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+ISA Drivers
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+-----------
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+
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+The following text is adapted from the commit message of the initial
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+commit of the ISA bus driver authored by Rene Herman.
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+
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+During the recent "isa drivers using platform devices" discussion it was
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+pointed out that (ALSA) ISA drivers ran into the problem of not having
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+the option to fail driver load (device registration rather) upon not
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+finding their hardware due to a probe() error not being passed up
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+through the driver model. In the course of that, I suggested a separate
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+ISA bus might be best; Russell King agreed and suggested this bus could
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+use the .match() method for the actual device discovery.
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+
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+The attached does this. For this old non (generically) discoverable ISA
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+hardware only the driver itself can do discovery so as a difference with
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+the platform_bus, this isa_bus also distributes match() up to the
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+driver.
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+
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+As another difference: these devices only exist in the driver model due
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+to the driver creating them because it might want to drive them, meaning
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+that all device creation has been made internal as well.
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+
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+The usage model this provides is nice, and has been acked from the ALSA
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+side by Takashi Iwai and Jaroslav Kysela. The ALSA driver module_init's
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+now (for oldisa-only drivers) become:
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+
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+static int __init alsa_card_foo_init(void)
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+{
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+ return isa_register_driver(&snd_foo_isa_driver, SNDRV_CARDS);
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+}
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+
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+static void __exit alsa_card_foo_exit(void)
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+{
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+ isa_unregister_driver(&snd_foo_isa_driver);
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+}
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+
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+Quite like the other bus models therefore. This removes a lot of
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+duplicated init code from the ALSA ISA drivers.
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+
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+The passed in isa_driver struct is the regular driver struct embedding a
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+struct device_driver, the normal probe/remove/shutdown/suspend/resume
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+callbacks, and as indicated that .match callback.
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+
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+The "SNDRV_CARDS" you see being passed in is a "unsigned int ndev"
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+parameter, indicating how many devices to create and call our methods
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+with.
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+
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+The platform_driver callbacks are called with a platform_device param;
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+the isa_driver callbacks are being called with a "struct device *dev,
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+unsigned int id" pair directly -- with the device creation completely
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+internal to the bus it's much cleaner to not leak isa_dev's by passing
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+them in at all. The id is the only thing we ever want other then the
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+struct device * anyways, and it makes for nicer code in the callbacks as
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+well.
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+
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+With this additional .match() callback ISA drivers have all options. If
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+ALSA would want to keep the old non-load behaviour, it could stick all
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+of the old .probe in .match, which would only keep them registered after
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+everything was found to be present and accounted for. If it wanted the
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+behaviour of always loading as it inadvertently did for a bit after the
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+changeover to platform devices, it could just not provide a .match() and
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+do everything in .probe() as before.
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+
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+If it, as Takashi Iwai already suggested earlier as a way of following
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+the model from saner buses more closely, wants to load when a later bind
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+could conceivably succeed, it could use .match() for the prerequisites
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+(such as checking the user wants the card enabled and that port/irq/dma
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+values have been passed in) and .probe() for everything else. This is
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+the nicest model.
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+
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+To the code...
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+
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+This exports only two functions; isa_{,un}register_driver().
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+
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+isa_register_driver() register's the struct device_driver, and then
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+loops over the passed in ndev creating devices and registering them.
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+This causes the bus match method to be called for them, which is:
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+
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+int isa_bus_match(struct device *dev, struct device_driver *driver)
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+{
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+ struct isa_driver *isa_driver = to_isa_driver(driver);
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+
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+ if (dev->platform_data == isa_driver) {
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+ if (!isa_driver->match ||
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+ isa_driver->match(dev, to_isa_dev(dev)->id))
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+ return 1;
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+ dev->platform_data = NULL;
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+ }
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+ return 0;
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+}
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+
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+The first thing this does is check if this device is in fact one of this
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+driver's devices by seeing if the device's platform_data pointer is set
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+to this driver. Platform devices compare strings, but we don't need to
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+do that with everything being internal, so isa_register_driver() abuses
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+dev->platform_data as a isa_driver pointer which we can then check here.
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+I believe platform_data is available for this, but if rather not, moving
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+the isa_driver pointer to the private struct isa_dev is ofcourse fine as
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+well.
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+
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+Then, if the the driver did not provide a .match, it matches. If it did,
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+the driver match() method is called to determine a match.
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+
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+If it did _not_ match, dev->platform_data is reset to indicate this to
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+isa_register_driver which can then unregister the device again.
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+
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+If during all this, there's any error, or no devices matched at all
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+everything is backed out again and the error, or -ENODEV, is returned.
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+
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+isa_unregister_driver() just unregisters the matched devices and the
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+driver itself.
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+
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+module_isa_driver is a helper macro for ISA drivers which do not do
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+anything special in module init/exit. This eliminates a lot of
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+boilerplate code. Each module may only use this macro once, and calling
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+it replaces module_init and module_exit.
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+
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+max_num_isa_dev is a macro to determine the maximum possible number of
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+ISA devices which may be registered in the I/O port address space given
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+the address extent of the ISA devices.
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William Breathitt Gray