Add kernel 5.15 for RPI4 support

root/target/linux/bcm27xx/patches-5.15/0550-regulator-rpi-panel-Remove-get_brightness-hook.patch

@@ -0,0 +1,59 @@
+From 81dda278c29bb42ac97b0d3bcca1e55181d6c331 Mon Sep 17 00:00:00 2001
+From: Dave Stevenson <dave.stevenson@raspberrypi.com>
+Date: Tue, 21 Sep 2021 15:32:50 +0100
+Subject: [PATCH 550/634] regulator: rpi-panel: Remove get_brightness hook
+
+The driver was implementing a get_brightness function that
+tried to read back the PWM setting of the display to report
+as the current brightness.
+The controller on the display does not support that, therefore
+we end up reporting a brightness of 0, and that confuses
+systemd's backlight service.
+
+Remove the hook so that the framework returns the current
+brightness automatically.
+
+Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
+---
+ .../regulator/rpi-panel-attiny-regulator.c    | 23 -------------------
+ 1 file changed, 23 deletions(-)
+
+diff --git a/drivers/regulator/rpi-panel-attiny-regulator.c b/drivers/regulator/rpi-panel-attiny-regulator.c
+index 998233f14085..8090b9a485b5 100644
+--- a/drivers/regulator/rpi-panel-attiny-regulator.c
++++ b/drivers/regulator/rpi-panel-attiny-regulator.c
+@@ -207,31 +207,8 @@ static int attiny_update_status(struct backlight_device *bl)
+ 	return ret;
+ }
+ 
+-static int attiny_get_brightness(struct backlight_device *bl)
+-{
+-	struct attiny_lcd *state = bl_get_data(bl);
+-	struct regmap *regmap = state->regmap;
+-	int ret, brightness, i;
+-
+-	mutex_lock(&state->lock);
+-
+-	for (i = 0; i < 10; i++) {
+-		ret = regmap_read(regmap, REG_PWM, &brightness);
+-		if (!ret)
+-			break;
+-	}
+-
+-	mutex_unlock(&state->lock);
+-
+-	if (ret)
+-		return ret;
+-
+-	return brightness;
+-}
+-
+ static const struct backlight_ops attiny_bl = {
+ 	.update_status	= attiny_update_status,
+-	.get_brightness	= attiny_get_brightness,
+ };
+ 
+ static int attiny_gpio_get_direction(struct gpio_chip *gc, unsigned int off)
+-- 
+2.33.1
+