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? Sarah Sharp -- 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
