Subject: [PATCH] clocksource: arm_arch_timer: fix system hang

From: [email-masked] (Mark Rutland)
Date: Sun, 19 Oct 2014 20:40:55 +0100

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 <[email-masked]>

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?

Thanks,
Mark.

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