linux-kernel - Re: [PATCH 12/12] [RFC] Introduce Alarm (hybrid) timers

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Date:	Thu, 6 Jan 2011 16:58:45 -0800
From:	Arve Hjønnevåg <arve@...roid.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	linux-kernel@...r.kernel.org, Brian Swetland <swetland@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alessandro Zummo <a.zummo@...ertech.it>
Subject: Re: [PATCH 12/12] [RFC] Introduce Alarm (hybrid) timers

2011/1/6 John Stultz <john.stultz@...aro.org>:
> On Wed, 2011-01-05 at 20:07 -0800, Arve Hjønnevåg wrote:
>> On Wed, Jan 5, 2011 at 6:15 PM, John Stultz <john.stultz@...aro.org> wrote:
>> > Its possible that I've missed some subtleties of the Android
>> > alarm driver interface, and that some of the interface decisions
>> > I've made may not allow Android to use this interface directly,
>> > I'd be very interested if those details could be pointed out,
>> > and hopefully we can find a good solution to get this useful
>> > functionality upstream.
>> >
>>
>> I don't know how suited the posix interface is for this, but I think
>> it is critical to prevent suspend while an alarm is pending. If an
>> alarm is important enough to wake the system up from suspend, it is
>> probably not safe to suspend right after it triggered. The android
>> alarm driver holds a wakelock until user-space calls back in to wait
>> for the next alarm, while in-kernel alarms are called from interrupt
>
> Hrm. I was hoping to avoid wakelock discussions for now. What happens if
> an app sets a single alarm and then never calls back in? I assume
> closing the device drops the wakelock?
>

Yes, but the current driver only supports a single client, so this
only happens when the system_server crashes, not when apps exits.

>> context. The apis provided in include/linux/pm_wakeup.h should provide
>> the functionality you need to prevent suspend until the alarms have
>> been fully processed, but I have not tried this api yet.
>
> Ok. I'll have to check out the pm_wakeup.h api and see if it can be
> used.
>
>> It would also be useful to still allow in-kernel alarms to be
>> activated from atomic context (we currently do this in a couple of
>> drivers to avoid using a second wakelock).
>
> This is useful. I think I was being overly cautious using a mutex
> instead of a spinlock for the base lock since I was worried about
> calling into the RTC code which require mutexes, but we only do that at
> suspend, so it should be ok to use a spinlock there. I'll revise and add
> that in.
>
> So otherwise, do you see any reason why android might not be able to
> adapt this code to replace the android alarm timers?
>

The user-space interface does not look appealing, but I don't see any
reason why the in-kernel interface(s) cannot be shared. Our user-space
code has a single thread that waits for alarms to trigger, while the
alarms can be modified from any thread. As far as I can tell, using
the posix interface would either require a thread per alarm (up to 5)
or using signals. Both make the user-space code more complicated, and
it is not clear if either of them provide a clear hand-off between
where the kernel needs to block suspend because the alarm has not been
delivered to user-space and where user-space needs to block suspend
because it is handling the alarm.

-- 
Arve Hjønnevåg
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