Commit c48c1c9b authored Apr 23, 2014 by Matt Wagantall

iopoll: Use ktime_get() instead of jiffies for timeout calculations



Presently, small timeout values will round up to 1 jiffy which,
on CONFIG_HZ=100 systems, corresponds to a full 10 milliseconds.
This is undesirable for drivers specifying timeouts of only a
few 10's of microseconds (which is common), since they may end
up tight-loop spinning for hundreds or thousands of times longer
than expected before reporting a timeout.

Additionally, jiffies cannot be reliably used with time_after()
when the value of jiffies is small (like 1). In rare but real
scenarios, jiffies may actually increment by more than 1 at a
time. Specifically, this will occur if interrupts are disabled
on for more than 1 jiffy (10 milliseconds for CONFIG_HZ=100) on
the CPU responsible for incrementing jiffies.

If interrupts are re-enabled on that CPU between the time the
iopoll code (on another CPU) calculates the timeout value of
jiffies and when the time_after() comparison is made, then the
iopoll APIs may return -ETIMEDOUT prematurely, even though the
specified timeout has not actually expired.

Using ktime_get() avoid this problem (which arguably also needs
a generic fix to avoid similar problems for other code which use
jiffies to calculate timeouts).

CRs-Fixed: 587801
Change-Id: I19150f41965b918c59c3fb98e29a8bd2e2c9609f
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>

parent f43774a8

include/linux/iopoll.h

+4 −4

Original line number	Diff line number	Diff line
		/*
		* Copyright (c) 2012-2013 The Linux Foundation. All rights reserved.
		* Copyright (c) 2012-2014 The Linux Foundation. All rights reserved.
		*
		* This program is free software; you can redistribute it and/or modify
		* it under the terms of the GNU General Public License version 2 and
		@@ -17,7 +17,7 @@

		#include <linux/kernel.h>
		#include <linux/types.h>
		#include <linux/jiffies.h>
		#include <linux/hrtimer.h>
		#include <linux/delay.h>
		#include <asm-generic/errno.h>
		#include <asm/io.h>
		@@ -36,13 +36,13 @@
		*/
		#define readl_poll_timeout(addr, val, cond, sleep_us, timeout_us) \
		({ \
		unsigned long timeout = jiffies + usecs_to_jiffies(timeout_us); \
		ktime_t timeout = ktime_add_us(ktime_get(), timeout_us); \
		might_sleep_if(timeout_us); \
		for (;;) { \
		(val) = readl(addr); \
		if (cond) \
		break; \
		if (timeout_us && time_after(jiffies, timeout)) { \
		if (timeout_us && ktime_compare(ktime_get(), timeout) > 0) { \
		(val) = readl(addr); \
		break; \
		} \