Re: USB D3 vs system S3

[David Brownell <david-b@pacbell.net>]

On Wednesday 09 January 2008, Alan Stern wrote:
> 
> > > The USB autosuspend code affects only the controller's USB interface --
> > > it doesn't touch the PCI side.  An autosuspended controller will remain
> > > in D0.  Until somebody tries writing autosuspend code for PCI
> > > devices...
> > 
> > Is this likely to happen?
> 
> I don't know of anybody working on it.  A minimal prerequisite is that 
> PCI runtime wakeup processing needs to work right -- which it doesn't.  
> See 
> 
>         http://bugzilla.kernel.org/show_bug.cgi?id=6892

And while most of the non-PCI USB host platforms to which I have access
don't have that type of issue, their hardware isn't actually set up to
offer an analogue of runtime PCI_D3 (for example) power states.

In more detail:  they generally have some clocks that could be disabled,
but for various reasons they need to be left running.  Disabling those
clocks prevents wakeup from working ... yes, a multi-MHz clock just to
detect the D+ (or D-) pullup as it kicks in.  Systems using an external
PHY will sometimes offer an alternative, when the PHY can issue those
wakeup IRQs by itself, but that seems oddly uncommon.

- Dave

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