On Mon, 27 Dec 2004, Christian Heimanns wrote:
> Thanks for the tipp: I've compiled the USBcore as a module now. Before
> it was compiled into the kernel. In the file /etc/modprobe.conf I've
> added the line: > options usbcore old_scheme_first=y <.
> It seems to work now :-) Can you please explain in a few sentences the
> difference between the "new" and the "old" USB scheme?
I'll try...
The "old scheme" is the way Linux worked before 2.6.10. When a new device
is plugged in, the system first assigns it an address, then reads the
initial 8 bytes of the device descriptor, then reads the entire 18-byte
device descriptor.
The "new scheme" is basically the way Windows works. (Not surprisingly,
some devices won't work any other way.) When a new device is plugged in,
the system first issues a 64-byte read request for the device descriptor,
then resets the device, then assigns it an address, and then reads the
actual 18-byte device descriptor.
The reason for these shenanigans is that with a full-speed device, the
maximum packet size for endpoint 0 (ep0maxpacket) isn't known beforehand.
It could be 8, 16, 32, or 64 bytes. (Low-speed devices must use 8, and
high-speed devices must use 64.) The ep0maxpacket value is stored in the
initial 8 bytes of the device descriptor, but to read the device
descriptor you have to use endpoint 0!
The two schemes above are the two common solutions to this chicken-and-egg
problem. The old scheme is the one recommended by the USB Implementors
Forum (which makes it the "Standard"); the new scheme is the one used by
Microsoft (which makes it another kind of "Standard"!). A well-designed
standards-compliant device will work okay with either scheme.
Unfortunately it seems that no matter which scheme you pick, some
badly-designed non-compliant devices won't work. There's an additional
usbcore.ko module parameter people can use in especially bad cases:
use_both_schemes=y
This will cause the system to try one of the schemes, and if it fails then
try the other scheme. (Maybe that should always be the default...)
Alan Stern
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