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/Documentation/i2c/porting-clients

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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.