Re: [PATCH] clocksource: arc_timer: RTC: allow registration despite SMP

[Daniel Lezcano]

On Wed, Feb 01, 2017 at 04:50:15PM -0800, Vineet Gupta wrote:
> So far we didn't allow CPU private 64-bit RTC timer to register in SMP
> as the individual counters may not be synchronized across cores.

That is the case for the other archs and the per cpu clocksource are used, the
kernel is supposed to be immune against clocks drift. There are mechanisms to
have a synchronized view of the clock when they are per cpu and the kernel does
not compare clocks between cpus. 

I didn't realize that when we did the modification around this check.

May be you can double check these clocksources are really not suitable for SMP,
because if it is not the case the if !CONFIG_SMP could be simply removed.
 
A sidenote: RTC is confusing and should be changed to something else.

> However there is a situation when we build SMP kernel but want to use
> the same image on UP as well as SMP hardware. Here we would certainly
> want to use RTC, but current code doesn't allow as it only uses build
> info (CONFIG_SMP) and not runtime info (num_online_cpus() or some such).
> 
> We can't possibly use num_online_cpus() anyways because clocksource
> probe happens before other cpus are brought online
> 
> The simple fix is allow ETC probe for SMP and rely on higher rating of
> GFRC to take over in general SMP case. We leave the pr_warn to notify
> the user.
> 
> Cc: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> ---
>  drivers/clocksource/arc_timer.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/clocksource/arc_timer.c b/drivers/clocksource/arc_timer.c
> index 7517f959cba7..87f193794bf2 100644
> --- a/drivers/clocksource/arc_timer.c
> +++ b/drivers/clocksource/arc_timer.c
> @@ -145,10 +145,8 @@ static int __init arc_cs_setup_rtc(struct device_node *node)
>  	}
>  
>  	/* Local to CPU hence not usable in SMP */
> -	if (IS_ENABLED(CONFIG_SMP)) {
> +	if (IS_ENABLED(CONFIG_SMP))
>  		pr_warn("Local-64-bit-Ctr not usable in SMP");
> -		return -EINVAL;
> -	}
>  
>  	ret = arc_get_timer_clk(node);
>  	if (ret)
> -- 
> 2.7.4
> 

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