linux-kernel - Re: [PATCH] checkpatch: Add warnings for use of mdelay()

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Message-Id: <20100727121610.64b38cfa.akpm@linux-foundation.org>
Date:	Tue, 27 Jul 2010 12:16:10 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Patrick Pannuto <ppannuto@...eaurora.org>
Cc:	Jonathan Corbet <corbet@....net>,
	Israel Schlesinger <israels@...eaurora.org>,
	linux-kernel@...r.kernel.org, apw@...onical.com, joe@...ches.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] checkpatch: Add warnings for use of mdelay()

On Tue, 27 Jul 2010 10:32:54 -0700
Patrick Pannuto <ppannuto@...eaurora.org> wrote:

> On 07/27/2010 10:31 AM, Jonathan Corbet wrote:
> > On Tue, 27 Jul 2010 10:11:11 -0700
> > Israel Schlesinger <israels@...eaurora.org> wrote:
> > 
> >> mdelay is a busy-wait loop which is wasteful. If at all possible,
> >> callers should use msleep instead of mdelay.
> >>
> >> The only time mdelay is really appropriate is in atomic context,
> >> however, delays of 1ms+ in atomic context are rather expensive, so
> >> a warning for this case is probably appropriate as well to encourage
> >> people to move such expensive delays outside of atomic context
> > 
> > Once upon a time, msleep(1) would sleep for 20ms, while mdelay(1) gave
> > a 1ms delay.  My patch to fix msleep() at that time didn't get in due
> > to concerns about the cost of using hrtimers.  Perhaps msleep() has
> > gotten better, but, if not, actually getting a 1ms delay remains a
> > valid reason for using mdelay() instead IMO.  It made a difference of a
> > few seconds at open time for a driver I was doing at the time.
> > 
> > jon
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/
> 
> Check out the recently added usleep in -tip, and the checkpatch patch
> pending in my queue that fixes that case (I'll send in a few hours ;) )
> 

The message should point people at usleep_range(), I'd suggest.  It's a
more power-friendly way of sleeping.

That assumes that the below patch gets merged - the people who handle
timer-related things are presently, err, asleep.


From: Patrick Pannuto <ppannuto@...eaurora.org>

usleep[_range] are finer precision implementations of msleep and are
designed to be drop-in replacements for udelay where a precise sleep /
busy-wait is unnecessary.  They also allow an easy interface to specify
slack when a precise (ish) wakeup is unnecessary to help minimize wakeups

Signed-off-by: Patrick Pannuto <ppannuto@...eaurora.org>
Acked-by: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 include/linux/delay.h |    6 ++++++
 kernel/timer.c        |   22 ++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff -puN include/linux/delay.h~timers-add-usleep-timer include/linux/delay.h
--- a/include/linux/delay.h~timers-add-usleep-timer
+++ a/include/linux/delay.h
@@ -45,6 +45,12 @@ extern unsigned long lpj_fine;
 void calibrate_delay(void);
 void msleep(unsigned int msecs);
 unsigned long msleep_interruptible(unsigned int msecs);
+void usleep_range(unsigned long min, unsigned long max);
+
+static inline void usleep(unsigned long usecs)
+{
+	usleep_range(usecs, usecs);
+}
 
 static inline void ssleep(unsigned int seconds)
 {
diff -puN kernel/timer.c~timers-add-usleep-timer kernel/timer.c
--- a/kernel/timer.c~timers-add-usleep-timer
+++ a/kernel/timer.c
@@ -1763,3 +1763,25 @@ unsigned long msleep_interruptible(unsig
 }
 
 EXPORT_SYMBOL(msleep_interruptible);
+
+static int __sched do_usleep_range(unsigned long min, unsigned long max)
+{
+	ktime_t kmin;
+	unsigned long delta;
+
+	kmin = ktime_set(0, min * NSEC_PER_USEC);
+	delta = max - min;
+	return schedule_hrtimeout_range(&kmin, delta, HRTIMER_MODE_REL);
+}
+
+/**
+ * usleep_range - Drop in replacement for udelay where wakeup is flexible
+ * @min: Minimum time in usecs to sleep
+ * @max: Maximum time in usecs to sleep
+ */
+void usleep_range(unsigned long min, unsigned long max)
+{
+	__set_current_state(TASK_UNINTERRUPTIBLE);
+	do_usleep_range(min, max);
+}
+EXPORT_SYMBOL(usleep_range);
_

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