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+================
+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-api/driver-model/driver.rst. 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-api/driver-model/device.rst)
+======== ============================================================
+
+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 disabled, 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.