Hi, On Mon, Oct 08, 2012 at 03:47:35PM -0700, Sarah Sharp wrote: > On Mon, Oct 08, 2012 at 04:05:00PM -0400, Alan Stern wrote: > > On Mon, 8 Oct 2012, Sarah Sharp wrote: > > > > > The Set SEL control transfer tells a device the exit latencies > > > associated with a device-initated U1 or U2 exit. Since a parent hub may > > > initiate a transition to U1 soon after a downstream port's U1 timeout is > > > set, we need to make sure the device receives the Set SEL transfer > > > before the parent hub timeout is set. > > > > Just a question, not a comment on the patch. Why is "Set SEL" needed? > > Doesn't the device already know its own exit latencies? After all, the > > values come from descriptors that were originally provided by the > > device. > > The device knows its own exit latency, but it doesn't know the exit > latencies for the path above it. It could be directly attached to the > roothub (which may have a bigger exit latency than the device) or a > couple tiers deep in the tree. If the device wants to be smart about > when it requests to come out of U1 or U2, then it needs to know the > whole path exit latency. > > For example, maybe there's an interrupt endpoint on a self-powered radio > device. As soon as the device doesn't have any data to send, it > transmits an NRDY (Not Ready) response to the request for data. Then it > puts its link into U1 or U2 (or maybe the host controller does this when > the U1/U2 timeouts expire). > > Later, the device starts receiving a packet. Let's assume it takes some > time to process that packet. If the device knows how long it takes to > wake up the link and send an ERDY to the host, it can parallelize the > link wakeup and the packet processing. It doesn't want to send an ERDY > too soon, because the host could ask for the data before it's ready. So > the device needs to know the time it takes for the link to come out of > U1 or U2, the time it takes for the host to process the ERDY, and the > time it takes for the request for data to reach the device. That's what > the Set SEL request does. > > Of course, I'm not a USB device designer, so I don't know how the Set > SEL command is really used. Felipe, do we use it at all in any of the > Linux USB 3.0 gadget drivers? sorry for the delay. Currently we're not using Set SEL at all, but we could use, eventually, to pass it to pm_qos ?!? Just a thought. -- balbi
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