Documentation/i2c/porting-clients

   1 Revision 4, 2004-03-30
   2 Jean Delvare <khali@linux-fr.org>
   3 Greg KH <greg@kroah.com>
   4
   5 This is a guide on how to convert I2C chip drivers from Linux 2.4 to
   6 Linux 2.6. I have been using existing drivers (lm75, lm78) as examples.
   7 Then I converted a driver myself (lm83) and updated this document.
   8
   9 There are two sets of points below. The first set concerns technical
  10 changes. The second set concerns coding policy. Both are mandatory.
  11
  12 Although reading this guide will help you porting drivers, I suggest
  13 you keep an eye on an already ported driver while porting your own
  14 driver. This will help you a lot understanding what this guide
  15 exactly means. Choose the chip driver that is the more similar to
  16 yours for best results.
  17
  18 Technical 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
 105 Coding 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.