[BUG] ARM Architected timers appear broken in 3.9-rc1

[Jon Medhurst (Tixy)]

On Thu, 2013-03-07 at 15:26 +0000, Mark Rutland wrote:
> On Thu, Mar 07, 2013 at 11:04:49AM +0000, Jon Medhurst (Tixy) wrote: 
> > When booting Versatile Express TC2 I am getting a null pointer
> > dereference which appears to be related to the recent changes in
> > architected timer support. 
> > 
> > The interesting part of the call stack is below (the full log is
> > attached). It shows that whilst entering CPU idle the code is calling
> > the NULL set_next_event function on the dummy timer setup by
> > broadcast_timer_setup.
> 
> As you say, we're calling the NULL set_next_event on the dummy timer,
> because the dummy is registered as the broadcast device, which makes no
> sense (as that means it's responsible for broadcasting to itself).
> 
> As far as I can see, the issue is that the dummy timer has a higher
> rating (400) than the sp804 (300) that would otherwise be used as the
> broadcast source, and tick_check_broadcast_device does not reject clocks
> with CLOCK_EVT_FEAT_DUMMY (which should *never* be used as a broadcast
> source). 
> 
> Giving the dummy timer a lower rating than the sp804 would prevent this,
> but it might make more sense to do something like the below and
> explicitly prevent dummy timers from being registered as broadcast
> sources (this seems to work for me with your kernel, though I hit an
> unrelated BUG() later due to bL_platform_power_ops *power_ops not being
> set).

The patch works for me too and I can boot successfully to a command
prompt. I think the BUG you are hitting could be due to different
device-trees, I'm using vexpress-v2p-ca15-tc2.dts in the branch I
pointed you to.

Thanks Mark for so quickly looking into this.

Tixy.

> ---->8----
> From 7432019cdfea9b2808e3b04489cd71a89266ca8f Mon Sep 17 00:00:00 2001
> From: Mark Rutland <mark.rutland at arm.com>
> Date: Thu, 7 Mar 2013 15:09:24 +0000
> Subject: [PATCH] clockevents: don't allow dummy broadcast timers
> 
> Currently tick_check_broadcast_device doesn't reject clock_event_devices
> with CLOCK_EVT_FEAT_DUMMY, and may select them in preference to real
> hardware if they have a higher rating value. In this situation, the
> dummy timer is responsible for broadcasting to itself, and the core
> clockevents code may attempt to call non-existent callbacks for
> programming the dummy, eventually leading to a panic.
> 
> This patch makes tick_check_broadcast_device always reject dummy timers,
> preventing this problem.
> 
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> ---
>  kernel/time/tick-broadcast.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
> index 2fb8cb8..7f32fe0 100644
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -67,7 +67,8 @@ static void tick_broadcast_start_periodic(struct clock_event_device *bc)
>   */
>  int tick_check_broadcast_device(struct clock_event_device *dev)
>  {
> -	if ((tick_broadcast_device.evtdev &&
> +	if ((dev->features & CLOCK_EVT_FEAT_DUMMY) ||
> +	    (tick_broadcast_device.evtdev &&
>  	     tick_broadcast_device.evtdev->rating >= dev->rating) ||
>  	     (dev->features & CLOCK_EVT_FEAT_C3STOP))
>  		return 0;