Documentation/isa.txt

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