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>

diff --git a/Documentation/i2c/upgrading-clients b/Documentation/i2c/upgrading-clients
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+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,
+};