如何从处于低功耗状态的USB设备获取字符串描述符？

Question

I am trying to write an application in C++ which lists Information obtained from a USB device. I am following USBView ( Github ) utility's code for it.

When device is in D0 power state (fully powered), I am able to get string descriptors, but when device goes to a low power state (D2) , I am not.

The IOCTL IOCTL_USB_GET_DESCRIPTOR_FROM_NODE_CONNECTION returns with the error "A device attached to the system is not functioning." USBView utility shows that string descriptors are not available when device is in a low power state.

This behaviour is odd, and nowhere in the USB spec did I find reference to it. I have checked with 2 devices, and I get the same behaviour. I am able to get other descriptors such as Device Descriptor, BOS Descriptor, etc. even in D2 state.

Is there a way to get string descriptors when USB device is in low power state? If not, is there a way to momentarily turn it to D0 power state? Probably ACPI is the answer, but it is a very low level API which I am finding overwhelming to understand. Does Windows provide any high level API to set power states?

Answer 1

The USB 2.0 specification defines suspend mode, a state where there is no traffic on the bus and devices go to sleep to save power. Since there is no traffic, you cannot request string descriptors from a device in suspend mode. You'd have to wake it up with special signalling before you can do that.

The only thing I can think of that might help you is disabling the "USB selective suspend setting", a feature of Windows that generally puts devices into suspend mode when they are not being used. It's pretty deep in the power settings part of the Control Panel so let me know if you have trouble finding it.

It seems to me that you ought to be able to wake the device up even if it is in selective suspend so that you can get information from it, but I am not sure exactly how to do that and it probably depends on what drivers your device uses.