===========
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.