Add kernel 5.15 for RPI4 support

root/target/linux/bcm27xx/patches-5.15/0021-drm-vc4-hdmi-Fix-HPD-GPIO-detection.patch

@@ -0,0 +1,43 @@
+From 84da606fe60662e046d6f7982d8a9281245b09f5 Mon Sep 17 00:00:00 2001
+From: Maxime Ripard <maxime@cerno.tech>
+Date: Fri, 25 Jun 2021 17:16:51 +0200
+Subject: [PATCH 021/634] drm/vc4: hdmi: Fix HPD GPIO detection
+
+Prior to commit 6800234ceee0 ("drm/vc4: hdmi: Convert to gpiod"), in the
+detect hook, if we had an HPD GPIO we would only rely on it and return
+whatever state it was in.
+
+However, that commit changed that by mistake to only consider the case
+where we have a GPIO and it returns a logical high, and would fall back
+to the other methods otherwise.
+
+Since we can read the EDIDs when the HPD signal is low on some displays,
+we changed the detection status from disconnected to connected, and we
+would ignore an HPD pulse.
+
+Fixes: 6800234ceee0 ("drm/vc4: hdmi: Convert to gpiod")
+Signed-off-by: Maxime Ripard <maxime@cerno.tech>
+---
+ drivers/gpu/drm/vc4/vc4_hdmi.c | 6 +++---
+ 1 file changed, 3 insertions(+), 3 deletions(-)
+
+diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
+index 918941a2bc58..50e9e1f3ac47 100644
+--- a/drivers/gpu/drm/vc4/vc4_hdmi.c
++++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
+@@ -190,9 +190,9 @@ vc4_hdmi_connector_detect(struct drm_connector *connector, bool force)
+ 
+ 	WARN_ON(pm_runtime_resume_and_get(&vc4_hdmi->pdev->dev));
+ 
+-	if (vc4_hdmi->hpd_gpio &&
+-	    gpiod_get_value_cansleep(vc4_hdmi->hpd_gpio)) {
+-		connected = true;
++	if (vc4_hdmi->hpd_gpio) {
++		if (gpiod_get_value_cansleep(vc4_hdmi->hpd_gpio))
++			connected = true;
+ 	} else {
+ 		unsigned long flags;
+ 		u32 hotplug;
+-- 
+2.33.1
+