[RFC 13/13] USB: Disable hub-initiated LPM for comms devices.

[Greg Kroah-Hartman]

On Wed, May 16, 2012 at 03:45:28PM -0700, Sarah Sharp wrote:
> [Resending with a smaller Cc list]
> 
> Hub-initiated LPM is not good for USB communications devices.  Comms
> devices should be able to tell when their link can go into a lower power
> state, because they know when an incoming transmission is finished.
> Ideally, these devices would slam their links into a lower power state,
> using the device-initiated LPM, after finishing the last packet of their
> data transfer.
> 
> If we enable the idle timeouts for the parent hubs to enable
> hub-initiated LPM, we will get a lot of useless LPM packets on the bus
> as the devices reject LPM transitions when they're in the middle of
> receiving data.  Worse, some devices might blindly accept the
> hub-initiated LPM and power down their radios while they're in the
> middle of receiving a transmission.
> 
> The Intel Windows folks are disabling hub-initiated LPM for all USB
> communications devices under a xHCI USB 3.0 host.  In order to keep
> the Linux behavior as close as possible to Windows, we need to do the
> same in Linux.

How is the USB core on Windows determining that LPM should be turned off
for these devices?  Surely they aren't modifying each individual driver
like this is, right?  Any way we also can do this in the core?

Or, turn it around the other way, and only enable it if we know it's
safe to do so, in each driver, but I guess that would be even messier.

thanks,

greg k-h