[PATCH 2/2] fbcon: Defer console takeover for splash screens to first switch

[Daniel van Vugt]

Until now, deferred console takeover only meant defer until there is
output. But that risks stepping on the toes of userspace splash screens,
as console messages may appear before the splash screen. So check for the
"splash" parameter (as used by Plymouth) and if present then extend the
deferral until the first switch.

Closes: https://bugs.launchpad.net/bugs/1970069
Cc: Mario Limonciello <mario.limonciello@xxxxxxx>
Signed-off-by: Daniel van Vugt <daniel.van.vugt@xxxxxxxxxxxxx>
---
 drivers/video/fbdev/core/fbcon.c | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 63af6ab034..5b9f7635f7 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -76,6 +76,7 @@
 #include <linux/crc32.h> /* For counting font checksums */
 #include <linux/uaccess.h>
 #include <asm/irq.h>
+#include <asm/cmdline.h>
 
 #include "fbcon.h"
 #include "fb_internal.h"
@@ -146,6 +147,7 @@ static inline void fbcon_map_override(void)
 
 #ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
 static bool deferred_takeover = true;
+static int initial_console = -1;
 #else
 #define deferred_takeover false
 #endif
@@ -3341,7 +3343,7 @@ static void fbcon_register_existing_fbs(struct work_struct *work)
 	console_unlock();
 }
 
-static struct notifier_block fbcon_output_nb;
+static struct notifier_block fbcon_output_nb, fbcon_switch_nb;
 static DECLARE_WORK(fbcon_deferred_takeover_work, fbcon_register_existing_fbs);
 
 static int fbcon_output_notifier(struct notifier_block *nb,
@@ -3358,6 +3360,21 @@ static int fbcon_output_notifier(struct notifier_block *nb,
 
 	return NOTIFY_OK;
 }
+
+static int fbcon_switch_notifier(struct notifier_block *nb,
+				 unsigned long action, void *data)
+{
+	struct vc_data *vc = data;
+
+	WARN_CONSOLE_UNLOCKED();
+
+	if (vc->vc_num != initial_console) {
+		dummycon_unregister_switch_notifier(&fbcon_switch_nb);
+		dummycon_register_output_notifier(&fbcon_output_nb);
+	}
+
+	return NOTIFY_OK;
+}
 #endif
 
 static void fbcon_start(void)
@@ -3370,7 +3387,14 @@ static void fbcon_start(void)
 
 	if (deferred_takeover) {
 		fbcon_output_nb.notifier_call = fbcon_output_notifier;
-		dummycon_register_output_notifier(&fbcon_output_nb);
+		fbcon_switch_nb.notifier_call = fbcon_switch_notifier;
+		initial_console = fg_console;
+
+		if (cmdline_find_option_bool(boot_command_line, "splash"))
+			dummycon_register_switch_notifier(&fbcon_switch_nb);
+		else
+			dummycon_register_output_notifier(&fbcon_output_nb);
+
 		return;
 	}
 #endif
@@ -3417,8 +3441,10 @@ void __exit fb_console_exit(void)
 {
 #ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
 	console_lock();
-	if (deferred_takeover)
+	if (deferred_takeover) {
 		dummycon_unregister_output_notifier(&fbcon_output_nb);
+		dummycon_unregister_switch_notifier(&fbcon_switch_nb);
+	}
 	console_unlock();
 
 	cancel_work_sync(&fbcon_deferred_takeover_work);
-- 
2.43.0