i2c: Documentation: upgrading clients HOWTO

Add a document describing how i2c clients on Linux 2.6 can be
moved from the old to the new driver model.
Signed-off-by: Ben Dooks <ben-linux@fluff.org>

Documentation/i2c/upgrading-clients

+Upgrading I2C Drivers to the new 2.6 Driver Model
+=================================================
+
+Ben Dooks <ben-linux@fluff.org>
+
+Introduction
+------------
+
+This guide outlines how to alter existing Linux 2.6 client drivers from
+the old to the new new binding methods.
+
+
+Example old-style driver
+------------------------
+
+
+struct example_state {
+	struct i2c_client	client;
+	....
+};
+
+static struct i2c_driver example_driver;
+
+static unsigned short ignore[] = { I2C_CLIENT_END };
+static unsigned short normal_addr[] = { OUR_ADDR, I2C_CLIENT_END };
+
+I2C_CLIENT_INSMOD;
+
+static int example_attach(struct i2c_adapter *adap, int addr, int kind)
+{
+	struct example_state *state;
+	struct device *dev = &adap->dev;  /* to use for dev_ reports */
+	int ret;
+
+	state = kzalloc(sizeof(struct example_state), GFP_KERNEL);
+	if (state == NULL) {
+		dev_err(dev, "failed to create our state\n");
+		return -ENOMEM;
+	}
+
+	example->client.addr    = addr;
+	example->client.flags   = 0;
+	example->client.adapter = adap;
+
+	i2c_set_clientdata(&state->i2c_client, state);
+	strlcpy(client->i2c_client.name, "example", I2C_NAME_SIZE);
+
+	ret = i2c_attach_client(&state->i2c_client);
+	if (ret < 0) {
+		dev_err(dev, "failed to attach client\n");
+		kfree(state);
+		return ret;
+	}
+
+	dev = &state->i2c_client.dev;
+
+	/* rest of the initialisation goes here. */
+
+	dev_info(dev, "example client created\n");
+
+	return 0;
+}
+
+static int __devexit example_detach(struct i2c_client *client)
+{
+	struct example_state *state = i2c_get_clientdata(client);
+
+	i2c_detach_client(client);
+	kfree(state);
+	return 0;
+}
+
+static int example_attach_adapter(struct i2c_adapter *adap)
+{
+	return i2c_probe(adap, &addr_data, example_attach);
+}
+
+static struct i2c_driver example_driver = {
+ 	.driver		= {
+		.owner		= THIS_MODULE,
+		.name		= "example",
+	},
+	.attach_adapter = example_attach_adapter,
+	.detach_client	= __devexit_p(example_detach),
+	.suspend	= example_suspend,
+	.resume		= example_resume,
+};
+
+
+Updating the client
+-------------------
+
+The new style binding model will check against a list of supported
+devices and their associated address supplied by the code registering
+the busses. This means that the driver .attach_adapter and
+.detach_adapter methods can be removed, along with the addr_data,
+as follows:
+
+- static struct i2c_driver example_driver;
+
+- static unsigned short ignore[] = { I2C_CLIENT_END };
+- static unsigned short normal_addr[] = { OUR_ADDR, I2C_CLIENT_END };
+
+- I2C_CLIENT_INSMOD;
+
+- static int example_attach_adapter(struct i2c_adapter *adap)
+- {
+- 	return i2c_probe(adap, &addr_data, example_attach);
+- }
+
+ static struct i2c_driver example_driver = {
+-	.attach_adapter = example_attach_adapter,
+-	.detach_client	= __devexit_p(example_detach),
+ }
+
+Add the probe and remove methods to the i2c_driver, as so:
+
+ static struct i2c_driver example_driver = {
++	.probe		= example_probe,
++	.remove		= __devexit_p(example_remove),
+ }
+
+Change the example_attach method to accept the new parameters
+which include the i2c_client that it will be working with:
+
+- static int example_attach(struct i2c_adapter *adap, int addr, int kind)
++ static int example_probe(struct i2c_client *client,
++			   const struct i2c_device_id *id)
+
+Change the name of example_attach to example_probe to align it with the
+i2c_driver entry names. The rest of the probe routine will now need to be
+changed as the i2c_client has already been setup for use.
+
+The necessary client fields have already been setup before
+the probe function is called, so the following client setup
+can be removed:
+
+-	example->client.addr    = addr;
+-	example->client.flags   = 0;
+-	example->client.adapter = adap;
+-
+-	strlcpy(client->i2c_client.name, "example", I2C_NAME_SIZE);
+
+The i2c_set_clientdata is now:
+
+-	i2c_set_clientdata(&state->client, state);
++	i2c_set_clientdata(client, state);
+
+The call to i2c_attach_client is no longer needed, if the probe
+routine exits successfully, then the driver will be automatically
+attached by the core. Change the probe routine as so:
+
+-	ret = i2c_attach_client(&state->i2c_client);
+-	if (ret < 0) {
+-		dev_err(dev, "failed to attach client\n");
+-		kfree(state);
+-		return ret;
+-	}
+
+
+Remove the storage of 'struct i2c_client' from the 'struct example_state'
+as we are provided with the i2c_client in our example_probe. Instead we
+store a pointer to it for when it is needed.
+
+struct example_state {
+-	struct i2c_client	client;
++	struct i2c_client	*client;
+
+the new i2c client as so:
+
+-	struct device *dev = &adap->dev;  /* to use for dev_ reports */
++ 	struct device *dev = &i2c_client->dev;  /* to use for dev_ reports */
+
+And remove the change after our client is attached, as the driver no
+longer needs to register a new client structure with the core:
+
+-	dev = &state->i2c_client.dev;
+
+In the probe routine, ensure that the new state has the client stored
+in it:
+
+static int example_probe(struct i2c_client *i2c_client,
+			 const struct i2c_device_id *id)
+{
+	struct example_state *state;
+ 	struct device *dev = &i2c_client->dev;
+	int ret;
+
+	state = kzalloc(sizeof(struct example_state), GFP_KERNEL);
+	if (state == NULL) {
+		dev_err(dev, "failed to create our state\n");
+		return -ENOMEM;
+	}
+
++	state->client = i2c_client;
+
+Update the detach method, by changing the name to _remove and
+to delete the i2c_detach_client call. It is possible that you
+can also remove the ret variable as it is not not needed for
+any of the core functions.
+
+- static int __devexit example_detach(struct i2c_client *client)
++ static int __devexit example_remove(struct i2c_client *client)
+{
+	struct example_state *state = i2c_get_clientdata(client);
+
+-	i2c_detach_client(client);
+
+And finally ensure that we have the correct ID table for the i2c-core
+and other utilities:
+
++ struct i2c_device_id example_idtable[] = {
++       { "example", 0 },
++       { }
++};
++
++MODULE_DEVICE_TABLE(i2c, example_idtable);
+
+static struct i2c_driver example_driver = {
+ 	.driver		= {
+		.owner		= THIS_MODULE,
+		.name		= "example",
+	},
++	.id_table	= example_ids,
+
+
+Our driver should now look like this:
+
+struct example_state {
+	struct i2c_client	*client;
+	....
+};
+
+static int example_probe(struct i2c_client *client,
+		     	 const struct i2c_device_id *id)
+{
+	struct example_state *state;
+	struct device *dev = &client->dev;
+
+	state = kzalloc(sizeof(struct example_state), GFP_KERNEL);
+	if (state == NULL) {
+		dev_err(dev, "failed to create our state\n");
+		return -ENOMEM;
+	}
+
+	state->client = client;
+	i2c_set_clientdata(client, state);
+
+	/* rest of the initialisation goes here. */
+
+	dev_info(dev, "example client created\n");
+
+	return 0;
+}
+
+static int __devexit example_remove(struct i2c_client *client)
+{
+	struct example_state *state = i2c_get_clientdata(client);
+
+	kfree(state);
+	return 0;
+}
+
+static struct i2c_device_id example_idtable[] = {
+	{ "example", 0 },
+	{ }
+};
+
+MODULE_DEVICE_TABLE(i2c, example_idtable);
+
+static struct i2c_driver example_driver = {
+ 	.driver		= {
+		.owner		= THIS_MODULE,
+		.name		= "example",
+	},
+	.id_table	= example_idtable,
+	.probe		= example_probe,
+	.remove		= __devexit_p(example_remove),
+	.suspend	= example_suspend,
+	.resume		= example_resume,
+};