Re: Changes to power management in ehci-tegra

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 04/18/2012 09:22 AM, Alan Stern wrote:
> On Tue, 17 Apr 2012, Stephen Warren wrote:
...
> In other words, if both the root hub and the controller are powered 
> down, then neither one wakes up when a device is plugged in?
> 
>> I assume this a bug in the Tegra EHCI driver's suspend implementation?
> 
> Actually it sounds like a bug in the controller's wakeup mechanism.  
> Maybe a hardware bug, maybe a software bug.
> 
> While looking through the code, I didn't notice anything about enabling
> wakeups.  But I wasn't looking very carefully, because I don't know how
> the Tegra platform works.  A quick search through the patch now seems
> to show that wakeups never get enabled at all!

I looked at our downstream ehci-tegra.c, and there's a lot more code
there for suspend/resume, wakeup, and a bunch of other stuff. Given
that, I'm not too surprised that the upstream driver isn't resuming from
suspend as expected.

I think we can still apply your patch if you want, but we additionally
need to make CONFIG_USB_EHCI_TEGRA depend on !CONFIG_USB_SUSPEND, which
will ensure nobody gets surprised by this fails-to-resume issue. Does
that seem reasonable? I don't think this will have any user-impact,
since (a) the controllers weren't suspending before this patch, and (b)
neither CONFIG_USB_SUSPEND nor CONFIG_PM_RUNTIME were enabled in
tegra_defconfig (although in 3.5 we'll enable CONFIG_PM_RUNTIME).

Those responsible for ehci-tegra.c in our downstream kernels are
supposed to be working on a plan to upstream the missing parts. When
that comes to fruition, we should be able to remove that Kconfig
restriction. I'm afraid I don't have an ETA for when that will be though.