suspend: make sync() on suspend-to-RAM build-time optional

Based on discussion resulting from...

https://lkml.org/lkml/2015/5/8/82
[PATCH 1/1] suspend: delete sys_sync()

https://lkml.org/lkml/2015/7/14/896
[PATCH v4 0/1] suspend: make sync() on suspend-to-RAM optional

this patch makes sys_sync() optional at build time, rather than deleting it entirely.

cheers,
Len Brown, Intel Open Source Technology Center

From: Len Brown <[email protected]>

The Linux kernel suspend path has traditionally invoked sys_sync() before freezing user threads.

But sys_sync() can be expensive, and some user-space OS's do not want the kernel to pay the cost of sys_sync() on every suspend -- preferring invoke sync() from user-space if/when they want it.

So make sys_sync on suspend build-time optional.

The default is unchanged.

---
 kernel/power/Kconfig   | 10 ++++++++++
 kernel/power/suspend.c |  2 ++
 2 files changed, 12 insertions(+)

index 7e01f78..80afa8a 100644
@@ -18,6 +18,16 @@ config SUSPEND_FREEZER
 
 	  Turning OFF this setting is NOT recommended! If in doubt, say Y.
 
+config SUSPEND_SKIP_SYNC
+	bool "Skip kernel's sys_sync() on suspend to RAM/standby"
+	depends on SUSPEND
+	depends on EXPERT
+	help
+	  Delete the kernel sys_sync() before freezing user processes.
+	  Some operating systems prefer not to pay this cost on every
+	  invocation of suspend, or they are content with invoking sync()
+	  from user-space before invoking suspend.
+
 config HIBERNATE_CALLBACKS
 	bool
 

index 8d7a1ef..4a942ee 100644
@@ -482,11 +482,13 @@ static int enter_state(suspend_state_t state)
 	if (state == PM_SUSPEND_FREEZE)
 		freeze_begin();
 
+#ifndef	CONFIG_SUSPEND_SKIP_SYNC
 	trace_suspend_resume(TPS("sync_filesystems"), 0, true);
 	printk(KERN_INFO "PM: Syncing filesystems ... ");
 	sys_sync();
 	printk("done.\n");
 	trace_suspend_resume(TPS("sync_filesystems"), 0, false);
+#endif
 
 	pr_debug("PM: Preparing system for %s sleep\n", pm_states[state]);
 	error = suspend_prepare(state);

2.5.0

OK, I'm replacing your previous sync patch with this one (modulo some minor changes in the help text).

Thanks,
Rafael