Subject: Re: next-20130627 breaks i.MX6 sabre sd UART console

From: Stephen Boyd <[email-masked]>
Date: Mon, 01 Jul 2013 13:54:36 -0700

On 07/01/13 13:14, Thomas Gleixner wrote:
> The issue is very subtle. What happens is:
>
> CPU0						CPU1
>
> Switch to oneshot mode
>
>  Copy the bits from tick_broadcast_mask to
>  tick_broadcast_oneshot_mask. We need to do
>  that so the other cpus reach the timer irq
>  and the softirq which switches them to
>  oneshot.
>
>  Kick the broadcast device into oneshot.
>
> 						Timer interrupt fires
> 						
> 						irq_enter sees the cpu in
> 						tick_broadcast_oneshot_mask and
> 						sets the device to oneshot mode
> 						
> 						handle_periodic:
> 						 Sees oneshot mode and adds
> 						 period to
> 						 dev->next_event(KTIME_MAX)
> 			

Yep. It is also racing with the timer interrupt so having more than two
CPUs must help widen the window (which is why we see it on the higher
numbered CPUs).

> 			 
> So we need two fixes:
>
> 1) The replacement of the dummy timer and the effect on the broadcast
>    mask/device
>
> 2) tick_check_oneshot_broadcast needs a sanity check
>
> Patch below.

Looks good.

Reviewed-by: Stephen Boyd <[email-masked]>

One minor typo in the comment below.

> +		switch (tick_broadcast_device.mode) {
> +		case TICKDEV_MODE_ONESHOT:
> +			/*
> +			 * If the system is in oneshot mode we can
> +			 * unconditionally clear the oneshot mask,
> +			 * because the CPU is running and therefor not

s/therefor/therefore/

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