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