Add kernel 5.15 for RPI4 support

root/target/linux/bcm27xx/patches-5.15/0571-gpio-bcm-virt-Fix-the-get-method.patch

@@ -0,0 +1,34 @@
+From a91d93b66590d912b1551f707b083348c7af4836 Mon Sep 17 00:00:00 2001
+From: Phil Elwell <phil@raspberrypi.com>
+Date: Tue, 19 Oct 2021 11:23:43 +0100
+Subject: [PATCH 571/634] gpio: bcm-virt: Fix the get() method
+
+The get() method does not understand the on-the-wire encoding of the
+remote GPIO states, thinking they are simple on/off bits when they are
+really pairs of 16-bit counts. Rewrite the get() handler to return the
+value last written, which will eventually match the actual GPIO state
+if there are no other changes.
+
+See: https://github.com/raspberrypi/linux/issues/4638
+
+Signed-off-by: Phil Elwell <phil@raspberrypi.com>
+---
+ drivers/gpio/gpio-bcm-virt.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/drivers/gpio/gpio-bcm-virt.c b/drivers/gpio/gpio-bcm-virt.c
+index 49e28ad9760e..55c40190e88a 100644
+--- a/drivers/gpio/gpio-bcm-virt.c
++++ b/drivers/gpio/gpio-bcm-virt.c
+@@ -49,7 +49,7 @@ static int brcmvirt_gpio_get(struct gpio_chip *gc, unsigned off)
+ 	unsigned v;
+ 	gpio = container_of(gc, struct brcmvirt_gpio, gc);
+ 	v = readl(gpio->ts_base + off);
+-	return (v >> off) & 1;
++	return (s16)((v >> 16) - v) > 0;
+ }
+ 
+ static void brcmvirt_gpio_set(struct gpio_chip *gc, unsigned off, int val)
+-- 
+2.33.1
+