Documentation/i2c/porting-clients

   1 Revision 7, 2007-04-19
   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 Note that this guide is strongly oriented towards hardware monitoring
   9 drivers. Many points are still valid for other type of drivers, but
  10 others may be irrelevant.
  11
  12 There are two sets of points below. The first set concerns technical
  13 changes. The second set concerns coding policy. Both are mandatory.
  14
  15 Although reading this guide will help you porting drivers, I suggest
  16 you keep an eye on an already ported driver while porting your own
  17 driver. This will help you a lot understanding what this guide
  18 exactly means. Choose the chip driver that is the more similar to
  19 yours for best results.
  20
  21 Technical changes:
  22
  23 * [Driver type] Any driver that was relying on i2c-isa has to be
  24   converted to a proper isa, platform or pci driver. This is not
  25   covered by this guide.
  26
  27 * [Includes] Get rid of "version.h" and <linux/i2c-proc.h>.
  28   Includes typically look like that:
  29   #include <linux/module.h>
  30   #include <linux/init.h>
  31   #include <linux/slab.h>
  32   #include <linux/jiffies.h>
  33   #include <linux/i2c.h>
  34   #include <linux/hwmon.h>      /* for hardware monitoring drivers */
  35   #include <linux/hwmon-sysfs.h>
  36   #include <linux/hwmon-vid.h>  /* if you need VRM support */
  37   #include <linux/err.h>        /* for class registration */
  38   Please respect this inclusion order. Some extra headers may be
  39   required for a given driver (e.g. "lm75.h").
  40
  41 * [Addresses] SENSORS_I2C_END becomes I2C_CLIENT_END, ISA addresses
  42   are no more handled by the i2c core. Address ranges are no more
  43   supported either, define each individual address separately.
  44   SENSORS_INSMOD_<n> becomes I2C_CLIENT_INSMOD_<n>.
  45
  46 * [Client data] Get rid of sysctl_id. Try using standard names for
  47   register values (for example, temp_os becomes temp_max). You're
  48   still relatively free here, but you *have* to follow the standard
  49   names for sysfs files (see the Sysctl section below).
  50
  51 * [Function prototypes] The detect functions loses its flags
  52   parameter. Sysctl (e.g. lm75_temp) and miscellaneous functions
  53   are off the list of prototypes. This usually leaves five
  54   prototypes:
  55   static int lm75_attach_adapter(struct i2c_adapter *adapter);
  56   static int lm75_detect(struct i2c_adapter *adapter, int address,
  57       int kind);
  58   static void lm75_init_client(struct i2c_client *client);
  59   static int lm75_detach_client(struct i2c_client *client);
  60   static struct lm75_data lm75_update_device(struct device *dev);
  61
  62 * [Sysctl] All sysctl stuff is of course gone (defines, ctl_table
  63   and functions). Instead, you have to define show and set functions for
  64   each sysfs file. Only define set for writable values. Take a look at an
  65   existing 2.6 driver for details (it87 for example). Don't forget
  66   to define the attributes for each file (this is that step that
  67   links callback functions). Use the file names specified in
  68   Documentation/hwmon/sysfs-interface for the individual files. Also
  69   convert the units these files read and write to the specified ones.
  70   If you need to add a new type of file, please discuss it on the
  71   sensors mailing list <lm-sensors@lm-sensors.org> by providing a
  72   patch to the Documentation/hwmon/sysfs-interface file.
  73
  74 * [Attach] The attach function should make sure that the adapter's
  75   class has I2C_CLASS_HWMON (or whatever class is suitable for your
  76   driver), using the following construct:
  77   if (!(adapter->class & I2C_CLASS_HWMON))
  78           return 0;
  79   Call i2c_probe() instead of i2c_detect().
  80
  81 * [Detect] As mentioned earlier, the flags parameter is gone.
  82   The type_name and client_name strings are replaced by a single
  83   name string, which will be filled with a lowercase, short string.
  84   The labels used for error paths are reduced to the number needed.
  85   It is advised that the labels are given descriptive names such as
  86   exit and exit_free. Don't forget to properly set err before
  87   jumping to error labels. By the way, labels should be left-aligned.
  88   Use kzalloc instead of kmalloc.
  89   Use i2c_set_clientdata to set the client data (as opposed to
  90   a direct access to client->data).
  91   Use strlcpy instead of strcpy or snprintf to copy the client name.
  92   Replace the sysctl directory registration by calls to
  93   device_create_file. Move the driver initialization before any
  94   sysfs file creation.
  95   Register the client with the hwmon class (using hwmon_device_register)
  96   if applicable.
  97   Drop client->id.
  98   Drop any 24RF08 corruption prevention you find, as this is now done
  99   at the i2c-core level, and doing it twice voids it.
 100   Don't add I2C_CLIENT_ALLOW_USE to client->flags, it's the default now.
 101
 102 * [Init] Limits must not be set by the driver (can be done later in
 103   user-space). Chip should not be reset default (although a module
 104   parameter may be used to force it), and initialization should be
 105   limited to the strictly necessary steps.
 106
 107 * [Detach] Remove the call to i2c_deregister_entry. Do not log an
 108   error message if i2c_detach_client fails, as i2c-core will now do
 109   it for you.
 110   Unregister from the hwmon class if applicable.
 111
 112 * [Update] The function prototype changed, it is now
 113   passed a device structure, which you have to convert to a client
 114   using to_i2c_client(dev). The update function should return a
 115   pointer to the client data.
 116   Don't access client->data directly, use i2c_get_clientdata(client)
 117   instead.
 118   Use time_after() instead of direct jiffies comparison.
 119
 120 * [Interface] Make sure there is a MODULE_LICENSE() line, at the bottom
 121   of the file (after MODULE_AUTHOR() and MODULE_DESCRIPTION(), in this
 122   order).
 123
 124 * [Driver] The flags field of the i2c_driver structure is gone.
 125   I2C_DF_NOTIFY is now the default behavior.
 126   The i2c_driver structure has a driver member, which is itself a
 127   structure, those name member should be initialized to a driver name
 128   string. i2c_driver itself has no name member anymore.
 129
 130 * [Driver model] Instead of shutdown or reboot notifiers, provide a
 131   shutdown() method in your driver.
 132
 133 * [Power management] Use the driver model suspend() and resume()
 134   callbacks instead of the obsolete pm_register() calls.
 135
 136 Coding policy:
 137
 138 * [Copyright] Use (C), not (c), for copyright.
 139
 140 * [Debug/log] Get rid of #ifdef DEBUG/#endif constructs whenever you
 141   can. Calls to printk for debugging purposes are replaced by calls to
 142   dev_dbg where possible, else to pr_debug. Here is an example of how
 143   to call it (taken from lm75_detect):
 144   dev_dbg(&client->dev, "Starting lm75 update\n");
 145   Replace other printk calls with the dev_info, dev_err or dev_warn
 146   function, as appropriate.
 147
 148 * [Constants] Constants defines (registers, conversions) should be
 149   aligned. This greatly improves readability.
 150   Alignments are achieved by the means of tabs, not spaces. Remember
 151   that tabs are set to 8 in the Linux kernel code.
 152
 153 * [Layout] Avoid extra empty lines between comments and what they
 154   comment. Respect the coding style (see Documentation/CodingStyle),
 155   in particular when it comes to placing curly braces.
 156
 157 * [Comments] Make sure that no comment refers to a file that isn't
 158   part of the Linux source tree (typically doc/chips/<chip name>),
 159   and that remaining comments still match the code. Merging comment
 160   lines when possible is encouraged.