Inter-driver #if dependencies

[Paul Goyette <paul%vps1.whooppee.com@localhost>]

My crusade for modularity has arrived at the pcppi(4) driver, and I've 
discovered that there are a number of places in the code where a #if is 
used to determine whether or not some _other_ driver is available to 
provide certain routines.  For pcppi(4), these dependencies are for the 
attimer(4) and pckbd(4) drivers.  (While I haven't yet gone searching, 
I'd be willing to wager that there are other similar examples in other 
drivers.)

These #if constructs make it very difficult to modularize these drivers.

I'd like to propose the following new kernel mechanism that will allow 
us to remove these #if dependencies.

1. Extend the struct cfattach to have an additional member, and create
   a new CFATTACH_DECL4_NEW macro to initialize it (and updates to the
   existing CFATTACH_* family to default the value to NULL).

	int	(*ca_callback)(device_t, int, void *);

   (This will require a kernel version bump.)

2. In kern/subr_device.c (or kern/subr_autoconfig) add a new routine
   to call the callback routines for a driver until something handles
   the request.  Note that the caller provides a device_t to identify
   itself to the callback routine.

	int
	config_device_callback(device_t caller, const char *name,
			int cb, void *args)
	{
		struct cfdriver *cd;
		struct cfattach *ca;

		cd = config_cfdriver_lookup(name);
		if (cd == NULL)
			return ENODEV;
		LIST_FOREACH(ca, &cd->cd_attach, ca_list) {
			if (ca->ca_callback != NULL) {
				rv = (*lca->callback)(caller, cb, args);
				if (rv != EPASSTHROUGH)
					return rv;
			}
		}
		/* No driver attachment handled the request - punt */
		return ENODEV;
	}

3. Replace all of the #if code with callbacks.  For example, in pcppi.c
   we can replace

	...
	#if NPCKBD > 0
		pckbd_unhook_bell(pcppi_pckbd_bell, sc);
	#endif
	...

	...
	#if NPCKBD > 0
		pckbd_hookup_bell(pcppi_pckbd_bell, sc);
	#endif
	...

   with

	...
	config_device_callback(self, "pckbd", 1, NULL);
	...

	...
	config_device_callback(self, "pckbd", 2, NULL);
	...

   and in the pckbd driver code we have

	...
	struct pcppi_softc *sc = device_private(caller);
	...
	switch (cmd) {
	case 1:
		pckbd_unhook_bell(pcppi_pckbd_bell, sc);
		break;
	case 2:
		pckbd_hookup_bell(pcppi_pckbd_bell, sc);
		break;
	...


This mechanism still requires the two drivers to coordinate with each 
other (for defining the "cmd" values, and the contents of "args" for 
each cmd), but it no longer requires compile-time checking to see if 
another driver exists.

Comments?  Alternative methods of unravelling the #if's?




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