G5 Xserve rackmeter broken?

[Benjamin Herrenschmidt]

On Thu, 2015-05-14 at 13:06 +0300, Aaro Koskinen wrote:
> Hi,
> 
> On Wed, May 13, 2015 at 06:39:40AM +1000, Benjamin Herrenschmidt wrote:
> > On Tue, 2015-05-12 at 20:55 +0300, Aaro Koskinen wrote:
> > > I'm running with HZ=100 so the values are still probably within
> > > jiffy resolution, so perhaps the calculation should first do
> > > idle = min(idle, total)?
> > 
> > Does it gives you a reasonable output if you do that ?
> 
> The below change fixes the idle system blinking behaviour.
> 
> I'm also able to reproduce the leds going off during full CPU load case.
> It seems there is some DMA error. Normally, reading rm->dma_regs->status
> in the IRQ handler gives 0x8400. In the failure cases I've seen values
> 0x8880 and 0x8980 - the IRQ will stop after this and it will need
> paused -> started cycle before it gets going again (but sometimes fails
> again soon after).

That's a bit worrysome, is that new ? Smells like faulting HW ...

Ben.

> A.
> 
> diff --git a/drivers/macintosh/rack-meter.c b/drivers/macintosh/rack-meter.c
> index 048901a..3381fa59 100644
> --- a/drivers/macintosh/rack-meter.c
> +++ b/drivers/macintosh/rack-meter.c
> @@ -227,6 +227,7 @@ static void rackmeter_do_timer(struct work_struct *work)
>  
>  	total_idle_ticks = get_cpu_idle_time(cpu);
>  	idle_ticks = (unsigned int) (total_idle_ticks - rcpu->prev_idle);
> +	idle_ticks = min(idle_ticks, total_ticks);
>  	rcpu->prev_idle = total_idle_ticks;
>  
>  	/* We do a very dumb calculation to update the LEDs for now,