Parallelization to improve boot times has been successful enough that race
conditions now exist between the init_post() open of /dev/console and
initialization of the console device. When this occurs, opening /dev/console
fails and any applications inherited for init have no standard in/out/err
devices. This is expected behavior if no console device is available, but
quite unfortunate in the case where the console is just a bit slow waking up.
Some buses, such as USB, offer no guarantees about how long it takes to
discover devices, so there is no reliable way to distinguish between a missing
console and a slow one. The pragmatic approach taken in this patch is to
wait for a while to see if a console shows up, and just go on if it doesn't.
The default delay interval is set to 500 msec (0.5 seconds) based on a sample
size of one device (cp2101) but is a tunable. If the console shows up sooner,
we will continue immediately.
History
v2 Wait for the preferred console rather than any console. Make the
delay interval a tunable.
v1 Initial version
Signed-off-by: David VomLehn <dvomlehn@xxxxxxxxx>
---
init/Kconfig | 10 ++++++++++
kernel/printk.c | 11 +++++++++++
2 files changed, 21 insertions(+), 0 deletions(-)
diff --git a/init/Kconfig b/init/Kconfig
index 7be4d38..5d5b0fa 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -835,6 +835,16 @@ config PRINTK
very difficult to diagnose system problems, saying N here is
strongly discouraged.
+config PRINTK_CONSOLE_WAIT
+ int "Number of milliseconds to wait for console device"
+ default 500
+ help
+ Some systems use console devices, such as USB serial devices, which
+ may not be present or which may take an unspecified amount of time
+ to be initialized. This setting determines the maximum number of
+ milliseconds the system will wait for a console to be registered.
+ Slow devices may require this value be increased.
+
config BUG
bool "BUG() support" if EMBEDDED
default y
diff --git a/kernel/printk.c b/kernel/printk.c
index 5052b54..3a05c8a 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -105,6 +105,9 @@ static unsigned log_start; /* Index into log_buf: next char to be read by syslog
static unsigned con_start; /* Index into log_buf: next char to be sent to consoles */
static unsigned log_end; /* Index into log_buf: most-recently-written-char + 1 */
+/* Wait queue controlling the wait for a console device */
+static DECLARE_WAIT_QUEUE_HEAD(console_wait);
+
/*
* Array of consoles built from command line options (console=)
*/
@@ -1081,6 +1084,13 @@ struct tty_driver *console_device(int *index)
struct console *c;
struct tty_driver *driver = NULL;
+ /* Wait a while for a console to show up. If one doesn't show up
+ * for too long, we'll just continue without a console. */
+ if (wait_event_timeout(console_wait, preferred_console >= 0,
+ CONFIG_PRINTK_CONSOLE_WAIT) == 0)
+ pr_warning("No preferred console after waiting %u msec; "
+ "continuing anyway\n", CONFIG_PRINTK_CONSOLE_WAIT);
+
acquire_console_sem();
for (c = console_drivers; c != NULL; c = c->next) {
if (!c->device)
@@ -1230,6 +1240,7 @@ void register_console(struct console *console)
spin_unlock_irqrestore(&logbuf_lock, flags);
}
release_console_sem();
+ wake_up_all(&console_wait);
}
EXPORT_SYMBOL(register_console);
--
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