From: John Ogness Date: Mon, 30 Nov 2020 01:42:09 +0106 Subject: [PATCH 27/28] printk: add console handover If earlyprintk is used, a boot console will print directly to the console immediately. The boot console will unregister itself as soon as a non-boot console registers. However, the non-boot console does not begin printing until its kthread has started. Since this happens much later, there is a long pause in the console output. If the ringbuffer is small, messages could even be dropped during the pause. Add a new CON_HANDOVER console flag to be used internally by printk in order to track which non-boot console took over from a boot console. If handover consoles have implemented write_atomic(), they are allowed to print directly to the console until their kthread can take over. Signed-off-by: John Ogness Signed-off-by: Sebastian Andrzej Siewior --- include/linux/console.h | 1 + kernel/printk/printk.c | 8 +++++++- 2 files changed, 8 insertions(+), 1 deletion(-) --- a/include/linux/console.h +++ b/include/linux/console.h @@ -137,6 +137,7 @@ static inline int con_debug_leave(void) #define CON_ANYTIME (16) /* Safe to call when cpu is offline */ #define CON_BRL (32) /* Used for a braille device */ #define CON_EXTENDED (64) /* Use the extended output format a la /dev/kmsg */ +#define CON_HANDOVER (128) /* Device was previously a boot console. */ struct console { char name[16]; --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1726,6 +1726,8 @@ static bool console_can_sync(struct cons return false; if (con->write_atomic && kernel_sync_mode()) return true; + if (con->write_atomic && (con->flags & CON_HANDOVER) && !con->thread) + return true; if (con->write && (con->flags & CON_BOOT) && !con->thread) return true; return false; @@ -1737,6 +1739,8 @@ static bool call_sync_console_driver(str return false; if (con->write_atomic && kernel_sync_mode()) con->write_atomic(con, text, text_len); + else if (con->write_atomic && (con->flags & CON_HANDOVER) && !con->thread) + con->write_atomic(con, text, text_len); else if (con->write && (con->flags & CON_BOOT) && !con->thread) con->write(con, text, text_len); else @@ -2829,8 +2833,10 @@ void register_console(struct console *ne * the real console are the same physical device, it's annoying to * see the beginning boot messages twice */ - if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV)) + if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV)) { newcon->flags &= ~CON_PRINTBUFFER; + newcon->flags |= CON_HANDOVER; + } /* * Put this console in the list - keep the