ISA Drivers
  -----------
  
  The following text is adapted from the commit message of the initial
  commit of the ISA bus driver authored by Rene Herman.
  
  During the recent "isa drivers using platform devices" discussion it was
  pointed out that (ALSA) ISA drivers ran into the problem of not having
  the option to fail driver load (device registration rather) upon not
  finding their hardware due to a probe() error not being passed up
  through the driver model. In the course of that, I suggested a separate
  ISA bus might be best; Russell King agreed and suggested this bus could
  use the .match() method for the actual device discovery.
  
  The attached does this. For this old non (generically) discoverable ISA
  hardware only the driver itself can do discovery so as a difference with
  the platform_bus, this isa_bus also distributes match() up to the
  driver.
  
  As another difference: these devices only exist in the driver model due
  to the driver creating them because it might want to drive them, meaning
  that all device creation has been made internal as well.
  
  The usage model this provides is nice, and has been acked from the ALSA
  side by Takashi Iwai and Jaroslav Kysela. The ALSA driver module_init's
  now (for oldisa-only drivers) become:
  
  static int __init alsa_card_foo_init(void)
  {
  	return isa_register_driver(&snd_foo_isa_driver, SNDRV_CARDS);
  }
  
  static void __exit alsa_card_foo_exit(void)
  {
  	isa_unregister_driver(&snd_foo_isa_driver);
  }
  
  Quite like the other bus models therefore. This removes a lot of
  duplicated init code from the ALSA ISA drivers.
  
  The passed in isa_driver struct is the regular driver struct embedding a
  struct device_driver, the normal probe/remove/shutdown/suspend/resume
  callbacks, and as indicated that .match callback.
  
  The "SNDRV_CARDS" you see being passed in is a "unsigned int ndev"
  parameter, indicating how many devices to create and call our methods
  with.
  
  The platform_driver callbacks are called with a platform_device param;
  the isa_driver callbacks are being called with a "struct device *dev,
  unsigned int id" pair directly -- with the device creation completely
  internal to the bus it's much cleaner to not leak isa_dev's by passing
  them in at all. The id is the only thing we ever want other then the
  struct device * anyways, and it makes for nicer code in the callbacks as
  well.
  
  With this additional .match() callback ISA drivers have all options. If
  ALSA would want to keep the old non-load behaviour, it could stick all
  of the old .probe in .match, which would only keep them registered after
  everything was found to be present and accounted for. If it wanted the
  behaviour of always loading as it inadvertently did for a bit after the
  changeover to platform devices, it could just not provide a .match() and
  do everything in .probe() as before.
  
  If it, as Takashi Iwai already suggested earlier as a way of following
  the model from saner buses more closely, wants to load when a later bind
  could conceivably succeed, it could use .match() for the prerequisites
  (such as checking the user wants the card enabled and that port/irq/dma
  values have been passed in) and .probe() for everything else. This is
  the nicest model.
  
  To the code...
  
  This exports only two functions; isa_{,un}register_driver().
  
  isa_register_driver() register's the struct device_driver, and then
  loops over the passed in ndev creating devices and registering them.
  This causes the bus match method to be called for them, which is:
  
  int isa_bus_match(struct device *dev, struct device_driver *driver)
  {
            struct isa_driver *isa_driver = to_isa_driver(driver);
  
            if (dev->platform_data == isa_driver) {
                    if (!isa_driver->match ||
                            isa_driver->match(dev, to_isa_dev(dev)->id))
                            return 1;
                    dev->platform_data = NULL;
            }
            return 0;
  }
  
  The first thing this does is check if this device is in fact one of this
  driver's devices by seeing if the device's platform_data pointer is set
  to this driver. Platform devices compare strings, but we don't need to
  do that with everything being internal, so isa_register_driver() abuses
  dev->platform_data as a isa_driver pointer which we can then check here.
  I believe platform_data is available for this, but if rather not, moving
  the isa_driver pointer to the private struct isa_dev is ofcourse fine as
  well.
  
  Then, if the the driver did not provide a .match, it matches. If it did,
  the driver match() method is called to determine a match.
  
  If it did _not_ match, dev->platform_data is reset to indicate this to
  isa_register_driver which can then unregister the device again.
  
  If during all this, there's any error, or no devices matched at all
  everything is backed out again and the error, or -ENODEV, is returned.
  
  isa_unregister_driver() just unregisters the matched devices and the
  driver itself.
  
  module_isa_driver is a helper macro for ISA drivers which do not do
  anything special in module init/exit. This eliminates a lot of
  boilerplate code. Each module may only use this macro once, and calling
  it replaces module_init and module_exit.
  
  max_num_isa_dev is a macro to determine the maximum possible number of
  ISA devices which may be registered in the I/O port address space given
  the address extent of the ISA devices.