Documentation: Add ISA bus driver documentation

This is a verbatim copy of the original commit message of the initial
commit of the ISA bus driver authored by Rene Herman. Descriptions of
the module_isa_driver macro and max_num_isa_dev macro are provided at
the end.
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/isa.txt

+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.

MAINTAINERS

@@ -5995,6 +5995,13 @@ F:	include/linux/irqdomain.h
 F:	kernel/irq/irqdomain.c
 F:	kernel/irq/msi.c
 
+ISA
+M:	William Breathitt Gray <vilhelm.gray@gmail.com>
+S:	Maintained
+F:	Documentation/isa.txt
+F:	drivers/base/isa.c
+F:	include/linux/isa.h
+
 ISAPNP
 M:	Jaroslav Kysela <perex@perex.cz>
 S:	Maintained