================
  EISA bus support
  ================
  
  :Author: Marc Zyngier <maz@wild-wind.fr.eu.org>
  
  This document groups random notes about porting EISA drivers to the
  new EISA/sysfs API.
  
  Starting from version 2.5.59, the EISA bus is almost given the same
  status as other much more mainstream busses such as PCI or USB. This
  has been possible through sysfs, which defines a nice enough set of
  abstractions to manage busses, devices and drivers.
  
  Although the new API is quite simple to use, converting existing
  drivers to the new infrastructure is not an easy task (mostly because
  detection code is generally also used to probe ISA cards). Moreover,
  most EISA drivers are among the oldest Linux drivers so, as you can
  imagine, some dust has settled here over the years.
  
  The EISA infrastructure is made up of three parts:
  
      - The bus code implements most of the generic code. It is shared
        among all the architectures that the EISA code runs on. It
        implements bus probing (detecting EISA cards available on the bus),
        allocates I/O resources, allows fancy naming through sysfs, and
        offers interfaces for driver to register.
  
      - The bus root driver implements the glue between the bus hardware
        and the generic bus code. It is responsible for discovering the
        device implementing the bus, and setting it up to be latter probed
        by the bus code. This can go from something as simple as reserving
        an I/O region on x86, to the rather more complex, like the hppa
        EISA code. This is the part to implement in order to have EISA
        running on an "new" platform.
  
      - The driver offers the bus a list of devices that it manages, and
        implements the necessary callbacks to probe and release devices
        whenever told to.
  
  Every function/structure below lives in <linux/eisa.h>, which depends
  heavily on <linux/device.h>.
  
  Bus root driver
  ===============
  
  ::
  
  	int eisa_root_register (struct eisa_root_device *root);
  
  The eisa_root_register function is used to declare a device as the
  root of an EISA bus. The eisa_root_device structure holds a reference
  to this device, as well as some parameters for probing purposes::
  
  	struct eisa_root_device {
  		struct device   *dev;	 /* Pointer to bridge device */
  		struct resource *res;
  		unsigned long    bus_base_addr;
  		int		 slots;  /* Max slot number */
  		int		 force_probe; /* Probe even when no slot 0 */
  		u64		 dma_mask; /* from bridge device */
  		int              bus_nr; /* Set by eisa_root_register */
  		struct resource  eisa_root_res;	/* ditto */
  	};
  
  ============= ======================================================
  node          used for eisa_root_register internal purpose
  dev           pointer to the root device
  res           root device I/O resource
  bus_base_addr slot 0 address on this bus
  slots	      max slot number to probe
  force_probe   Probe even when slot 0 is empty (no EISA mainboard)
  dma_mask      Default DMA mask. Usually the bridge device dma_mask.
  bus_nr	      unique bus id, set by eisa_root_register
  ============= ======================================================
  
  Driver
  ======
  
  ::
  
  	int eisa_driver_register (struct eisa_driver *edrv);
  	void eisa_driver_unregister (struct eisa_driver *edrv);
  
  Clear enough ?
  
  ::
  
  	struct eisa_device_id {
  		char sig[EISA_SIG_LEN];
  		unsigned long driver_data;
  	};
  
  	struct eisa_driver {
  		const struct eisa_device_id *id_table;
  		struct device_driver         driver;
  	};
  
  =============== ====================================================
  id_table	an array of NULL terminated EISA id strings,
  		followed by an empty string. Each string can
  		optionally be paired with a driver-dependent value
  		(driver_data).
  
  driver		a generic driver, such as described in
  		Documentation/driver-model/driver.txt. Only .name,
  		.probe and .remove members are mandatory.
  =============== ====================================================
  
  An example is the 3c59x driver::
  
  	static struct eisa_device_id vortex_eisa_ids[] = {
  		{ "TCM5920", EISA_3C592_OFFSET },
  		{ "TCM5970", EISA_3C597_OFFSET },
  		{ "" }
  	};
  
  	static struct eisa_driver vortex_eisa_driver = {
  		.id_table = vortex_eisa_ids,
  		.driver   = {
  			.name    = "3c59x",
  			.probe   = vortex_eisa_probe,
  			.remove  = vortex_eisa_remove
  		}
  	};
  
  Device
  ======
  
  The sysfs framework calls .probe and .remove functions upon device
  discovery and removal (note that the .remove function is only called
  when driver is built as a module).
  
  Both functions are passed a pointer to a 'struct device', which is
  encapsulated in a 'struct eisa_device' described as follows::
  
  	struct eisa_device {
  		struct eisa_device_id id;
  		int                   slot;
  		int                   state;
  		unsigned long         base_addr;
  		struct resource       res[EISA_MAX_RESOURCES];
  		u64                   dma_mask;
  		struct device         dev; /* generic device */
  	};
  
  ======== ============================================================
  id	 EISA id, as read from device. id.driver_data is set from the
  	 matching driver EISA id.
  slot	 slot number which the device was detected on
  state    set of flags indicating the state of the device. Current
  	 flags are EISA_CONFIG_ENABLED and EISA_CONFIG_FORCED.
  res	 set of four 256 bytes I/O regions allocated to this device
  dma_mask DMA mask set from the parent device.
  dev	 generic device (see Documentation/driver-model/device.txt)
  ======== ============================================================
  
  You can get the 'struct eisa_device' from 'struct device' using the
  'to_eisa_device' macro.
  
  Misc stuff
  ==========
  
  ::
  
  	void eisa_set_drvdata (struct eisa_device *edev, void *data);
  
  Stores data into the device's driver_data area.
  
  ::
  
  	void *eisa_get_drvdata (struct eisa_device *edev):
  
  Gets the pointer previously stored into the device's driver_data area.
  
  ::
  
  	int eisa_get_region_index (void *addr);
  
  Returns the region number (0 <= x < EISA_MAX_RESOURCES) of a given
  address.
  
  Kernel parameters
  =================
  
  eisa_bus.enable_dev
  	A comma-separated list of slots to be enabled, even if the firmware
  	set the card as disabled. The driver must be able to properly
  	initialize the device in such conditions.
  
  eisa_bus.disable_dev
  	A comma-separated list of slots to be enabled, even if the firmware
  	set the card as enabled. The driver won't be called to handle this
  	device.
  
  virtual_root.force_probe
  	Force the probing code to probe EISA slots even when it cannot find an
  	EISA compliant mainboard (nothing appears on slot 0). Defaults to 0
  	(don't force), and set to 1 (force probing) when either
  	CONFIG_ALPHA_JENSEN or CONFIG_EISA_VLB_PRIMING are set.
  
  Random notes
  ============
  
  Converting an EISA driver to the new API mostly involves *deleting*
  code (since probing is now in the core EISA code). Unfortunately, most
  drivers share their probing routine between ISA, and EISA. Special
  care must be taken when ripping out the EISA code, so other busses
  won't suffer from these surgical strikes...
  
  You *must not* expect any EISA device to be detected when returning
  from eisa_driver_register, since the chances are that the bus has not
  yet been probed. In fact, that's what happens most of the time (the
  bus root driver usually kicks in rather late in the boot process).
  Unfortunately, most drivers are doing the probing by themselves, and
  expect to have explored the whole machine when they exit their probe
  routine.
  
  For example, switching your favorite EISA SCSI card to the "hotplug"
  model is "the right thing"(tm).
  
  Thanks
  ======
  
  I'd like to thank the following people for their help:
  
  - Xavier Benigni for lending me a wonderful Alpha Jensen,
  - James Bottomley, Jeff Garzik for getting this stuff into the kernel,
  - Andries Brouwer for contributing numerous EISA ids,
  - Catrin Jones for coping with far too many machines at home.