On Sat, 21 Feb 2004, Greg KH wrote:

> > One thing that would be good, whether this change gets made or not, is to
> > remove all assumptions from drivers about the order in which interfaces
> > are stored (use usb_ifnum_to_if()) and the order in which altsettings are
> > stored (replace intf.act_altsetting with a pointer and create
> > usb_altnum_to_alt() analogous to usb_ifnum_to_if()).  There are plenty of
> > drivers that will need to be fixed up.
>
> I'd be glad to take patches to fix up any drivers that still have this
> problem right now.

Here's a start.  This patch begins the conversion process by adding
usbcore support for cur_altsetting and deprecating act_altsetting.

So long as we assumed that altsetting numbers range from 0 to
num_altsetting-1 and that the number matches its index in the altsetting
array, there was no harm in using act_altsetting.  But without that
assumption act_altsetting is merely an invitation to errors.  Although the
kerneldoc says that act_altsetting is the _index_ of the active
altsetting, it's all too easy to confuse it with the _number_ of the
active altsetting.  Using cur_altsetting instead (a pointer rather than a
number) will prevent that confusion.

Until all the drivers have been converted to use cur_altsetting, the core
will have to maintain act_altsetting in parallel with it.  Eventually we
will be able to remove act_altsetting, but fixing all the drivers will
take a while.

Included in this patch:

	Add cur_altsetting to struct usb_interface and deprecate
	act_altsetting.

	Add comments and kerneldoc explaining the changes.  Also remove
	the comments in front of struct usb_host_config (they seem to
	have been left behind when usb_ch9.h was split out) and add
	kerneldoc for that structure.

	Add usb_altnum_to_altsetting() to help look up altsettings based
	on their number.

	Convert the usb_set_interface(), usb_set_configuration(), and
	usb_reset_configuration() routines to support cur_altsetting
	and act_altsetting in parallel.  Convert a few others to use
	cur_altsetting rather than act_altsetting.

	Rename a few local variables to make their meaning a little
	clearer.  It would be nice to change struct usb_host_interface
	to something like usb_host_altsetting, but that's a patch for
	another time.