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+Revision 4, 2004-03-30
+Jean Delvare <khali@linux-fr.org>
+Greg KH <greg@kroah.com>
+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.
+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". Replace <linux/i2c-proc.h> with
+ <linux/i2c-sensor.h>. Includes typically look like that:
+ #include <linux/module.h>
+ #include <linux/init.h>
+ #include <linux/slab.h>
+ #include <linux/i2c.h>
+ #include <linux/i2c-sensor.h>
+ #include <linux/i2c-vid.h> /* if you need VRM support */
+ #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").
+ becomes I2C_CLIENT_ISA_END.
+* [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
+ prototypes:
+ 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 void lm75_update_client(struct i2c_client *client);
+* [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 (lm78 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/i2c/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 <sensors@stimpy.netroedge.com> by providing a
+ patch to the Documentation/i2c/sysfs-interface file.
+* [Attach] For I2C drivers, the attach function should make sure
+ that the adapter's class has I2C_CLASS_HWMON, using the
+ following construct:
+ if (!(adapter->class & I2C_CLASS_HWMON))
+ return 0;
+ ISA-only drivers of course don't need this.
+* [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
+ (typically the driver name, e.g. "lm75").
+ In i2c-only drivers, drop the i2c_is_isa_adapter check, it's
+ useless.
+ The errorN labels are reduced to the number needed. If that number
+ is 2 (i2c-only drivers), it is advised that the labels are named
+ exit and exit_free. For i2c+isa drivers, labels should be named
+ ERROR0, ERROR1 and ERROR2. Don't forget to properly set err before
+ jumping to error labels. By the way, labels should be left-aligned.
+ Use memset to fill the client and data area with 0x00.
+ Use i2c_set_clientdata to set the client data (as opposed to
+ a direct access to client->data).
+ Use strlcpy instead of strcpy 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.
+ Drop client->id.
+* [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 is), and initialization should be
+ limited to the strictly necessary steps.
+* [Detach] Get rid of data, remove the call to
+ i2c_deregister_entry.
+* [Update] Don't access client->data directly, use
+ i2c_get_clientdata(client) instead.
+* [Interface] Init function should not print anything. Make sure
+ there is a MODULE_LICENSE() line, at the bottom of the file
+ (after MODULE_AUTHOR() and MODULE_DESCRIPTION(), in this order).
+Coding policy:
+* [Copyright] Use (C), not (c), for copyright.
+* [Debug/log] Get rid of #ifdef DEBUG/#endif constructs whenever you
+ can. Calls to printk/pr_debug for debugging purposes are replaced
+ by calls to dev_dbg. Here is an example on 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, initial
+ values) should be aligned. This greatly improves readability.
+ Same goes for variables declarations. Alignments are achieved by the
+ means of tabs, not spaces. Remember that tabs are set to 8 in the
+ Linux kernel code.
+* [Structure definition] The name field should be standardized. All
+ lowercase and as simple as the driver name itself (e.g. "lm75").
+* [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.