[PATCH 03/12] arm: omap3: Only sleep in cpuidle driver if I/O wake-ups work

Paul Walmsley paul at pwsan.com
Wed Apr 11 18:47:31 EDT 2012

cc Govindraj

On Wed, 11 Apr 2012, Mark A. Greer wrote:

> On Wed, Apr 11, 2012 at 03:37:47PM -0600, Paul Walmsley wrote:
> > I don't quite understand this patch.  Are you saying that AM3517/3505 
> > can't wake from WFI?  That would seem odd.
> No, I'm saying that I/O doesn't seem to wake it up from the WFI.
> I've learned to not trust the am35x TRM much so I'm pretty much
> feeling my way along in the dark.

Hehe, no worries, just curious.

> I do know that without this patch, the system is extremely slow which I 
> believe is from it only returning from the WFI because of a timer 
> expiration or something like that.

How are you trying to wake it up -- from an incoming character via 
the UART?

Does the system pause in the middle of UART transmits, or does it get the 
transmit buffers out cleanly and just not respond promptly to incoming 

> > There are other sources of wakeup on the system other than I/O wakeup.  
> > I/O wakeup only applies to wakeups from the I/O pads when the chip is in 
> > RETENTION or OFF.  And as I understand it, neither of those apply to 
> > AM3517/3505?
> Hmm, its true that RETENTION and OFF aren't supported.  It does wake up
> occasionally (as mentioned above) it just seems that I/O doesn't wake
> it up.  I may be mistaken.  I'm only going from what I've seen since and
> the TRM seems to have lots of errors so I'm not sure what to trust in it.

OK no problem, just trying to understand what's going on.  Sounds like 
you've gotten tossed into the deep end :-)

> > Even if I/O wakeups aren't supported, many of the IP blocks on the system 
> > should be able to cause the ARM to exit WFI by asserting their 
> > SWAKEUP lines and raising their interrupt lines.
> Okay, but that doesn't seem to be working.  I'll look some more to see
> why they aren't working.

There could be a hidden dependency on I/O wakeup being present in the UART 
driver. We've been going through some UART driver angst recently...

- Paul

More information about the linux-arm-kernel mailing list