linux-kernel - Re: [PATCH] - race-free suspend. Was: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Date:	Wed, 2 Jun 2010 02:53:15 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Neil Brown <neilb@...e.de>, "Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	Felipe Balbi <felipe.balbi@...ia.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul@...p1.linux-foundation.org" <Paul@...p1.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	James Bottomley <James.Bottomley@...e.de>
Subject: Re: [PATCH] - race-free suspend. Was: Re: [linux-pm] [PATCH 0/8] 
	Suspend block api (version 8)

2010/6/2 Thomas Gleixner <tglx@...utronix.de>:
> On Wed, 2 Jun 2010, Arve Hjønnevåg wrote:
>> 2010/6/2 Neil Brown <neilb@...e.de>:
>> > There would still need to be some sort of communication between the the
>> > suspend daemon on any event daemon to ensure that the events had been
>> > processed.  This could be very light weight interaction.  The point though is
>> > that with this patch it becomes possible to avoid races.  Possible is better
>> > than impossible.
>> >
>>
>> We already have a solution. I don't think rejecting our solution but
>> merging a worse solution should be the goal.
>
> That's not the goal at all. We want a solution which is acceptable for
> android and OTOH does not get into the way of other approaches.
>

I don't actually think the suspend blocker patchset get in the way of
anything else.

> The main problem I have is that suspend blockers are only addressing
> one particular problem space of power management.
>
> We have more requirements than that, e.g. an active device transfer
> requires to prevent the idle code to select a deep power state due to
> latency requirements.
>
> So we then have to implement two mechanisms in the relevant drivers:
>
>   1) telling the idle code to limit latency
>   2) telling the suspend code not to suspend

And 3) telling the idle code to not enter low power modes that disrupt
active interrupts or clocks.

Our wakelock code handles 2 and 3, but I removed support for 3 on
request since you can hack it by specifying a latency value that you
know the low power mode cannot support.

>
> My main interest is to limit it to one mechanism, which is QoS based
> and let idle and suspend make the appropriate decisions based on that
> information.
>

We can use one mechanism for this, but we still have to specify both.
To me this is just another naming argument and not a good reason to
not merge the suspend blocker code. You have to modify the same
drivers if you call suspend_block() as if you call
pm_qos_update_requirement(don't suspend). We have to specify when it
is not safe to suspend independent of when it is not safe to enter low
power idle modes so unless you want to have a bitmap of constraints
you don't save any calls. And, if we later get a constraint framework
that supports everything, we can switch to it then and we will then
already have some drivers annotated.

-- 
Arve Hjønnevåg
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