1================
2EISA bus support
3================
4
5:Author: Marc Zyngier <maz@wild-wind.fr.eu.org>
6
7This document groups random notes about porting EISA drivers to the
8new EISA/sysfs API.
9
10Starting from version 2.5.59, the EISA bus is almost given the same
11status as other much more mainstream busses such as PCI or USB. This
12has been possible through sysfs, which defines a nice enough set of
13abstractions to manage busses, devices and drivers.
14
15Although the new API is quite simple to use, converting existing
16drivers to the new infrastructure is not an easy task (mostly because
17detection code is generally also used to probe ISA cards). Moreover,
18most EISA drivers are among the oldest Linux drivers so, as you can
19imagine, some dust has settled here over the years.
20
21The EISA infrastructure is made up of three parts:
22
23    - The bus code implements most of the generic code. It is shared
24      among all the architectures that the EISA code runs on. It
25      implements bus probing (detecting EISA cards available on the bus),
26      allocates I/O resources, allows fancy naming through sysfs, and
27      offers interfaces for driver to register.
28
29    - The bus root driver implements the glue between the bus hardware
30      and the generic bus code. It is responsible for discovering the
31      device implementing the bus, and setting it up to be latter probed
32      by the bus code. This can go from something as simple as reserving
33      an I/O region on x86, to the rather more complex, like the hppa
34      EISA code. This is the part to implement in order to have EISA
35      running on an "new" platform.
36
37    - The driver offers the bus a list of devices that it manages, and
38      implements the necessary callbacks to probe and release devices
39      whenever told to.
40
41Every function/structure below lives in <linux/eisa.h>, which depends
42heavily on <linux/device.h>.
43
44Bus root driver
45===============
46
47::
48
49	int eisa_root_register (struct eisa_root_device *root);
50
51The eisa_root_register function is used to declare a device as the
52root of an EISA bus. The eisa_root_device structure holds a reference
53to this device, as well as some parameters for probing purposes::
54
55	struct eisa_root_device {
56		struct device   *dev;	 /* Pointer to bridge device */
57		struct resource *res;
58		unsigned long    bus_base_addr;
59		int		 slots;  /* Max slot number */
60		int		 force_probe; /* Probe even when no slot 0 */
61		u64		 dma_mask; /* from bridge device */
62		int              bus_nr; /* Set by eisa_root_register */
63		struct resource  eisa_root_res;	/* ditto */
64	};
65
66============= ======================================================
67node          used for eisa_root_register internal purpose
68dev           pointer to the root device
69res           root device I/O resource
70bus_base_addr slot 0 address on this bus
71slots	      max slot number to probe
72force_probe   Probe even when slot 0 is empty (no EISA mainboard)
73dma_mask      Default DMA mask. Usually the bridge device dma_mask.
74bus_nr	      unique bus id, set by eisa_root_register
75============= ======================================================
76
77Driver
78======
79
80::
81
82	int eisa_driver_register (struct eisa_driver *edrv);
83	void eisa_driver_unregister (struct eisa_driver *edrv);
84
85Clear enough ?
86
87::
88
89	struct eisa_device_id {
90		char sig[EISA_SIG_LEN];
91		unsigned long driver_data;
92	};
93
94	struct eisa_driver {
95		const struct eisa_device_id *id_table;
96		struct device_driver         driver;
97	};
98
99=============== ====================================================
100id_table	an array of NULL terminated EISA id strings,
101		followed by an empty string. Each string can
102		optionally be paired with a driver-dependent value
103		(driver_data).
104
105driver		a generic driver, such as described in
106		Documentation/driver-api/driver-model/driver.rst. Only .name,
107		.probe and .remove members are mandatory.
108=============== ====================================================
109
110An example is the 3c59x driver::
111
112	static struct eisa_device_id vortex_eisa_ids[] = {
113		{ "TCM5920", EISA_3C592_OFFSET },
114		{ "TCM5970", EISA_3C597_OFFSET },
115		{ "" }
116	};
117
118	static struct eisa_driver vortex_eisa_driver = {
119		.id_table = vortex_eisa_ids,
120		.driver   = {
121			.name    = "3c59x",
122			.probe   = vortex_eisa_probe,
123			.remove  = vortex_eisa_remove
124		}
125	};
126
127Device
128======
129
130The sysfs framework calls .probe and .remove functions upon device
131discovery and removal (note that the .remove function is only called
132when driver is built as a module).
133
134Both functions are passed a pointer to a 'struct device', which is
135encapsulated in a 'struct eisa_device' described as follows::
136
137	struct eisa_device {
138		struct eisa_device_id id;
139		int                   slot;
140		int                   state;
141		unsigned long         base_addr;
142		struct resource       res[EISA_MAX_RESOURCES];
143		u64                   dma_mask;
144		struct device         dev; /* generic device */
145	};
146
147======== ============================================================
148id	 EISA id, as read from device. id.driver_data is set from the
149	 matching driver EISA id.
150slot	 slot number which the device was detected on
151state    set of flags indicating the state of the device. Current
152	 flags are EISA_CONFIG_ENABLED and EISA_CONFIG_FORCED.
153res	 set of four 256 bytes I/O regions allocated to this device
154dma_mask DMA mask set from the parent device.
155dev	 generic device (see Documentation/driver-api/driver-model/device.rst)
156======== ============================================================
157
158You can get the 'struct eisa_device' from 'struct device' using the
159'to_eisa_device' macro.
160
161Misc stuff
162==========
163
164::
165
166	void eisa_set_drvdata (struct eisa_device *edev, void *data);
167
168Stores data into the device's driver_data area.
169
170::
171
172	void *eisa_get_drvdata (struct eisa_device *edev):
173
174Gets the pointer previously stored into the device's driver_data area.
175
176::
177
178	int eisa_get_region_index (void *addr);
179
180Returns the region number (0 <= x < EISA_MAX_RESOURCES) of a given
181address.
182
183Kernel parameters
184=================
185
186eisa_bus.enable_dev
187	A comma-separated list of slots to be enabled, even if the firmware
188	set the card as disabled. The driver must be able to properly
189	initialize the device in such conditions.
190
191eisa_bus.disable_dev
192	A comma-separated list of slots to be enabled, even if the firmware
193	set the card as enabled. The driver won't be called to handle this
194	device.
195
196virtual_root.force_probe
197	Force the probing code to probe EISA slots even when it cannot find an
198	EISA compliant mainboard (nothing appears on slot 0). Defaults to 0
199	(don't force), and set to 1 (force probing) when either
200	CONFIG_ALPHA_JENSEN or CONFIG_EISA_VLB_PRIMING are set.
201
202Random notes
203============
204
205Converting an EISA driver to the new API mostly involves *deleting*
206code (since probing is now in the core EISA code). Unfortunately, most
207drivers share their probing routine between ISA, and EISA. Special
208care must be taken when ripping out the EISA code, so other busses
209won't suffer from these surgical strikes...
210
211You *must not* expect any EISA device to be detected when returning
212from eisa_driver_register, since the chances are that the bus has not
213yet been probed. In fact, that's what happens most of the time (the
214bus root driver usually kicks in rather late in the boot process).
215Unfortunately, most drivers are doing the probing by themselves, and
216expect to have explored the whole machine when they exit their probe
217routine.
218
219For example, switching your favorite EISA SCSI card to the "hotplug"
220model is "the right thing"(tm).
221
222Thanks
223======
224
225I'd like to thank the following people for their help:
226
227- Xavier Benigni for lending me a wonderful Alpha Jensen,
228- James Bottomley, Jeff Garzik for getting this stuff into the kernel,
229- Andries Brouwer for contributing numerous EISA ids,
230- Catrin Jones for coping with far too many machines at home.
231