https://bugzilla.kernel.org/show_bug.cgi?id=219111 --- Comment #38 from Alan Stern (stern@xxxxxxxxxxxxxxxxxxx) --- It's more than just the driver knowing what configuration to use. The device has to be _told_ to use that configuration; until it is told it won't use any configuration at all and won't function. Your usbmon traces and Windows packet captures all show the computer telling the device to use configuration 1. (If you made a usbmon trace with old_scheme_first=Y, presumably it would show the computer telling the device to use configuration 2.) The configuration matters because the data sent over the USB connection is formatted differently depending on the number of channels. No matter what the driver may know about the possible configs, as long as the device is using config 1, it will provide only two-channel stereo. And as long as it is using config 2, it will provide only four-channel stereo. The difference between the old scheme and the new scheme is rather technical. It has to do with how the device is initialized when it is first plugged in (and also whenever it gets reset). With the old scheme, after performing an initial reset the computer asks the device to send just the first 8 bytes of its device descriptor, then the computer tells the device what address to use, and then it reads the complete device descriptor and all the other descriptors. With the new scheme, after performing the initial reset the computer asks the device to send the first 64 bytes of its device descriptor. Since the device descriptor is only 18 bytes long, the response is always a short one. The computer then resets the device a second time, tells it what address to use, and reads all the descriptors. (That's why I thought skipping the second reset might cause the device to make both configurations available, like it does with the old scheme.) The old scheme is what the USB 2.0 specification recommends. The new scheme is what Microsoft implemented early on, however, and as a result quite a few devices would work with the new scheme but not the old one. In theory any device should work properly with either scheme, but in practice they don't always do so. As in this case. -- You may reply to this email to add a comment. You are receiving this mail because: You are watching the assignee of the bug.
