fbcon: Defer AND delay console takeover

From 35be19e113dae19d9c86484620d57f628554afa9 Mon Sep 17 00:00:00 2001
From: Daniel van Vugt <daniel.van.vugt@canonical.com>
Date: Fri, 19 Jan 2024 21:42:57 +0800
Subject: [PATCH] fbcon: Defer AND delay console takeover

While the existing deferred takeover means to wait until there are
messages to display, we add an additional delay here to prevent even
those from appearing in the first N seconds. If "splash" is present then
N is 10 seconds, otherwise it's zero. You can override the delay value
with: fbcon=delay:N[:M]

M is a secondary (default 1 second) delay which starts at primary
framebuffer registration and overrides N. This ensures fbcon takes over
the console quick enough that even a fast typer won't find their VTs are
missing, but M provides a slight delay so as to ensure it doesn't win a
race with Plymouth that's also waiting on primary (DRM) framebuffer
registration. We prefer Plymouth takes over the framebuffer first.

This patch exists to work around the limitations of Plymouth which
refuses to display until DRM has started. And Plymouth will wait up to
8 seconds for that.

TL;DR - This patch makes fbcon takeover happen one second after Plymouth
has started animating, or 10 seconds after init in case that didn't
happen, or if "splash" is removed then immediately on init.

After SimpleDRM is enabled, this patch *might* not be necessary, and
the current design is very Plymouth-centric (it works out of the box with
no other system changes required) so isn't being proposed upstream just
yet.

Bug-Ubuntu: https://launchpad.net/bugs/1970069
Signed-off-by: Daniel van Vugt <daniel.van.vugt@canonical.com>
---
 drivers/video/fbdev/core/fbcon.c | 50 +++++++++++++++++++++++++++++++-
 1 file changed, 49 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 63af6ab03..1994cca81 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 +147,8 @@  static inline void fbcon_map_override(void)
 
 #ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
 static bool deferred_takeover = true;
+static int start_delay = -1;  /* seconds from init */
+static int primary_fb_takeover_delay = 1;  /* seconds from FB registration */
 #else
 #define deferred_takeover false
 #endif
@@ -470,6 +473,17 @@  static int __init fb_console_setup(char *this_opt)
 			deferred_takeover = false;
 			continue;
 		}
+
+		if (!strncmp(options, "delay:", 6)) {
+			options += 6;
+			if (*options)
+				start_delay = simple_strtol(options, &options, 0);
+			if (*options == ':') {
+				options++;
+				primary_fb_takeover_delay = simple_strtol(options, &options, 0);
+			}
+			continue;
+		}
 #endif
 
 #ifdef CONFIG_LOGO
@@ -2973,6 +2987,19 @@  module_param_named_unsafe(lockless_register_fb, lockless_register_fb, bool, 0400
 MODULE_PARM_DESC(lockless_register_fb,
 	"Lockless framebuffer registration for debugging [default=off]");
 
+#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+static void fbcon_start(void);
+
+static void fbcon_delayed_start(struct work_struct *work)
+{
+	console_lock();
+	fbcon_start();
+	console_unlock();
+}
+
+static DECLARE_DELAYED_WORK(fbcon_delayed_start_work, fbcon_delayed_start);
+#endif
+
 /* called with console_lock held */
 static int do_fb_registered(struct fb_info *info)
 {
@@ -2986,10 +3013,17 @@  static int do_fb_registered(struct fb_info *info)
 	idx = info->node;
 	fbcon_select_primary(info);
 
+#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
 	if (deferred_takeover) {
-		pr_info("fbcon: Deferring console take-over\n");
+		if (start_delay > 0 && primary_device >= 0) {
+			mod_delayed_work(system_wq, &fbcon_delayed_start_work, primary_fb_takeover_delay * HZ);
+			pr_info("fbcon: Delayed console takeover reduced to %ds after primary framebuffer registration\n", primary_fb_takeover_delay);
+		} else {
+			pr_info("fbcon: Deferring console takeover\n");
+		}
 		return 0;
 	}
+#endif
 
 	if (info_idx == -1) {
 		for (i = first_fb_vc; i <= last_fb_vc; i++) {
@@ -3395,6 +3429,20 @@  void __init fb_console_init(void)
 	for (i = 0; i < MAX_NR_CONSOLES; i++)
 		con2fb_map[i] = -1;
 
+#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+	if (!deferred_takeover)
+		start_delay = 0;
+	else if (start_delay < 0)
+		start_delay = cmdline_find_option_bool(boot_command_line, "splash") ? 10 : 0;
+
+	if (start_delay > 0) {
+		pr_info("fbcon: Delaying console takeover by up to %ds\n", start_delay);
+		schedule_delayed_work(&fbcon_delayed_start_work, start_delay * HZ);
+		console_unlock();
+		return;
+	}
+#endif
+
 	fbcon_start();
 	console_unlock();
 }
-- 
2.43.0