i2c: Document the message size limit

i2c_master_send & i2c_master_recv do not support more than 64 kb
transfer, since msg.len is u16.

Signed-off-by: Zhangfei Gao <zgao6@marvell.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

Documentation/i2c/writing-clients

@@ -318,8 +318,9 @@ Plain I2C communication
 These routines read and write some bytes from/to a client. The client
 contains the i2c address, so you do not have to include it. The second
 parameter contains the bytes to read/write, the third the number of bytes
-to read/write (must be less than the length of the buffer.) Returned is
-the actual number of bytes read/written.
+to read/write (must be less than the length of the buffer, also should be
+less than 64k since msg.len is u16.) Returned is the actual number of bytes
+read/written.
 
 	int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
 			 int num);

drivers/i2c/i2c-core.c

@@ -1183,7 +1183,7 @@ EXPORT_SYMBOL(i2c_transfer);
  * i2c_master_send - issue a single I2C message in master transmit mode
  * @client: Handle to slave device
  * @buf: Data that will be written to the slave
- * @count: How many bytes to write
+ * @count: How many bytes to write, must be less than 64k since msg.len is u16
  *
  * Returns negative errno, or else the number of bytes written.
  */
@@ -1210,7 +1210,7 @@ EXPORT_SYMBOL(i2c_master_send);
  * i2c_master_recv - issue a single I2C message in master receive mode
  * @client: Handle to slave device
  * @buf: Where to store data read from slave
- * @count: How many bytes to read
+ * @count: How many bytes to read, must be less than 64k since msg.len is u16
  *
  * Returns negative errno, or else the number of bytes read.
  */

include/linux/i2c.h

@@ -53,6 +53,7 @@ struct i2c_board_info;
  * on a bus (or read from them). Apart from two basic transfer functions to
  * transmit one message at a time, a more complex version can be used to
  * transmit an arbitrary number of messages without interruption.
+ * @count must be be less than 64k since msg.len is u16.
  */
 extern int i2c_master_send(struct i2c_client *client, const char *buf,
 			   int count);