[PATCH] clocksource: arm_arch_timer: fix system hang

[msalter@redhat.com (Mark Salter)]

On Sun, 2014-10-19 at 20:40 +0100, Mark Rutland wrote:
> Hi Mark,
> 
> On Sun, Oct 19, 2014 at 04:22:44PM +0100, Mark Salter wrote:
> > Arm allows for two possible architectural clock sources. One memory mapped
> > and the other coprocessor based. If both timers exist, then the driver waits
> > for both to be probed before registering a clocksource.
> > 
> > Commit c387f07e6205 ("clocksource: arm_arch_timer: Discard unavailable timers
> > correctly") attempted to fix a hang occurring when one of the two possible
> > timers had a device node, but was disabled. In that case, the second probe
> > would never occur and the system would hang without a clocksource being
> > registered.
> > 
> > Unfortunately, incorrect logic in that commit made things worse such that
> > a hang would occur unless both timers had a device node and were enabled.
> > This patch fixes the logic so that we don't wait to probe a second timer
> > unless it exists and is enabled.
> > 
> > Signed-off-by: Mark Salter <msalter@redhat.com>
> 
> Marc Zyngier had a similar fix for this issue a few days ago [1].
> 
> > ---
> >  drivers/clocksource/arm_arch_timer.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> > index d1a5e35..b73392b 100644
> > --- a/drivers/clocksource/arm_arch_timer.c
> > +++ b/drivers/clocksource/arm_arch_timer.c
> > @@ -666,13 +666,14 @@ static bool __init
> >  arch_timer_probed(int type, const struct of_device_id *matches)
> >  {
> >  	struct device_node *dn;
> > -	bool probed = false;
> > +	bool probed = true;
> >  
> >  	dn = of_find_matching_node(NULL, matches);
> > -	if (dn && of_device_is_available(dn) && (arch_timers_present & type))
> > -		probed = true;
> > -	of_node_put(dn);
> > -
> > +	if (dn) {
> > +		if (of_device_is_available(dn) && !(arch_timers_present & type))
> > +			probed = false;
> > +		of_node_put(dn);
> > +	}
> 
> Other than the addition of the NULL check, this looks identical to
> Marc's fix. There's already a NULL check in of_device_is_available, so I
> don't think it's necessary to add one here. Are you seeing some failure
> with a NULL np?

No. It just looked weird to do the of_node_put unconditionally even
though of_node_put does check for NULL also.

> 
> Thanks,
> Mark.
> 
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-October/294744.html
> 
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