linux-kernel - Re: ACPI: Idle Processor PM Improvements

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Message-Id: <1158233396.5871.169.camel@localhost.localdomain>
Date:	Thu, 14 Sep 2006 07:29:55 -0400
From:	Jim Gettys <jg@...top.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Jordan Crouse <jordan.crouse@....com>,
	"Brown, Len" <len.brown@...el.com>,
	Linux Kernel ML <linux-kernel@...r.kernel.org>,
	Dominik Brodowski <linux@...inikbrodowski.net>,
	ACPI ML <linux-acpi@...r.kernel.org>,
	Adam Belay <abelay@...ell.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Arjan van de Ven <arjan@...ux.intel.com>, devel@...top.org,
	Bjorn Helgaas <bjorn.helgaas@...com>
Subject: Re: ACPI: Idle Processor PM Improvements

On Thu, 2006-09-14 at 11:18 +0200, Pavel Machek wrote:
> On Tue, 2006-09-12 at 11:21 +0200, Pavel Machek wrote:
> > 
> > > Ok, so what is needed is message to X "we are suspending", and X
> needs
> > > to respond "okay, I'm ready, no need for console switch".
> > 
> > This presumes an external agent to X controlling the fast
> > suspend/resume, with messages having to flow to and from X, and to
> and
> > from the kernel, with the kernel in the middle.
> > 
> > Another simpler option is X itself just telling the kernel to suspend
> > without console switch, as the handoff of the display to the DCON chip
> > has to be done with X and with an interrupt signaling completion of the
> > handoff.  This would be triggered by an inactivity timeout in the X
> > server.
> 
> Whoa... that's a hack.. but yes, you can probably do that, and I think
> kernel even has neccessary interfaces already. (They were needed for
> uswsusp).

Glad you like it ;-).  Dunno which way we'll go yet, though it will get
to the top of the pile to implement this fall.  I suspect we may go this
route to get going, but explore the more general solution as we get more
sophisticated power management policies and standards in place.

> 
> > > Alternatively, hack kernel to take control from X without actually
> > > switching consoles. That should be possible even with current
> > > interface.
> > 
> > This would require saving/restoring all graphics state in the kernel
> > (and X already has that state internally).  Feasible, but seems like
> 
> Hmm, save/restore graphics state from the kernel would of course be
> clean solution, but you should have that anyway... what if someone
> suspends without X running?

X knows its graphics state; it has to remember it all to know when it
has to be changed; on resume, resume can reinit the graphics state to
what the console wants/needs.

If you VT switch back to X, X can restore the graphics state to what it
remembers.

> 
> And of course you can just cheat, and not do kernel save-state on your
> system.

Yup, though it isn't clear to me I'd call it cheating.  In some ways,
what I just described to handle suspends when X is not running is really
robust and simple.  And you don't have divided responsibility for
remembering the state. Simple == good in my book.

                                  - Jim

-- 
Jim Gettys
One Laptop Per Child


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