu.org

Author: agx

drivers/gpu/drm/panel/panel-visionox-rm69299.c

@@ -3,6 +3,7 @@
  * Copyright (c) 2019, The Linux Foundation. All rights reserved.
  */
 
+#include <linux/backlight.h>
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/property.h>
@@ -20,6 +21,8 @@ struct visionox_rm69299_panel_desc {
 	const struct drm_display_mode *mode;
 	const u8 *init_seq;
 	unsigned int init_seq_len;
+	int max_brightness;
+	int initial_brightness;
 };
 
 struct visionox_rm69299 {
@@ -285,6 +288,63 @@ static const struct drm_panel_funcs visionox_rm69299_drm_funcs = {
 	.get_modes = visionox_rm69299_get_modes,
 };
 
+static int visionox_rm69299_bl_get_brightness(struct backlight_device *bl)
+{
+	struct mipi_dsi_device *dsi = bl_get_data(bl);
+	u16 brightness;
+	int ret;
+
+	dsi->mode_flags &= ~MIPI_DSI_MODE_LPM;
+
+	ret = mipi_dsi_dcs_get_display_brightness(dsi, &brightness);
+	if (ret < 0)
+		return ret;
+
+	dsi->mode_flags |= MIPI_DSI_MODE_LPM;
+
+	return brightness;
+}
+
+static int visionox_rm69299_bl_update_status(struct backlight_device *bl)
+{
+	struct mipi_dsi_device *dsi = bl_get_data(bl);
+	u16 brightness = backlight_get_brightness(bl);
+	int ret;
+
+	dsi->mode_flags &= ~MIPI_DSI_MODE_LPM;
+
+	ret = mipi_dsi_dcs_set_display_brightness(dsi, brightness);
+	if (ret < 0)
+		return ret;
+
+	dsi->mode_flags |= MIPI_DSI_MODE_LPM;
+
+	return 0;
+}
+
+static const struct backlight_ops visionox_rm69299_bl_ops = {
+	.update_status = visionox_rm69299_bl_update_status,
+	.get_brightness = visionox_rm69299_bl_get_brightness,
+};
+
+static struct backlight_device *
+visionox_rm69299_create_backlight(struct visionox_rm69299 *ctx)
+{
+	struct device *dev = &ctx->dsi->dev;
+	const struct backlight_properties props = {
+		.type = BACKLIGHT_RAW,
+		.brightness = ctx->desc->initial_brightness,
+		.max_brightness = ctx->desc->max_brightness,
+	};
+
+	if (!ctx->desc->max_brightness)
+		return 0;
+
+	return devm_backlight_device_register(dev, dev_name(dev), dev, ctx->dsi,
+					      &visionox_rm69299_bl_ops,
+					      &props);
+}
+
 static int visionox_rm69299_probe(struct mipi_dsi_device *dsi)
 {
 	struct device *dev = &dsi->dev;
@@ -316,6 +376,11 @@ static int visionox_rm69299_probe(struct mipi_dsi_device *dsi)
 		return PTR_ERR(ctx->reset_gpio);
 	}
 
+	ctx->panel.backlight = visionox_rm69299_create_backlight(ctx);
+	if (IS_ERR(ctx->panel.backlight))
+		return dev_err_probe(dev, PTR_ERR(ctx->panel.backlight),
+				     "Failed to create backlight\n");
+
 	drm_panel_add(&ctx->panel);
 
 	dsi->lanes = 4;
@@ -353,6 +418,8 @@ const struct visionox_rm69299_panel_desc visionox_rm69299_shift_desc = {
 	.mode = &visionox_rm69299_1080x2160_60hz,
 	.init_seq = (const u8 *)visionox_rm69299_1080x2160_60hz_init_seq,
 	.init_seq_len = ARRAY_SIZE(visionox_rm69299_1080x2160_60hz_init_seq),
+	.max_brightness = 255,
+	.initial_brightness = 50,
 };
 
 static const struct of_device_id visionox_rm69299_of_match[] = {