drm/amd/display: use correct scale for actual_brightness

Documentation for sysfs backlight level interface requires that
values in both 'brightness' and 'actual_brightness' files are
interpreted to be in range from 0 to the value given in the
'max_brightness' file.

With amdgpu, max_brightness gives 255, and values written by the user
into 'brightness' are internally rescaled to a wider range. However,
reading from 'actual_brightness' gives the raw register value without
inverse rescaling. This causes issues for various userspace tools such
as PowerTop and systemd that expect the value to be in the correct
range.

Introduce a helper to retrieve internal backlight range. Use it to
reimplement 'convert_brightness' as 'convert_brightness_from_user' and
introduce 'convert_brightness_to_user'.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=203905
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1242
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alexander Monakov <amonakov@ispras.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -2894,51 +2894,50 @@ static int set_backlight_via_aux(struct dc_link *link, uint32_t brightness)
 	return rc ? 0 : 1;
 }
 
-static u32 convert_brightness(const struct amdgpu_dm_backlight_caps *caps,
-			      const uint32_t user_brightness)
+static int get_brightness_range(const struct amdgpu_dm_backlight_caps *caps,
+				unsigned *min, unsigned *max)
 {
-	u32 min, max, conversion_pace;
-	u32 brightness = user_brightness;
-
 	if (!caps)
-		goto out;
+		return 0;
 
-	if (!caps->aux_support) {
-		max = caps->max_input_signal;
-		min = caps->min_input_signal;
-		/*
-		 * The brightness input is in the range 0-255
-		 * It needs to be rescaled to be between the
-		 * requested min and max input signal
-		 * It also needs to be scaled up by 0x101 to
-		 * match the DC interface which has a range of
-		 * 0 to 0xffff
-		 */
-		conversion_pace = 0x101;
-		brightness =
-			user_brightness
-			* conversion_pace
-			* (max - min)
-			/ AMDGPU_MAX_BL_LEVEL
-			+ min * conversion_pace;
+	if (caps->aux_support) {
+		// Firmware limits are in nits, DC API wants millinits.
+		*max = 1000 * caps->aux_max_input_signal;
+		*min = 1000 * caps->aux_min_input_signal;
 	} else {
-		/* TODO
-		 * We are doing a linear interpolation here, which is OK but
-		 * does not provide the optimal result. We probably want
-		 * something close to the Perceptual Quantizer (PQ) curve.
-		 */
-		max = caps->aux_max_input_signal;
-		min = caps->aux_min_input_signal;
-
-		brightness = (AMDGPU_MAX_BL_LEVEL - user_brightness) * min
-			       + user_brightness * max;
-		// Multiple the value by 1000 since we use millinits
-		brightness *= 1000;
-		brightness = DIV_ROUND_CLOSEST(brightness, AMDGPU_MAX_BL_LEVEL);
+		// Firmware limits are 8-bit, PWM control is 16-bit.
+		*max = 0x101 * caps->max_input_signal;
+		*min = 0x101 * caps->min_input_signal;
 	}
+	return 1;
+}
+
+static u32 convert_brightness_from_user(const struct amdgpu_dm_backlight_caps *caps,
+					uint32_t brightness)
+{
+	unsigned min, max;
 
-out:
+	if (!get_brightness_range(caps, &min, &max))
 		return brightness;
+
+	// Rescale 0..255 to min..max
+	return min + DIV_ROUND_CLOSEST((max - min) * brightness,
+				       AMDGPU_MAX_BL_LEVEL);
+}
+
+static u32 convert_brightness_to_user(const struct amdgpu_dm_backlight_caps *caps,
+				      uint32_t brightness)
+{
+	unsigned min, max;
+
+	if (!get_brightness_range(caps, &min, &max))
+		return brightness;
+
+	if (brightness < min)
+		return 0;
+	// Rescale min..max to 0..255
+	return DIV_ROUND_CLOSEST(AMDGPU_MAX_BL_LEVEL * (brightness - min),
+				 max - min);
 }
 
 static int amdgpu_dm_backlight_update_status(struct backlight_device *bd)
@@ -2954,7 +2953,7 @@ static int amdgpu_dm_backlight_update_status(struct backlight_device *bd)
 
 	link = (struct dc_link *)dm->backlight_link;
 
-	brightness = convert_brightness(&caps, bd->props.brightness);
+	brightness = convert_brightness_from_user(&caps, bd->props.brightness);
 	// Change brightness based on AUX property
 	if (caps.aux_support)
 		return set_backlight_via_aux(link, brightness);
@@ -2971,7 +2970,7 @@ static int amdgpu_dm_backlight_get_brightness(struct backlight_device *bd)
 
 	if (ret == DC_ERROR_UNEXPECTED)
 		return bd->props.brightness;
-	return ret;
+	return convert_brightness_to_user(&dm->backlight_caps, ret);
 }
 
 static const struct backlight_ops amdgpu_dm_backlight_ops = {