[PATCH,RFC] mmp clockevent handling race
nico at fluxnic.net
Tue Jun 7 14:29:08 EDT 2011
On Tue, 7 Jun 2011, Lennert Buytenhek wrote:
> The patch below has been sitting around in the OLPC XO-1.75 (MMP2-based
> XO) bringup kernel tree for some time now, and upon recent rebasing of
> this tree to 3.0, it was discovered that something like this patch is
> still needed.
> Symptoms: e.g. as described here: http://dev.laptop.org/ticket/10945
> -- applications hang for a couple of minutes at a time, and sometimes
> there are several-minute hangs during the boot process.
> >From the ticket:
> The problem in the current upstream mmp timer handling code
> that appears to be triggered here is that it handles clockevents
> by setting up a comparator on the free-running clocksource timer
> to match and trigger an interrupt at 'current_time + delta',
> which if delta is small enough can lead to 'current_time + delta'
> already being in the past when comparator setup has finished,
> and the requested event will not trigger.
This is a classical issue that was solved on the SA1100 more than 10
> What this patch does is to rewrite the timer handling code to use
> two timers, one for the free-running clocksource, and one to trigger
> clockevents with, which is more or less the standard approach to this.
> It's kind of invasive, though (certainly more invasive than it strictly
> needs to be, as it reorganises time.c somewhat at the same time), and
> something like this is probably too invasive for 3.0 at this point.
> A safer "fix" for 3.0 may be to disallow NO_HZ/HIGH_RES_TIMERS on mmp.
> Any thoughts?
What about simply this:
diff --git a/arch/arm/mach-mmp/time.c b/arch/arm/mach-mmp/time.c
index 99833b9..8b8f99a 100644
@@ -39,7 +39,7 @@
#define TIMERS_VIRT_BASE TIMERS1_VIRT_BASE
-#define MAX_DELTA (0xfffffffe)
+#define MAX_DELTA (0xfffffffe - 16)
#define MIN_DELTA (16)
@@ -85,7 +85,7 @@ static irqreturn_t timer_interrupt(int irq, void *dev_id)
static int timer_set_next_event(unsigned long delta,
struct clock_event_device *dev)
- unsigned long flags, next;
+ unsigned long flags, next, now;
@@ -95,9 +95,10 @@ static int timer_set_next_event(unsigned long delta,
next = timer_read() + delta;
__raw_writel(next, TIMERS_VIRT_BASE + TMR_TN_MM(0, 0));
+ now = timer_read();
- return 0;
+ return (signed)(next - now) <= MIN_DELTA ? -ETIME : 0;
static void timer_set_mode(enum clock_event_mode mode,
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