Hi everybody,
I have a question about a possible problem of accessing composite
devices using interface association from libusbx. Not sure, but the
problem might be related to the maling list thread
"libusb_claim_interface() error with usbK driver"
(http://sourceforge.net/mailarchive/message.php?msg_id=30123276) started
last year by Markus, who also complained that he's not able to claim
other interfaces but the first one. The answer there was that it's
misconception and each interface acts separately (needing separate
driver). But I would like to highlight the use of interface association
in this case. I found few other similar-sounding topics in the mailing
list and bug tracker but none of them seemed to mention interface
association directly.
To summarize the problem, I have a composite device using three
associated interfaces (described by an IAD descriptor). Windows by
default assigns usbccgp.sys driver which in turn presents the interfaces
as subdevices wanting own drivers. Because usbccgp.sys recognizes the
interface association, it wants driver just for the first interface of
the associated group ("MI_00"). After installing a driver (e.g. WinUSB,
using Zadig) for that first interface, libusbx allows to claim this
first interface but not the other two associated ones, similar as
described by Markus.
Some brief observation of libusbx internals shows me that during device
discovery the library assigns USB_API_COMPOSITE API to the device's
composite parent. It also assigns (set_composite_interface()) correct
SubAPI to the first interface (e.g. SUB_API_WINUSB), but no SubAPI to
the other two interfaces - because the system only uses one driver per
interface association and the libusbx library makes no attempt to look
at the IAD and interface association. As a result, these interfaces are
recognized as using USB_API_UNSUPPORTED and attempts to claim such
interfaces end with error LIBUSB_ERROR_NOT_SUPPORTED (-12) as described
by Markus (the call is routed through composite_claim_interface() to the
unsupported_claim_interface() for the two "associated" interfaces).
If I replace the system-default driver for the composite parent
(usbccgp.sys) with WinUSB directly, I can access all the interfaces, but
this workaround isn't a good solution in the long run for me, because
other versions of the device might use also other separate (independent,
different driver) interfaces besides the associated group or even
multiple association groups.
Now I'd like to ask if I'm doing something completely wrong,
misunderstanding the library interface or if this is a possible problem
that needs to be approached by the library (e.g. considering interface
association in set_composite_interface() during device enumeration).
Please let me know if further information is required, thanks, Jan
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