Subject: printk: add console handover From: John Ogness Date: Mon Nov 30 01:42:09 2020 +0106 From: John Ogness 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 Signed-off-by: Thomas Gleixner --- include/linux/console.h | 1 + kernel/printk/printk.c | 15 +++++++++++++-- 2 files changed, 14 insertions(+), 2 deletions(-) --- --- a/include/linux/console.h +++ b/include/linux/console.h @@ -143,6 +143,7 @@ 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 @@ -1746,6 +1746,8 @@ static bool console_may_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; @@ -1761,7 +1763,14 @@ static bool call_sync_console_driver(str return true; } - if (con->write && (con->flags & CON_BOOT) && !con->thread) { + if (con->write_atomic && (con->flags & CON_HANDOVER) && !con->thread) { + if (console_trylock()) { + con->write_atomic(con, text, text_len); + console_unlock(); + return true; + } + + } else if (con->write && (con->flags & CON_BOOT) && !con->thread) { if (console_trylock()) { con->write(con, text, text_len); console_unlock(); @@ -2891,8 +2900,10 @@ 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