linux-kernel - Re: [PATCH] ARM: timer: Shutdown clock event device when stopping local timer

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Message-ID: <CAH3Oq6T=xjEPnfdG2mP121ifwRAsZpopMKbW8CAi3pHDBZNX5w@mail.gmail.com>
Date:	Sun, 31 Mar 2013 21:11:25 +0800
From:	Ning Jiang <ning.n.jiang@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Stephen Boyd <sboyd@...eaurora.org>, kgene.kim@...sung.com,
	davidb@...eaurora.org, dwalker@...o99.com, bryanh@...eaurora.org,
	john.stultz@...aro.org, tglx@...utronix.de,
	linus.walleij@...aro.org, shawn.guo@...aro.org,
	rob.herring@...xeda.com, arnd@...db.de,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-samsung-soc@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] ARM: timer: Shutdown clock event device when stopping
 local timer

2013/3/30 Ning Jiang <ning.n.jiang@...il.com>:
> 2013/3/30 Russell King - ARM Linux <linux@....linux.org.uk>:
>> On Sat, Mar 30, 2013 at 05:57:38PM +0800, Ning Jiang wrote:
>>> 2013/3/30 Stephen Boyd <sboyd@...eaurora.org>:
>>> > On 03/29/13 02:24, ning.n.jiang@...il.com wrote:
>>> >> From: Ning Jiang <ning.n.jiang@...il.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.
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