Re: [RFC 13/13] USB: Disable hub-initiated LPM for comms devices.
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On Wed, May 16, 2012 at 09:52:20PM -0700, Sarah Sharp wrote: > On Wed, May 16, 2012 at 04:20:19PM -0700, Greg Kroah-Hartman wrote: > > 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? > > No, I don't think they're modifying individual drivers. Maybe they > placed a shim/filter driver below other drivers? They can do this in their driver by just watching the device class type. > Basically, I don't know the exact details of what the Windows folks are > doing. The recommendation from the Intel Windows team was simply to > turn hub-initiated LPM off for "all communications devices". Perhaps > the Windows USB core is looking for specific USB class codes? Or maybe > it has some older API that lets the core know it's a communications > device? > > I'm not really sure we can do it in the USB core with out basically > duplicating all the class/PID/VID matching in the communications driver. > I think just adding a flag might be the best way. I'm open to > suggestions though. You can detect something as "simple" as a class type, which I bet is all that Windows is going to be able to do as well. > > 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. > > Yeah, I think it would be messier. Ok, this is probably the best solution for us as well, sorry for the noise. greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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