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1Revision 4, 2004-03-30
2Jean Delvare <khali@linux-fr.org>
3Greg KH <greg@kroah.com>
4
5This is a guide on how to convert I2C chip drivers from Linux 2.4 to
6Linux 2.6. I have been using existing drivers (lm75, lm78) as examples.
7Then I converted a driver myself (lm83) and updated this document.
8
9There are two sets of points below. The first set concerns technical
10changes. The second set concerns coding policy. Both are mandatory.
11
12Although reading this guide will help you porting drivers, I suggest
13you keep an eye on an already ported driver while porting your own
14driver. This will help you a lot understanding what this guide
15exactly means. Choose the chip driver that is the more similar to
16yours for best results.
17
18Technical changes:
19
20* [Includes] Get rid of "version.h". Replace <linux/i2c-proc.h> with
21 <linux/i2c-sensor.h>. Includes typically look like that:
22 #include <linux/module.h>
23 #include <linux/init.h>
24 #include <linux/slab.h>
25 #include <linux/i2c.h>
26 #include <linux/i2c-sensor.h>
27 #include <linux/i2c-vid.h> /* if you need VRM support */
28 #include <asm/io.h> /* if you have I/O operations */
29 Please respect this inclusion order. Some extra headers may be
30 required for a given driver (e.g. "lm75.h").
31
32* [Addresses] SENSORS_I2C_END becomes I2C_CLIENT_END, SENSORS_ISA_END
33 becomes I2C_CLIENT_ISA_END.
34
35* [Client data] Get rid of sysctl_id. Try using standard names for
36 register values (for example, temp_os becomes temp_max). You're
37 still relatively free here, but you *have* to follow the standard
38 names for sysfs files (see the Sysctl section below).
39
40* [Function prototypes] The detect functions loses its flags
41 parameter. Sysctl (e.g. lm75_temp) and miscellaneous functions
42 are off the list of prototypes. This usually leaves five
43 prototypes:
44 static int lm75_attach_adapter(struct i2c_adapter *adapter);
45 static int lm75_detect(struct i2c_adapter *adapter, int address,
46 int kind);
47 static void lm75_init_client(struct i2c_client *client);
48 static int lm75_detach_client(struct i2c_client *client);
49 static void lm75_update_client(struct i2c_client *client);
50
51* [Sysctl] All sysctl stuff is of course gone (defines, ctl_table
52 and functions). Instead, you have to define show and set functions for
53 each sysfs file. Only define set for writable values. Take a look at an
54 existing 2.6 driver for details (lm78 for example). Don't forget
55 to define the attributes for each file (this is that step that
56 links callback functions). Use the file names specified in
57 Documentation/i2c/sysfs-interface for the individual files. Also
58 convert the units these files read and write to the specified ones.
59 If you need to add a new type of file, please discuss it on the
60 sensors mailing list <sensors@stimpy.netroedge.com> by providing a
61 patch to the Documentation/i2c/sysfs-interface file.
62
63* [Attach] For I2C drivers, the attach function should make sure
64 that the adapter's class has I2C_CLASS_HWMON, using the
65 following construct:
66 if (!(adapter->class & I2C_CLASS_HWMON))
67 return 0;
68 ISA-only drivers of course don't need this.
69
70* [Detect] As mentioned earlier, the flags parameter is gone.
71 The type_name and client_name strings are replaced by a single
72 name string, which will be filled with a lowercase, short string
73 (typically the driver name, e.g. "lm75").
74 In i2c-only drivers, drop the i2c_is_isa_adapter check, it's
75 useless.
76 The errorN labels are reduced to the number needed. If that number
77 is 2 (i2c-only drivers), it is advised that the labels are named
78 exit and exit_free. For i2c+isa drivers, labels should be named
79 ERROR0, ERROR1 and ERROR2. Don't forget to properly set err before
80 jumping to error labels. By the way, labels should be left-aligned.
81 Use memset to fill the client and data area with 0x00.
82 Use i2c_set_clientdata to set the client data (as opposed to
83 a direct access to client->data).
84 Use strlcpy instead of strcpy to copy the client name.
85 Replace the sysctl directory registration by calls to
86 device_create_file. Move the driver initialization before any
87 sysfs file creation.
88 Drop client->id.
89
90* [Init] Limits must not be set by the driver (can be done later in
91 user-space). Chip should not be reset default (although a module
92 parameter may be used to force is), and initialization should be
93 limited to the strictly necessary steps.
94
95* [Detach] Get rid of data, remove the call to
96 i2c_deregister_entry.
97
98* [Update] Don't access client->data directly, use
99 i2c_get_clientdata(client) instead.
100
101* [Interface] Init function should not print anything. Make sure
102 there is a MODULE_LICENSE() line, at the bottom of the file
103 (after MODULE_AUTHOR() and MODULE_DESCRIPTION(), in this order).
104
105Coding policy:
106
107* [Copyright] Use (C), not (c), for copyright.
108
109* [Debug/log] Get rid of #ifdef DEBUG/#endif constructs whenever you
110 can. Calls to printk/pr_debug for debugging purposes are replaced
111 by calls to dev_dbg. Here is an example on how to call it (taken
112 from lm75_detect):
113 dev_dbg(&client->dev, "Starting lm75 update\n");
114 Replace other printk calls with the dev_info, dev_err or dev_warn
115 function, as appropriate.
116
117* [Constants] Constants defines (registers, conversions, initial
118 values) should be aligned. This greatly improves readability.
119 Same goes for variables declarations. Alignments are achieved by the
120 means of tabs, not spaces. Remember that tabs are set to 8 in the
121 Linux kernel code.
122
123* [Structure definition] The name field should be standardized. All
124 lowercase and as simple as the driver name itself (e.g. "lm75").
125
126* [Layout] Avoid extra empty lines between comments and what they
127 comment. Respect the coding style (see Documentation/CodingStyle),
128 in particular when it comes to placing curly braces.
129
130* [Comments] Make sure that no comment refers to a file that isn't
131 part of the Linux source tree (typically doc/chips/<chip name>),
132 and that remaining comments still match the code. Merging comment
133 lines when possible is encouraged.