ect has not been able to tell me how to calculate. There's no way to get HSD from the xHCI registers either, so I'm simply ignoring it. The Path Exit Latency (PEL) is the worst-case time it will take for a device-initiate exit to U0 to place all the links from the device to the host into U0. The System Exit Latency (SEL) is another device-initiated exit latency. SEL is useful for USB 3.0 devices that need to send data to the host at specific intervals. The device may send an NRDY to indicate it isn't ready to send data, then put its link into a lower power state. If it needs to have that data transmitted at a specific time, it can use SEL to back calculate when it will need to bring the link back into U0 to meet its deadlines. SEL is the worst-case time from the device-initiated exit to U0, to when the device will receive a packet from the host controller. It includes PEL, the time it takes for an ERDY to get to the host, a host-specific delay for the host to process that ERDY, and the time it takes for the packet to traverse the path to the device. See Figure C-2 in the USB 3.0 bus specification. Note: I have not been able to get good answers about what the host-specific delay to process the ERDY should be. The Intel HW developers say it will be specific to the platform the xHCI host is integrated into, and they say it's negligible. Ignore this too. Separate from these four exit latencies are the U1/U2 timeout values we program into the parent hubs. These timeouts tell the hub to attempt to place the device into a lower power link state after the link has been idle for that amount of time. Create two arrays (one for U1 and one for U2) to store mel, pel, sel, and the timeout values. Store the exit latency values in nanosecond units, since that's the smallest units used (DEL is in us, but the Hub Header Decode Latency is in ns). If a USB 3.0 device doesn't have a SuperSpeed Extended Capabilities BOS descriptor, it's highly unlikely it will be able to handle LPM requests properly. So it's best to disable LPM for devices that don't have this descriptor, and any children beneath it, if it's a USB 3.0 hub. Warn users when that happens, since it means they have a non-compliant USB 3.0 device or hub. This patch assumes a simplified design where links deep in the tree will not have U1 or U2 enabled unless all their parent links have the corresponding LPM state enabled. Eventually, we might want to allow a different policy, and we can revisit this patch when that happens. Signed-off-by: Sarah Sharp Cc: Alan Stern $̝3x