I have a pretty simple question. If a physical device advertises a
function with class 255, subclass 1, protocol 0, and another with
class 255 subclass 2, protocol 0, is it possible to access both of
these functions at the same time?
I presume by "function" you mean the device has these interfaces.
If these are differently numbered interfaces within the same
configuration, then they should be available at the same time.
However, this is vendor specific (that's what 255 means) so the
actual behaviour is vendor specific. They really should be available
at the same time.
A device driver should have no difficulty in accessing such
interfaces at once. A device driver does not have to worry about the
separate interfaces and can go straight to the endpoints contained
within the interfaces. (The interfaces must not be repeated within
the different interfaces, if the device is to spec.)
Such a device should also be able to be addressed by 2 separate
interface drivers at the same time. Each driver will act
independently on its interfaces. If such a device has its
class/subclass as 0/0 the composite driver should even go looking for
the individual interface drivers. If the class is 255, the composite
driver will not do that automatically, but I think its easily
persuaded to do that.
If the interfaces are within the same configuration and not
differently numbered, then they must be Alternatives for the same
interface. These will not be available at the same time, only one
Alternative of one interface is available at one time. One interface
driver (or device driver) will be responsible for choosing which
interface is active at one time.
If these interfaces are in different configurations, then they are
not available at the same time. Only one configuration can be active
at one time. The device driver is responsible for configuring the
device by making one configuration active.
--
Barry Twycross
email@hidden
---
USB, it's not a Dyslexic BUS. (Thanks to TC.)
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