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123 lines
5.1 KiB
ReStructuredText
123 lines
5.1 KiB
ReStructuredText
===========
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ISA Drivers
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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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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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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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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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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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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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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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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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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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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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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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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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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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To the code...
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This exports only two functions; isa_{,un}register_driver().
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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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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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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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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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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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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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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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isa_unregister_driver() just unregisters the matched devices and the
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driver itself.
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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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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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