doc: Add OPAL_I2C_REQUEST documentation

Fixes: #132
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

doc/opal-api/opal-i2c-request-109.rst

@@ -0,0 +1,64 @@
+.. _OPAL_I2C_REQUEST:
+
+OPAL_I2C_REQUEST
+================
+
+.. code-block:: c
+
+   #define OPAL_I2C_REQUEST			109
+
+   /* OPAL I2C request */
+   struct opal_i2c_request {
+		uint8_t	type;
+   #define OPAL_I2C_RAW_READ	0
+   #define OPAL_I2C_RAW_WRITE	1
+   #define OPAL_I2C_SM_READ	2
+   #define OPAL_I2C_SM_WRITE	3
+		uint8_t flags;
+   #define OPAL_I2C_ADDR_10	0x01	/* Not supported yet */
+		uint8_t	subaddr_sz;		/* Max 4 */
+		uint8_t reserved;
+		__be16 addr;			/* 7 or 10 bit address */
+		__be16 reserved2;
+		__be32 subaddr;		/* Sub-address if any */
+		__be32 size;			/* Data size */
+		__be64 buffer_ra;		/* Buffer real address */
+   };
+
+   int opal_i2c_request(uint64_t async_token, uint32_t bus_id,
+                        struct opal_i2c_request *oreq);
+
+Initiate I2C request using i2c master that OPAL controls.
+
+Return Codes
+------------
+
+Most return codes will come through as part of async completion.
+
+:ref:`OPAL_PARAMETER`
+     Invalid request pointer, or bus ID.
+:ref:`OPAL_UNSUPPORTED`
+     Unsupported operation. e.g. 10 bit addresses not yet supported.
+:ref:`OPAL_NO_MEM`
+     Not enough free memory in OPAL to initiate request.
+:ref:`OPAL_ASYNC_COMPLETION`
+     Operation will complete asynchronously.
+:ref:`OPAL_I2C_TIMEOUT`
+     I2C operation initiated successfully, but timed out.
+:ref:`OPAL_I2C_INVALID_CMD`
+     Invalid i2c Command.
+:ref:`OPAL_I2C_LBUS_PARITY`
+     I2C LBUS Parity error
+:ref:`OPAL_I2C_BKEND_OVERRUN`
+     I2C Backend overrun.
+:ref:`OPAL_I2C_BKEND_ACCESS`
+     I2C Backend Access error.
+:ref:`OPAL_I2C_ARBT_LOST`
+     I2C Bus Arbitration lost.
+:ref:`OPAL_I2C_NACK_RCVD`
+     I2C NACK received.
+:ref:`OPAL_I2C_STOP_ERR`
+     I2C STOP error.
+:ref:`OPAL_SUCCESS`
+     I2C operation completed successfully. Typically only as part of
+     async completion.