linux-kernel - Re: at91sam9260 wakeup on serial port

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Date:	Wed, 30 Jan 2008 21:34:22 +0100
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	David Brownell <david-b@...bell.net>
Cc:	michael <trimarchi@...dalf.sssup.it>, linux-kernel@...r.kernel.org,
	Andrew Victor <linux@...im.org.za>
Subject: Re: at91sam9260 wakeup on serial port

On Wed, 30 Jan 2008 10:56:12 -0800
David Brownell <david-b@...bell.net> wrote:
> On Wednesday 30 January 2008, Haavard Skinnemoen wrote:
> > Yeah, although the nasty thing about UARTs is that you never know when
> > DMA really is idle.
> 
> If the UART isn't open, its DMA should be inactive.  :)
> 
> Also, after suspend() it should normally be inactive.
> (That latter is somewhat platform-specific.)

True, but can a closed or suspended UART wake the system? I guess it
could if it's really a GPIO interrupt that triggers the wakeup :-)

> > > The closest analogue to the AT91 support would map /sys/power/state:
> > > 
> > > 	standby --> to AP7 "Frozen"
> > > 	mem --> to AP7 "Stop"
> > 
> > Yes, that looks reasonable. We can also do something in between by
> > stopping most peripherals and busses. For example, keep one peripheral
> > bus and one USART running from OSC0 with everything else stopped.
> 
> Wouldn't that just be a variant of "Frozen"?  The clock API should
> be fully capable of disabling unused clocks, PLLs, and oscillators
> when the platform supports it.  It's common for lots of clocks to be
> disable even in non-suspended system states.

Yes, indeed. I was just pointing out that "Frozen" doesn't necessarily
mean what the datasheet says -- we can disable a lot of clocks manually.

> > I think we need some chip- or family-specific sleep code that knows how
> > to enter a given power state. But the specifics about how to wake the
> > system up must necessarily be board-specific (or even run-time
> > configurable.)
> 
> The sysfs wakeup attributes are the runtiime config mechanism for all
> events associated with a single device.

Right. I'm not all that familiar with the power management mechanisms
in the kernel yet, but this thread has made a few things much clearer.

Haavard
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