I've got a patch that's almost ready for submission, to change the way USB
devices are initialized. The trickiest part is determining the maxpacket
value for endpoint 0. The number is fixed at 8 for low-speed and 64 for
high-speed devices, but for full-speed devices it can be any power of two
between 8 and 64. And of course, without knowing the correct value it's
impossible to transmit any messages longer than 8 bytes.
The current scheme (the "Linux" scheme) goes like this:
Reset the device
Set endpoint 0 maxpacket to {8, 8, 64} for {low, full, high}-speed
Send SET-ADDRESS (an 8-byte SETUP with no message body)
Send 8-byte GET-DEVICE-DESCRIPTOR request; the bMaxPacketSize0
value is stored in the eighth byte of the descriptor
Set endpoint 0 maxpacket to the correct value
Send full 18-byte GET-DEVICE-DESCRIPTOR request
There are a couple of loops and retries in there that I've left out, and
the entire sequence is repeated if it fails.
The new scheme (the "Windows" scheme) is the one used by current Microsoft
operating systems (and therefore supported by virtually every USB device,
one would hope):
Reset the device
Set endpoint 0 maxpacket to {8, 64, 64} for {low, full, high}-speed
Send 64-byte GET-DEVICE-DESCRIPTOR request. Since the
descriptor is only 18 bytes long, this will terminate
with a short packet. But if the maxpacket size is 8
or 16 the device may be waiting to transmit some more.
Reset the device again (clears out any data the device is waiting
to send)
Set endpoint 0 maxpacket to the correct value
Send SET-ADDRESS
Send full 18-byte GET-DEVICE-DESCRIPTOR request
The patch has been tested and it works. In particular, Sony camcorders
work with the Windows scheme but not with the Linux scheme. Probably
other devices will act similarly; it's common for vendors to test only for
Windows compatibility and not for compliance with the USB spec.
Clearly we should support both schemes. One thing my patch does is change
the total number of tries (SET_CONFIG_TRIES) from 2 to 4, trying the Linux
scheme twice and the Windows scheme twice.
The question is, in what order should the schemes be tried? The
conservative approach would be to use the Linux scheme twice, then the
Windows scheme twice. But maybe that's not such a good choice; the Sony
camcorders respond to the 8-byte GET-DEVICE-DESCRIPTOR request by trying
to send the entire descriptor, thereby causing a Babble error. With a VIA
host controller this would ruin any chance of using the device.
Other possibilities are to try the Windows scheme twice, then the Linux
scheme twice, or to alternate. Any suggestions?
Alan Stern
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