[PATCH v2 2/2] fbcon: Defer console takeover for splash screens to first switch

[Daniel van Vugt <daniel.van.vugt@canonical.com>]

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 the
command line for the expectation of userspace splash and if present then
extend the deferral until after the first switch.

V2: Added Kconfig option instead of hard coding "splash".

Closes: https://bugs.launchpad.net/bugs/1970069
Cc: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Daniel van Vugt <daniel.van.vugt@canonical.com>
---
 drivers/video/console/Kconfig    | 13 +++++++++++
 drivers/video/fbdev/core/fbcon.c | 38 ++++++++++++++++++++++++++++++--
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
index bc31db6ef7..a6e371bfb4 100644
--- a/drivers/video/console/Kconfig
+++ b/drivers/video/console/Kconfig
@@ -138,6 +138,19 @@ config FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
 	  by the firmware in place, rather then replacing the contents with a
 	  black screen as soon as fbcon loads.
 
+config FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION
+	string "Framebuffer Console Deferred Takeover Condition"
+	depends on FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+	default "splash"
+	help
+	  If enabled this defers further the framebuffer console taking over
+	  until the first console switch has occurred. And even then only if
+	  text has been output, and only if the specified parameter is found
+	  on the command line. This ensures fbcon does not interrupt userspace
+	  splash screens such as Plymouth which may be yet to start rendering
+	  at the time of the first console output. "splash" is the simplest
+	  distro-agnostic condition for this that Plymouth checks for.
+
 config STI_CONSOLE
 	bool "STI text console"
 	depends on PARISC && HAS_IOMEM
diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 63af6ab034..98155d2256 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"
@@ -3358,6 +3359,26 @@ static int fbcon_output_notifier(struct notifier_block *nb,
 
 	return NOTIFY_OK;
 }
+
+#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION
+static int initial_console;
+static struct notifier_block fbcon_switch_nb;
+
+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
 #endif
 
 static void fbcon_start(void)
@@ -3370,7 +3391,16 @@ static void fbcon_start(void)
 
 	if (deferred_takeover) {
 		fbcon_output_nb.notifier_call = fbcon_output_notifier;
-		dummycon_register_output_notifier(&fbcon_output_nb);
+#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION
+		if (cmdline_find_option_bool(boot_command_line,
+		      CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION)) {
+			initial_console = fg_console;
+			fbcon_switch_nb.notifier_call = fbcon_switch_notifier;
+			dummycon_register_switch_notifier(&fbcon_switch_nb);
+		} else
+#endif
+			dummycon_register_output_notifier(&fbcon_output_nb);
+
 		return;
 	}
 #endif
@@ -3417,8 +3447,12 @@ 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);
+#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION
+		dummycon_unregister_switch_notifier(&fbcon_switch_nb);
+#endif
+	}
 	console_unlock();
 
 	cancel_work_sync(&fbcon_deferred_takeover_work);
-- 
2.43.0