linux-kernel - Re: A desktop environment[1] kernel wishlist

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Message-ID: <20141103141740.2d913583@alan.etchedpixels.co.uk>
Date:	Mon, 3 Nov 2014 14:17:40 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Bastien Nocera <hadess@...ess.net>
Cc:	John Stultz <john.stultz@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: A desktop environment[1] kernel wishlist

> > It's a reasonable ask but answers even if available are likely
> > to be things like "because GPE36" and GPE36 will just be some connection
> > to something that could be anything from a lid switch to a light sensor
> > or even a smart wifi chip deciding it wants the CPU to help out because
> > you are out of range of the base station. We may not even know what it
> > relates to.
> 
> But the device or platform driver would know that, presumably.

Quite often it has no idea - maybe the firmware knows but it isn't
telling us. It's an internal detail.
> 
> > A non suspend system will exit deep idle type status because they got
> > an IRQ or perhaps some DMA needed the cache coherency. That doesn't mean
> > they've got the foggiest which IRQ kicked them out if idle, just that hey
> > I'm awake and there are four pending interrupts. That of course is
> > assuming it even noticed it entered a deep idle state - you don't want to
> > wake an idle CPU to tell it that its more idle than it was before.
> 
> Sure, the CPU might not be the best example of a device for which we
> need to track the wakeup reason. The device drivers however...

You keep assuming a wakeup is "special" - it's quite possibly not. The
RTC driver knows whether an alarm went off, it's got no idea if that
cause a wakeup or even what a wake up is or if the platform has wakeups
or just deep sleeps.

Same for most other stuff - plugging in a display may well bump a machine
out of deep sleep but the graphics driver isn't going to know anything
other than "I'm handling a cable change".

Events can also be processed entirely by firmware so we just wake up, look
round, scratch out head and go back to sleep.

So I really think "why did I wake up" is actually the wrong question to
be asking.

What you probably should be asking (and what the kernel effectively asks)
is "Why am I not idle ?". That plus "what state changes have occurred
that I care about". Both of those are questions you can ask at any time
without caring how sleeping may or may not happen.

ie you don't care if a lid event woke you, you care if the lid is open or
shut. You don't care whether a wireless event woke you, you care that the
wireless has been lost etc..

Ie instead of doing

	if (woken && cause == BATTERY_LOW)
		suspend_to_disk()

you want to be doing (as part of the normal flow)

	if (battery < BATTERY_LOW && battery_prev >= BATTERY_LOW +
						HYSTERISIS)
		suspend_to_disk();

because it's kind of irrelevant whether it woke you for this, you need to
do it anyway.

Alan
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