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1.. SPDX-License-Identifier: GPL-2.0
2
3================
4EISA bus support
5================
6
7:Author: Marc Zyngier <[email protected]>
8
9This document groups random notes about porting EISA drivers to the
10new EISA/sysfs API.
11
12Starting from version 2.5.59, the EISA bus is almost given the same
13status as other much more mainstream buses such as PCI or USB. This
14has been possible through sysfs, which defines a nice enough set of
15abstractions to manage buses, devices and drivers.
16
17Although the new API is quite simple to use, converting existing
18drivers to the new infrastructure is not an easy task (mostly because
19detection code is generally also used to probe ISA cards). Moreover,
20most EISA drivers are among the oldest Linux drivers so, as you can
21imagine, some dust has settled here over the years.
22
23The EISA infrastructure is made up of three parts:
24
25 - The bus code implements most of the generic code. It is shared
26 among all the architectures that the EISA code runs on. It
27 implements bus probing (detecting EISA cards available on the bus),
28 allocates I/O resources, allows fancy naming through sysfs, and
29 offers interfaces for driver to register.
30
31 - The bus root driver implements the glue between the bus hardware
32 and the generic bus code. It is responsible for discovering the
33 device implementing the bus, and setting it up to be latter probed
34 by the bus code. This can go from something as simple as reserving
35 an I/O region on x86, to the rather more complex, like the hppa
36 EISA code. This is the part to implement in order to have EISA
37 running on an "new" platform.
38
39 - The driver offers the bus a list of devices that it manages, and
40 implements the necessary callbacks to probe and release devices
41 whenever told to.
42
43Every function/structure below lives in <linux/eisa.h>, which depends
44heavily on <linux/device.h>.
45
46Bus root driver
47===============
48
49::
50
51 int eisa_root_register (struct eisa_root_device *root);
52
53The eisa_root_register function is used to declare a device as the
54root of an EISA bus. The eisa_root_device structure holds a reference
55to this device, as well as some parameters for probing purposes::
56
57 struct eisa_root_device {
58 struct device *dev; /* Pointer to bridge device */
59 struct resource *res;
60 unsigned long bus_base_addr;
61 int slots; /* Max slot number */
62 int force_probe; /* Probe even when no slot 0 */
63 u64 dma_mask; /* from bridge device */
64 int bus_nr; /* Set by eisa_root_register */
65 struct resource eisa_root_res; /* ditto */
66 };
67
68============= ======================================================
69node used for eisa_root_register internal purpose
70dev pointer to the root device
71res root device I/O resource
72bus_base_addr slot 0 address on this bus
73slots max slot number to probe
74force_probe Probe even when slot 0 is empty (no EISA mainboard)
75dma_mask Default DMA mask. Usually the bridge device dma_mask.
76bus_nr unique bus id, set by eisa_root_register
77============= ======================================================
78
79Driver
80======
81
82::
83
84 int eisa_driver_register (struct eisa_driver *edrv);
85 void eisa_driver_unregister (struct eisa_driver *edrv);
86
87Clear enough ?
88
89::
90
91 struct eisa_device_id {
92 char sig[EISA_SIG_LEN];
93 unsigned long driver_data;
94 };
95
96 struct eisa_driver {
97 const struct eisa_device_id *id_table;
98 struct device_driver driver;
99 };
100
101=============== ====================================================
102id_table an array of NULL terminated EISA id strings,
103 followed by an empty string. Each string can
104 optionally be paired with a driver-dependent value
105 (driver_data).
106
107driver a generic driver, such as described in
108 Documentation/driver-api/driver-model/driver.rst. Only .name,
109 .probe and .remove members are mandatory.
110=============== ====================================================
111
112An example is the 3c59x driver::
113
114 static struct eisa_device_id vortex_eisa_ids[] = {
115 { "TCM5920", EISA_3C592_OFFSET },
116 { "TCM5970", EISA_3C597_OFFSET },
117 { "" }
118 };
119
120 static struct eisa_driver vortex_eisa_driver = {
121 .id_table = vortex_eisa_ids,
122 .driver = {
123 .name = "3c59x",
124 .probe = vortex_eisa_probe,
125 .remove = vortex_eisa_remove
126 }
127 };
128
129Device
130======
131
132The sysfs framework calls .probe and .remove functions upon device
133discovery and removal (note that the .remove function is only called
134when driver is built as a module).
135
136Both functions are passed a pointer to a 'struct device', which is
137encapsulated in a 'struct eisa_device' described as follows::
138
139 struct eisa_device {
140 struct eisa_device_id id;
141 int slot;
142 int state;
143 unsigned long base_addr;
144 struct resource res[EISA_MAX_RESOURCES];
145 u64 dma_mask;
146 struct device dev; /* generic device */
147 };
148
149======== ============================================================
150id EISA id, as read from device. id.driver_data is set from the
151 matching driver EISA id.
152slot slot number which the device was detected on
153state set of flags indicating the state of the device. Current
154 flags are EISA_CONFIG_ENABLED and EISA_CONFIG_FORCED.
155res set of four 256 bytes I/O regions allocated to this device
156dma_mask DMA mask set from the parent device.
157dev generic device (see Documentation/driver-api/driver-model/device.rst)
158======== ============================================================
159
160You can get the 'struct eisa_device' from 'struct device' using the
161'to_eisa_device' macro.
162
163Misc stuff
164==========
165
166::
167
168 void eisa_set_drvdata (struct eisa_device *edev, void *data);
169
170Stores data into the device's driver_data area.
171
172::
173
174 void *eisa_get_drvdata (struct eisa_device *edev):
175
176Gets the pointer previously stored into the device's driver_data area.
177
178::
179
180 int eisa_get_region_index (void *addr);
181
182Returns the region number (0 <= x < EISA_MAX_RESOURCES) of a given
183address.
184
185Kernel parameters
186=================
187
188eisa_bus.enable_dev
189 A comma-separated list of slots to be enabled, even if the firmware
190 set the card as disabled. The driver must be able to properly
191 initialize the device in such conditions.
192
193eisa_bus.disable_dev
194 A comma-separated list of slots to be disabled, even if the firmware
195 set the card as enabled. The driver won't be called to handle this
196 device.
197
198virtual_root.force_probe
199 Force the probing code to probe EISA slots even when it cannot find an
200 EISA compliant mainboard (nothing appears on slot 0). Defaults to 0
201 (don't force), and set to 1 (force probing) when
202 CONFIG_EISA_VLB_PRIMING is set.
203
204Random notes
205============
206
207Converting an EISA driver to the new API mostly involves *deleting*
208code (since probing is now in the core EISA code). Unfortunately, most
209drivers share their probing routine between ISA, and EISA. Special
210care must be taken when ripping out the EISA code, so other buses
211won't suffer from these surgical strikes...
212
213You *must not* expect any EISA device to be detected when returning
214from eisa_driver_register, since the chances are that the bus has not
215yet been probed. In fact, that's what happens most of the time (the
216bus root driver usually kicks in rather late in the boot process).
217Unfortunately, most drivers are doing the probing by themselves, and
218expect to have explored the whole machine when they exit their probe
219routine.
220
221For example, switching your favorite EISA SCSI card to the "hotplug"
222model is "the right thing"(tm).
223
224Thanks
225======
226
227I'd like to thank the following people for their help:
228
229- Xavier Benigni for lending me a wonderful Alpha Jensen,
230- James Bottomley, Jeff Garzik for getting this stuff into the kernel,
231- Andries Brouwer for contributing numerous EISA ids,
232- Catrin Jones for coping with far too many machines at home.