Documentation/i2c/porting-clients

   1 Revision 5, 2005-07-29
   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" and <linux/i2c-proc.h>.
  21   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/hwmon.h>      /* for hardware monitoring drivers */
  27   #include <linux/hwmon-sysfs.h>
  28   #include <linux/hwmon-vid.h>  /* if you need VRM support */
  29   #include <asm/io.h>           /* if you have I/O operations */
  30   Please respect this inclusion order. Some extra headers may be
  31   required for a given driver (e.g. "lm75.h").
  32
  33 * [Addresses] SENSORS_I2C_END becomes I2C_CLIENT_END, ISA addresses
  34   are no more handled by the i2c core.
  35   SENSORS_INSMOD_<n> becomes I2C_CLIENT_INSMOD_<n>.
  36
  37 * [Client data] Get rid of sysctl_id. Try using standard names for
  38   register values (for example, temp_os becomes temp_max). You're
  39   still relatively free here, but you *have* to follow the standard
  40   names for sysfs files (see the Sysctl section below).
  41
  42 * [Function prototypes] The detect functions loses its flags
  43   parameter. Sysctl (e.g. lm75_temp) and miscellaneous functions
  44   are off the list of prototypes. This usually leaves five
  45   prototypes:
  46   static int lm75_attach_adapter(struct i2c_adapter *adapter);
  47   static int lm75_detect(struct i2c_adapter *adapter, int address,
  48       int kind);
  49   static void lm75_init_client(struct i2c_client *client);
  50   static int lm75_detach_client(struct i2c_client *client);
  51   static void lm75_update_client(struct i2c_client *client);
  52
  53 * [Sysctl] All sysctl stuff is of course gone (defines, ctl_table
  54   and functions). Instead, you have to define show and set functions for
  55   each sysfs file. Only define set for writable values. Take a look at an
  56   existing 2.6 driver for details (lm78 for example). Don't forget
  57   to define the attributes for each file (this is that step that
  58   links callback functions). Use the file names specified in
  59   Documentation/i2c/sysfs-interface for the individual files. Also
  60   convert the units these files read and write to the specified ones.
  61   If you need to add a new type of file, please discuss it on the
  62   sensors mailing list <lm-sensors@lm-sensors.org> by providing a
  63   patch to the Documentation/i2c/sysfs-interface file.
  64
  65 * [Attach] For I2C drivers, the attach function should make sure
  66   that the adapter's class has I2C_CLASS_HWMON, using the
  67   following construct:
  68   if (!(adapter->class & I2C_CLASS_HWMON))
  69           return 0;
  70   ISA-only drivers of course don't need this.
  71   Call i2c_probe() instead of i2c_detect().
  72
  73 * [Detect] As mentioned earlier, the flags parameter is gone.
  74   The type_name and client_name strings are replaced by a single
  75   name string, which will be filled with a lowercase, short string
  76   (typically the driver name, e.g. "lm75").
  77   In i2c-only drivers, drop the i2c_is_isa_adapter check, it's
  78   useless. Same for isa-only drivers, as the test would always be
  79   true. Only hybrid drivers (which are quite rare) still need it.
  80   The errorN labels are reduced to the number needed. If that number
  81   is 2 (i2c-only drivers), it is advised that the labels are named
  82   exit and exit_free. For i2c+isa drivers, labels should be named
  83   ERROR0, ERROR1 and ERROR2. Don't forget to properly set err before
  84   jumping to error labels. By the way, labels should be left-aligned.
  85   Use kzalloc instead of kmalloc.
  86   Use i2c_set_clientdata to set the client data (as opposed to
  87   a direct access to client->data).
  88   Use strlcpy instead of strcpy to copy the client name.
  89   Replace the sysctl directory registration by calls to
  90   device_create_file. Move the driver initialization before any
  91   sysfs file creation.
  92   Drop client->id.
  93   Drop any 24RF08 corruption prevention you find, as this is now done
  94   at the i2c-core level, and doing it twice voids it.
  95   Don't add I2C_CLIENT_ALLOW_USE to client->flags, it's the default now.
  96
  97 * [Init] Limits must not be set by the driver (can be done later in
  98   user-space). Chip should not be reset default (although a module
  99   parameter may be used to force is), and initialization should be
 100   limited to the strictly necessary steps.
 101
 102 * [Detach] Get rid of data, remove the call to
 103   i2c_deregister_entry. Do not log an error message if
 104   i2c_detach_client fails, as i2c-core will now do it for you.
 105
 106 * [Update] Don't access client->data directly, use
 107   i2c_get_clientdata(client) instead.
 108
 109 * [Interface] Init function should not print anything. Make sure
 110   there is a MODULE_LICENSE() line, at the bottom of the file
 111   (after MODULE_AUTHOR() and MODULE_DESCRIPTION(), in this order).
 112
 113 * [Driver] The flags field of the i2c_driver structure is gone.
 114   I2C_DF_NOTIFY is now the default behavior.
 115
 116 Coding policy:
 117
 118 * [Copyright] Use (C), not (c), for copyright.
 119
 120 * [Debug/log] Get rid of #ifdef DEBUG/#endif constructs whenever you
 121   can. Calls to printk/pr_debug for debugging purposes are replaced
 122   by calls to dev_dbg. Here is an example on how to call it (taken
 123   from lm75_detect):
 124   dev_dbg(&client->dev, "Starting lm75 update\n");
 125   Replace other printk calls with the dev_info, dev_err or dev_warn
 126   function, as appropriate.
 127
 128 * [Constants] Constants defines (registers, conversions, initial
 129   values) should be aligned. This greatly improves readability.
 130   Same goes for variables declarations. Alignments are achieved by the
 131   means of tabs, not spaces. Remember that tabs are set to 8 in the
 132   Linux kernel code.
 133
 134 * [Structure definition] The name field should be standardized. All
 135   lowercase and as simple as the driver name itself (e.g. "lm75").
 136
 137 * [Layout] Avoid extra empty lines between comments and what they
 138   comment. Respect the coding style (see Documentation/CodingStyle),
 139   in particular when it comes to placing curly braces.
 140
 141 * [Comments] Make sure that no comment refers to a file that isn't
 142   part of the Linux source tree (typically doc/chips/<chip name>),
 143   and that remaining comments still match the code. Merging comment
 144   lines when possible is encouraged.