i2c: Document struct i2c_msg

Clarify use of the I2C_M_* flags by highlighting the fact that
most of them depend on I2C_FUNC_PROTOCOL_MANGLING.

Also provide kerneldoc for i2c_smbus_read_block_data() and also
for "struct i2c_msg".

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

2 files changed, 58 insertions, 10 deletions

diff --git a/drivers/i2c/i2c-core.c b/drivers/i2c/i2c-core.c
index f95b1b65761..5e58b5641a3 100644
--- a/drivers/i2c/i2c-core.c
+++ b/drivers/i2c/i2c-core.c
@@ -1307,7 +1307,22 @@ s32 i2c_smbus_write_word_data(struct i2c_client *client, u8 command, u16 value)
 }
 EXPORT_SYMBOL(i2c_smbus_write_word_data);
 
-/* Returns the number of read bytes */
+/**
+ * i2c_smbus_read_block_data - SMBus block read request
+ * @client: Handle to slave device
+ * @command: Command byte issued to let the slave know what data should
+ *	be returned
+ * @values: Byte array into which data will be read; big enough to hold
+ *	the data returned by the slave.  SMBus allows at most 32 bytes.
+ *
+ * Returns the number of bytes read in the slave's response, else a
+ * negative number to indicate some kind of error.
+ *
+ * Note that using this function requires that the client's adapter support
+ * the I2C_FUNC_SMBUS_READ_BLOCK_DATA functionality.  Not all adapter drivers
+ * support this; its emulation through I2C messaging relies on a specific
+ * mechanism (I2C_M_RECV_LEN) which may not be implemented.
+ */
 s32 i2c_smbus_read_block_data(struct i2c_client *client, u8 command,
 			      u8 *values)
 {
diff --git a/include/linux/i2c.h b/include/linux/i2c.h
index d4b63171d5d..8fc4310f071 100644
--- a/include/linux/i2c.h
+++ b/include/linux/i2c.h
@@ -443,19 +443,52 @@ static inline int i2c_adapter_id(struct i2c_adapter *adap)
 }
 #endif /* __KERNEL__ */
 
-/*
- * I2C Message - used for pure i2c transaction, also from /dev interface
+/**
+ * struct i2c_msg - an I2C transaction segment beginning with START
+ * @addr: Slave address, either seven or ten bits.  When this is a ten
+ *	bit address, I2C_M_TEN must be set in @flags and the adapter
+ *	must support I2C_FUNC_10BIT_ADDR.
+ * @flags: I2C_M_RD is handled by all adapters.  No other flags may be
+ *	provided unless the adapter exported the relevant I2C_FUNC_*
+ *	flags through i2c_check_functionality().
+ * @len: Number of data bytes in @buf being read from or written to the
+ *	I2C slave address.  For read transactions where I2C_M_RECV_LEN
+ *	is set, the caller guarantees that this buffer can hold up to
+ *	32 bytes in addition to the initial length byte sent by the
+ *	slave (plus, if used, the SMBus PEC); and this value will be
+ *	incremented by the number of block data bytes received.
+ * @buf: The buffer into which data is read, or from which it's written.
+ *
+ * An i2c_msg is the low level representation of one segment of an I2C
+ * transaction.  It is visible to drivers in the @i2c_transfer() procedure,
+ * to userspace from i2c-dev, and to I2C adapter drivers through the
+ * @i2c_adapter.@master_xfer() method.
+ *
+ * Except when I2C "protocol mangling" is used, all I2C adapters implement
+ * the standard rules for I2C transactions.  Each transaction begins with a
+ * START.  That is followed by the slave address, and a bit encoding read
+ * versus write.  Then follow all the data bytes, possibly including a byte
+ * with SMBus PEC.  The transfer terminates with a NAK, or when all those
+ * bytes have been transferred and ACKed.  If this is the last message in a
+ * group, it is followed by a STOP.  Otherwise it is followed by the next
+ * @i2c_msg transaction segment, beginning with a (repeated) START.
+ *
+ * Alternatively, when the adapter supports I2C_FUNC_PROTOCOL_MANGLING then
+ * passing certain @flags may have changed those standard protocol behaviors.
+ * Those flags are only for use with broken/nonconforming slaves, and with
+ * adapters which are known to support the specific mangling options they
+ * need (one or more of IGNORE_NAK, NO_RD_ACK, NOSTART, and REV_DIR_ADDR).
  */
 struct i2c_msg {
 	__u16 addr;	/* slave address			*/
 	__u16 flags;
-#define I2C_M_TEN	0x10	/* we have a ten bit chip address	*/
-#define I2C_M_RD	0x01
-#define I2C_M_NOSTART	0x4000
-#define I2C_M_REV_DIR_ADDR	0x2000
-#define I2C_M_IGNORE_NAK	0x1000
-#define I2C_M_NO_RD_ACK		0x0800
-#define I2C_M_RECV_LEN		0x0400 /* length will be first received byte */
+#define I2C_M_TEN		0x0010	/* this is a ten bit chip address */
+#define I2C_M_RD		0x0001	/* read data, from slave to master */
+#define I2C_M_NOSTART		0x4000	/* if I2C_FUNC_PROTOCOL_MANGLING */
+#define I2C_M_REV_DIR_ADDR	0x2000	/* if I2C_FUNC_PROTOCOL_MANGLING */
+#define I2C_M_IGNORE_NAK	0x1000	/* if I2C_FUNC_PROTOCOL_MANGLING */
+#define I2C_M_NO_RD_ACK		0x0800	/* if I2C_FUNC_PROTOCOL_MANGLING */
+#define I2C_M_RECV_LEN		0x0400	/* length will be first received byte */
 	__u16 len;		/* msg length				*/
 	__u8 *buf;		/* pointer to msg data			*/
 };