[RFC][PATCHv6 06/12] PM: switch between printk emergency modes

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

It's not always possible/safe to wake_up() printk kernel
thread. For example, late suspend/early resume may printk()
while timekeeping is not initialized yet, so calling into the
scheduler may result in recursive warnings.

Another thing to notice is the fact that PM at some point
freezes user space and kernel threads: freeze_processes()
and freeze_kernel_threads(), correspondingly. Thus we need
printk() to operate in emergency mode there and attempt to
immediately flush pending kernel message to the console.

This patch adds printk emergency switches to dpm.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Suggested-by: Rafael J. Wysocki <rjw@rjwysocki.net>
---
 drivers/base/power/main.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
index c0d5f4a3611d..5bc2cf1f812c 100644
--- a/drivers/base/power/main.c
+++ b/drivers/base/power/main.c
@@ -34,6 +34,7 @@
 #include <linux/cpufreq.h>
 #include <linux/cpuidle.h>
 #include <linux/timer.h>
+#include <linux/console.h>
 
 #include "../base.h"
 #include "power.h"
@@ -1059,6 +1060,8 @@ void dpm_complete(pm_message_t state)
 	/* Allow device probing and trigger re-probing of deferred devices */
 	device_unblock_probing();
 	trace_suspend_resume(TPS("dpm_complete"), state.event, false);
+
+	printk_emergency_end_sync();
 }
 
 /**
@@ -1789,6 +1792,8 @@ int dpm_prepare(pm_message_t state)
 	trace_suspend_resume(TPS("dpm_prepare"), state.event, true);
 	might_sleep();
 
+	printk_emergency_begin_sync();
+
 	/*
 	 * Give a chance for the known devices to complete their probes, before
 	 * disable probing of devices. This sync point is important at least
-- 
2.15.1