[PATCH] ARM: timer: Shutdown clock event device when stopping local timer

[Ning Jiang]

2013/3/30 Ning Jiang <ning.n.jiang at gmail.com>:
> 2013/3/30 Russell King - ARM Linux <linux at arm.linux.org.uk>:
>> On Sat, Mar 30, 2013 at 05:57:38PM +0800, Ning Jiang wrote:
>>> 2013/3/30 Stephen Boyd <sboyd at codeaurora.org>:
>>> > On 03/29/13 02:24, ning.n.jiang at gmail.com wrote:
>>> >> From: Ning Jiang <ning.n.jiang at gmail.com>
>>> >>
>>> >> Currently there are two problems when we try to stop local timer.
>>> >> First, it calls set_mode function directly so mode state is not
>>> >> updated for the clock event device. Second, it makes the device
>>> >> unused instead of shutdown.
>>> >
>>> > What device is this a problem on? I believe this only matters to drivers
>>> > which enable their timer in their set_next_event() callback? But even
>>> > then, does anything actually happen because the interrupt should have
>>> > been disabled in the local timer stop callback.
>>> >
>>>
>>> Right. Drivers which enable timer in set_next_event() will have this problem.
>>> It will not have functional problem in my case. But my device cannot enter
>>> low power mode with a pending interrupt even if it is disabled.
>>
>> You're not telling us what you have discovered.  How does set_next_event()
>> get called after we've set the mode to UNUSED in the current code?
>
> In the current code we did not set the mode to UNUSED but only call
> set_mode callback function for the clock event device. This normally
> disables current clock event device. The dying CPU eventually will
> switch to idle thread, call tick_nohz_idle_enter(), try to cancel the
> sched_timer and reprogram the next event. Then set_next_event() gets
> called. The call stack will be like:
>
> tick_nohz_idle_enter
>   -> __tick_nohz_idle_enter
>     -> tick_nohz_stop_sched_tick
>       -> hrtimer_cancel
>         -> hrtimer_try_to_cancel
>           -> remove_hrtimer
>             -> __remove_hrtimer
>               -> hrtimer_force_reprogram
>                 -> tick_program_event
>                   -> clockevents_program_event
>                     -> set_next_event
>
> In set_next_event() we'll re-enable and re-program the clock event device.

I think there are two problems here:
1. We should use clockevents_set_mode() instead of calling set_mode
callback directly. This is the issue my patch was trying to fix.
2. We shouldn't program a clock event device for a dying CPU anyway. I
can submit another patch if agreed.