drm/i915/dsi: Retrieve max brightness level from VBT

So far, DCS backlight driver hardcode (0xFF) for max brightness level.
MIPI DCS spec allow max 0xFFFF for set_display_brightness (51h) command.
And VBT brightness precision bits can support 8 ~ 16 bits.

We should set correct precision bits in VBT that meet panel's request.
Driver can refer to this setting then configure max brightness level
in DCS backlight driver properly.

v2: modify variable name brightness_precision_bits instead of
    max_brightness_level.
v3: fix checkpatch warning.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Cooper Chiou <cooper.chiou@intel.com>
Cc: William Tseng <william.tseng@intel.com>
Signed-off-by: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908115607.9633-5-shawn.c.lee@intel.com

drivers/gpu/drm/i915/display/intel_bios.c

@@ -483,6 +483,9 @@ parse_lfp_backlight(struct drm_i915_private *i915,
 			level = 255;
 		}
 		i915->vbt.backlight.min_brightness = min_level;
+
+		i915->vbt.backlight.brightness_precision_bits =
+			backlight_data->brightness_precision_bits[panel_type];
 	} else {
 		level = backlight_data->level[panel_type];
 		i915->vbt.backlight.min_brightness = entry->min_brightness;

drivers/gpu/drm/i915/display/intel_dsi_dcs_backlight.c

@@ -147,10 +147,16 @@ static void dcs_enable_backlight(const struct intel_crtc_state *crtc_state,
 static int dcs_setup_backlight(struct intel_connector *connector,
 			       enum pipe unused)
 {
+	struct drm_device *dev = connector->base.dev;
+	struct drm_i915_private *dev_priv = to_i915(dev);
 	struct intel_panel *panel = &connector->panel;
 
+	if (dev_priv->vbt.backlight.brightness_precision_bits > 8)
+		panel->backlight.max = (1 << dev_priv->vbt.backlight.brightness_precision_bits) - 1;
+	else
 		panel->backlight.max = PANEL_PWM_MAX_VALUE;
-	panel->backlight.level = PANEL_PWM_MAX_VALUE;
+
+	panel->backlight.level = panel->backlight.max;
 
 	return 0;
 }

drivers/gpu/drm/i915/i915_drv.h

@@ -689,6 +689,7 @@ struct intel_vbt_data {
 
 	struct {
 		u16 pwm_freq_hz;
+		u16 brightness_precision_bits;
 		bool present;
 		bool active_low_pwm;
 		u8 min_brightness;	/* min_brightness/255 of max */