[PATCH] doc: add note on usleep_range range

[Nicholas Mc Guire]

useleep_range() with a delta of 0 makes no sense and only prevents the
timer subsystem from optimizing interrupts. As any user of usleep_range()
is in non-atomic context the timer jitter is in the range of 10s of 
microseconds anyway.

This adds a note making it clear that a range of 0 is a bad idea.

Signed-off-by: Nicholas Mc Guire <hofrat@xxxxxxxxx>
---

as of 4.9.0 there are about 20 cases of usleep_ranges() that have 
min==max and none of them really look like they are necessary, so 
it does seem like a relatively common misunderstanding worth
noting in the documentation.

Patch is against 4.9.0 (localversion-next is 20161212)

 Documentation/timers/timers-howto.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/timers/timers-howto.txt b/Documentation/timers/timers-howto.txt
index 038f8c7..b5cdf82 100644
--- a/Documentation/timers/timers-howto.txt
+++ b/Documentation/timers/timers-howto.txt
@@ -93,6 +93,13 @@ NON-ATOMIC CONTEXT:
 			tolerances here are very situation specific, thus it
 			is left to the caller to determine a reasonable range.
 
+			A range of 0, that is usleep_range(100,100) or the 
+			like, do not make sense as this code is in a 
+			non-atomic section and a system can not be expected 
+			to have jitter 0. For any non-RT code any delta
+			less than 50 microseconds probably is only preventing
+			timer subsystem optimization but providing no benefit.
+
 	SLEEPING FOR LARGER MSECS ( 10ms+ )
 		* Use msleep or possibly msleep_interruptible
 
-- 
2.1.4