On Tue, Oct 29, 2013 at 08:13:45PM -0200, Wander Lairson Costa wrote:
> 2013/10/29 Tormod Volden <lists.tor...@gmail.com>:
> > On Tue, Oct 29, 2013 at 9:06 PM, Wander Lairson Costa wrote:
> >> 2013/10/29 Johannes Stezenbach:
> >>> On Tue, Oct 29, 2013 at 02:58:45PM -0200, Wander Lairson Costa wrote:
> >>>> 2013/10/29 Johannes Stezenbach:
> >>> We're running in cirecles... maybe I don't understand
> >>> what the (0,0) index in cfg[(0,0)] means? My assumption
> >>> is it means the first Interface Descriptor as listed
> >>> in lsusb -v output.
> >>>
> >>> And if the first descriptor has bInterfaceNumber==2 and
> >>> you look it up with usb.util.find_descriptor() you would
> >>> get the same first descriptor.
> >>>
> >>
> >> I am affraid you are confusing the relative order in which the
> >> interface descriptor appears inside a configuration descriptor (which
> >> is irrelevant to USB spec), and the interface number, which is
> >> actually used for USB, say, for interface claiming. They are not
> >> necessarily equal...
> >
> > No, he is not. He is just saying that the code in question will simply
> > look up the first (in appearing order) interface, whatever its
> > interface number is. So the question is why go via the interface
> > number in this example.
> >
>
> Ah, now I got it, thanks Tormod. It is needed because because we want
> the interface descriptor related to the current active alternate
> setting for the first interface found.
I'm still not getting it.
cfg = dev.get_active_configuration()
# now we have the Configuration Descriptor for some arbitrary configuration
intf = cfg[(0,0)]
# first altsetting of first interface in the configuration
The usb.util.find_descriptor() in the tutorial would be useful
if we would know bInterfaceNumber for the desired interface
by some other means. A more realistic example for the
tutorial would search the interface by bInterfaceClass,
bInterfaceSubClass and bInterfaceProtocol. Or, for the
case of Linux g_zero, find the configuration with
iConfiguration pointing to "source and sink data" and
use the first (and only) interface.
Thanks,
Johannes
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