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Revision 6, 2005-11-20
Jean Delvare <>
Greg KH <>
This is a guide on how to convert I2C chip drivers from Linux 2.4 to
Linux 2.6. I have been using existing drivers (lm75, lm78) as examples.
Then I converted a driver myself (lm83) and updated this document.
Note that this guide is strongly oriented towards hardware monitoring
drivers. Many points are still valid for other type of drivers, but
others may be irrelevant.
There are two sets of points below. The first set concerns technical
changes. The second set concerns coding policy. Both are mandatory.
Although reading this guide will help you porting drivers, I suggest
you keep an eye on an already ported driver while porting your own
driver. This will help you a lot understanding what this guide
exactly means. Choose the chip driver that is the more similar to
yours for best results.
Technical changes:
* [Includes] Get rid of "version.h" and <linux/i2c-proc.h>.
Includes typically look like that:
#include <linux/module.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/jiffies.h>
#include <linux/i2c.h>
#include <linux/i2c-isa.h> /* for ISA drivers */
#include <linux/hwmon.h> /* for hardware monitoring drivers */
#include <linux/hwmon-sysfs.h>
#include <linux/hwmon-vid.h> /* if you need VRM support */
#include <linux/err.h> /* for class registration */
#include <asm/io.h> /* if you have I/O operations */
Please respect this inclusion order. Some extra headers may be
required for a given driver (e.g. "lm75.h").
* [Addresses] SENSORS_I2C_END becomes I2C_CLIENT_END, ISA addresses
are no more handled by the i2c core. Address ranges are no more
supported either, define each individual address separately.
* [Client data] Get rid of sysctl_id. Try using standard names for
register values (for example, temp_os becomes temp_max). You're
still relatively free here, but you *have* to follow the standard
names for sysfs files (see the Sysctl section below).
* [Function prototypes] The detect functions loses its flags
parameter. Sysctl (e.g. lm75_temp) and miscellaneous functions
are off the list of prototypes. This usually leaves five
static int lm75_attach_adapter(struct i2c_adapter *adapter);
static int lm75_detect(struct i2c_adapter *adapter, int address,
int kind);
static void lm75_init_client(struct i2c_client *client);
static int lm75_detach_client(struct i2c_client *client);
static struct lm75_data lm75_update_device(struct device *dev);
* [Sysctl] All sysctl stuff is of course gone (defines, ctl_table
and functions). Instead, you have to define show and set functions for
each sysfs file. Only define set for writable values. Take a look at an
existing 2.6 driver for details (it87 for example). Don't forget
to define the attributes for each file (this is that step that
links callback functions). Use the file names specified in
Documentation/hwmon/sysfs-interface for the individual files. Also
convert the units these files read and write to the specified ones.
If you need to add a new type of file, please discuss it on the
sensors mailing list <> by providing a
patch to the Documentation/hwmon/sysfs-interface file.
* [Attach] For I2C drivers, the attach function should make sure
that the adapter's class has I2C_CLASS_HWMON (or whatever class is
suitable for your driver), using the following construct:
if (!(adapter->class & I2C_CLASS_HWMON))
return 0;
ISA-only drivers of course don't need this.
Call i2c_probe() instead of i2c_detect().
* [Detect] As mentioned earlier, the flags parameter is gone.
The type_name and client_name strings are replaced by a single
name string, which will be filled with a lowercase, short string.
In i2c-only drivers, drop the i2c_is_isa_adapter check, it's
useless. Same for isa-only drivers, as the test would always be
true. Only hybrid drivers (which are quite rare) still need it.
The labels used for error paths are reduced to the number needed.
It is advised that the labels are given descriptive names such as
exit and exit_free. Don't forget to properly set err before
jumping to error labels. By the way, labels should be left-aligned.
Use kzalloc instead of kmalloc.
Use i2c_set_clientdata to set the client data (as opposed to
a direct access to client->data).
Use strlcpy instead of strcpy or snprintf to copy the client name.
Replace the sysctl directory registration by calls to
device_create_file. Move the driver initialization before any
sysfs file creation.
Register the client with the hwmon class (using hwmon_device_register)
if applicable.
Drop client->id.
Drop any 24RF08 corruption prevention you find, as this is now done
at the i2c-core level, and doing it twice voids it.
Don't add I2C_CLIENT_ALLOW_USE to client->flags, it's the default now.
* [Init] Limits must not be set by the driver (can be done later in
user-space). Chip should not be reset default (although a module
parameter may be used to force it), and initialization should be
limited to the strictly necessary steps.
* [Detach] Remove the call to i2c_deregister_entry. Do not log an
error message if i2c_detach_client fails, as i2c-core will now do
it for you.
Unregister from the hwmon class if applicable.
* [Update] The function prototype changed, it is now
passed a device structure, which you have to convert to a client
using to_i2c_client(dev). The update function should return a
pointer to the client data.
Don't access client->data directly, use i2c_get_clientdata(client)
Use time_after() instead of direct jiffies comparison.
* [Interface] Make sure there is a MODULE_LICENSE() line, at the bottom
of the file (after MODULE_AUTHOR() and MODULE_DESCRIPTION(), in this
* [Driver] The flags field of the i2c_driver structure is gone.
I2C_DF_NOTIFY is now the default behavior.
The i2c_driver structure has a driver member, which is itself a
structure, those name member should be initialized to a driver name
string. i2c_driver itself has no name member anymore.
Coding policy:
* [Copyright] Use (C), not (c), for copyright.
* [Debug/log] Get rid of #ifdef DEBUG/#endif constructs whenever you
can. Calls to printk for debugging purposes are replaced by calls to
dev_dbg where possible, else to pr_debug. Here is an example of how
to call it (taken from lm75_detect):
dev_dbg(&client->dev, "Starting lm75 update\n");
Replace other printk calls with the dev_info, dev_err or dev_warn
function, as appropriate.
* [Constants] Constants defines (registers, conversions) should be
aligned. This greatly improves readability.
Alignments are achieved by the means of tabs, not spaces. Remember
that tabs are set to 8 in the Linux kernel code.
* [Layout] Avoid extra empty lines between comments and what they
comment. Respect the coding style (see Documentation/CodingStyle),
in particular when it comes to placing curly braces.
* [Comments] Make sure that no comment refers to a file that isn't
part of the Linux source tree (typically doc/chips/<chip name>),
and that remaining comments still match the code. Merging comment
lines when possible is encouraged.