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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	===========
2	ISA Drivers
3	===========
4	
5	The following text is adapted from the commit message of the initial
6	commit of the ISA bus driver authored by Rene Herman.
7	
8	During the recent "isa drivers using platform devices" discussion it was
9	pointed out that (ALSA) ISA drivers ran into the problem of not having
10	the option to fail driver load (device registration rather) upon not
11	finding their hardware due to a probe() error not being passed up
12	through the driver model. In the course of that, I suggested a separate
13	ISA bus might be best; Russell King agreed and suggested this bus could
14	use the .match() method for the actual device discovery.
15	
16	The attached does this. For this old non (generically) discoverable ISA
17	hardware only the driver itself can do discovery so as a difference with
18	the platform_bus, this isa_bus also distributes match() up to the
19	driver.
20	
21	As another difference: these devices only exist in the driver model due
22	to the driver creating them because it might want to drive them, meaning
23	that all device creation has been made internal as well.
24	
25	The usage model this provides is nice, and has been acked from the ALSA
26	side by Takashi Iwai and Jaroslav Kysela. The ALSA driver module_init's
27	now (for oldisa-only drivers) become::
28	
29		static int __init alsa_card_foo_init(void)
30		{
31			return isa_register_driver(&snd_foo_isa_driver, SNDRV_CARDS);
32		}
33	
34		static void __exit alsa_card_foo_exit(void)
35		{
36			isa_unregister_driver(&snd_foo_isa_driver);
37		}
38	
39	Quite like the other bus models therefore. This removes a lot of
40	duplicated init code from the ALSA ISA drivers.
41	
42	The passed in isa_driver struct is the regular driver struct embedding a
43	struct device_driver, the normal probe/remove/shutdown/suspend/resume
44	callbacks, and as indicated that .match callback.
45	
46	The "SNDRV_CARDS" you see being passed in is a "unsigned int ndev"
47	parameter, indicating how many devices to create and call our methods
48	with.
49	
50	The platform_driver callbacks are called with a platform_device param;
51	the isa_driver callbacks are being called with a ``struct device *dev,
52	unsigned int id`` pair directly -- with the device creation completely
53	internal to the bus it's much cleaner to not leak isa_dev's by passing
54	them in at all. The id is the only thing we ever want other then the
55	struct device anyways, and it makes for nicer code in the callbacks as
56	well.
57	
58	With this additional .match() callback ISA drivers have all options. If
59	ALSA would want to keep the old non-load behaviour, it could stick all
60	of the old .probe in .match, which would only keep them registered after
61	everything was found to be present and accounted for. If it wanted the
62	behaviour of always loading as it inadvertently did for a bit after the
63	changeover to platform devices, it could just not provide a .match() and
64	do everything in .probe() as before.
65	
66	If it, as Takashi Iwai already suggested earlier as a way of following
67	the model from saner buses more closely, wants to load when a later bind
68	could conceivably succeed, it could use .match() for the prerequisites
69	(such as checking the user wants the card enabled and that port/irq/dma
70	values have been passed in) and .probe() for everything else. This is
71	the nicest model.
72	
73	To the code...
74	
75	This exports only two functions; isa_{,un}register_driver().
76	
77	isa_register_driver() register's the struct device_driver, and then
78	loops over the passed in ndev creating devices and registering them.
79	This causes the bus match method to be called for them, which is::
80	
81		int isa_bus_match(struct device *dev, struct device_driver *driver)
82		{
83			struct isa_driver *isa_driver = to_isa_driver(driver);
84	
85			if (dev->platform_data == isa_driver) {
86				if (!isa_driver->match ||
87					isa_driver->match(dev, to_isa_dev(dev)->id))
88					return 1;
89				dev->platform_data = NULL;
90			}
91			return 0;
92		}
93	
94	The first thing this does is check if this device is in fact one of this
95	driver's devices by seeing if the device's platform_data pointer is set
96	to this driver. Platform devices compare strings, but we don't need to
97	do that with everything being internal, so isa_register_driver() abuses
98	dev->platform_data as a isa_driver pointer which we can then check here.
99	I believe platform_data is available for this, but if rather not, moving
100	the isa_driver pointer to the private struct isa_dev is ofcourse fine as
101	well.
102	
103	Then, if the the driver did not provide a .match, it matches. If it did,
104	the driver match() method is called to determine a match.
105	
106	If it did **not** match, dev->platform_data is reset to indicate this to
107	isa_register_driver which can then unregister the device again.
108	
109	If during all this, there's any error, or no devices matched at all
110	everything is backed out again and the error, or -ENODEV, is returned.
111	
112	isa_unregister_driver() just unregisters the matched devices and the
113	driver itself.
114	
115	module_isa_driver is a helper macro for ISA drivers which do not do
116	anything special in module init/exit. This eliminates a lot of
117	boilerplate code. Each module may only use this macro once, and calling
118	it replaces module_init and module_exit.
119	
120	max_num_isa_dev is a macro to determine the maximum possible number of
121	ISA devices which may be registered in the I/O port address space given
122	the address extent of the ISA devices.
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