linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Date:	Thu, 27 May 2010 18:49:18 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Matthew Garrett <mjg59@...f.ucam.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Arve Hjønnevåg 
	<arve@...roid.com>, Florian Mickler <florian@...kler.org>,
	Vitaly Wool <vitalywool@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul@...p1.linux-foundation.org, felipe.balbi@...ia.com,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

> What is a "Correctly implemented driver" in this case? One that receives 
> a wakeup event and then prevents suspend being entered until userspace 
> has acknowledged that event? Because that's what an in-kernel suspend 
> blocker is.

Kernel side maybe - but even then its a subset of expressing
latency/lowest level requirements. That bit isn't really too contentious.
You need a kernel object to hang a constraint off.

> ACPI provides no guarantees about what level of hardware functionality 
> remains during S3. You don't have any useful ability to determine which 
> events will generate wakeups. And from a purely practical point of view, 
> since the latency is in the range of seconds, you'll never have a low 
> enough wakeup rate to hit it.

So PCs with current ACPI don't get opportunistic suspend capability. It
probably won't be supported on the Commodore Amiga either - your point ?

> Suspend blockers are the mechanism for the 
> driver to indicate whether the wakeup event has been handled. That's 
> what they're there for. The in-kernel ones don't paper over anything.

Semantically the in kernel blockers and the in kernel expression of
device driven constraints are the same thing except that instead of 
yes/no you replace the boolean with information.


So we go from

	block_suspend() / unblock_suspend()

to
	add_pm_constraint(latency, level) 
	remove_pm_constraint(latency, level);


And if Android choses to interpret that in its policy code as

	if (latency > MAGIC)
		suspend_is_cool();
	else
		suspend_isnt_cool();

that's now isolated in droidspace policy

Alan


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