I wonder if that SCSI fix (restoring a wrongly deleted mem clear) helps
get rid of this oops too?
On Wednesday 08 March 2006 7:30 am, Alan Stern wrote:
> > > a) How come we're only considering the zeroth slot in that array in here?
> >
> > We start out with the first interface setting, as we always know we have
> > one of them as per the USB spec (I think, anyone from linux-usb-devel
> > want to verify this?)
>
> In this case it wouldn't make any difference, since all the altsettings
> for a particular interface are supposed to have the same bInterfaceClass,
> bInterfaceSubClass, and bInterfaceProtocol. Although I don't think the
> USB spec actually says this anywhere..
I'd have stopped at "wouldn't make any difference"; the kernel must make
some initial choice, but userspace is free to revise it. Agreed it would
be odd if altsettings had different class/subclass/protocol, but I don't
see any good reason to make that illegal.
> The bMaxPower value could be different for different altsettings.
Erm, no; that's a per-configuration thing, not a per-altsetting thing.
It's checking the config descriptor, not the interface descriptor,
for that particular concern.
> > > b) How do we know that there's actually anything _there_? The length of
> > > that variable-sized array doesn't seem to have been stored anywhere
> > > obvious by usb_parse_configuration() and choose_configuration() doesn't
> > > check. What happens if the length was zero?
> >
> > I don't think it is allowed to be, as all USB devices have to have at
> > least 1 interface.
I think that's not true, and it would be worth verifying that it's not
a no-interfaces device even if the USB spec required it. It's trivial
to create device firmware that advertises no-interfaces, and those should
never be able to make Linux hiccup (much less oops).
> The code in usb_parse_configuration() guarantees that the number of
> entries in the altsettings array is at least 1, because it sets nalts[n]
> to 1 initially and never decreases it. The whole idea of an interface
> without altsettings makes no sense...
Right; there's always at least one setting. Calling them "alt" settings
can be confusing; any one of them could be the "main" setting.
- Dave
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