i2c: Allow I2C devices to NAK start events

Add a return value to the event handler.  Some I2C devices will
NAK if they have no data, so allow them to do this.  This required
the following changes:

Go through all the event handlers and change them to return int
and return 0.

Modify i2c_start_transfer to terminate the transaction on a NAK.

Modify smbus handing to not assert if a NAK occurs on a second
operation, and terminate the transaction and return -1 instead.

Add some information on semantics to I2CSlaveClass.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/include/hw/i2c/i2c.h b/include/hw/i2c/i2c.h
index c4085aa366..2ce611d4c8 100644
--- a/include/hw/i2c/i2c.h
+++ b/include/hw/i2c/i2c.h
@@ -32,14 +32,22 @@ typedef struct I2CSlaveClass
     /* Callbacks provided by the device.  */
     int (*init)(I2CSlave *dev);
 
-    /* Master to slave.  */
+    /* Master to slave. Returns non-zero for a NAK, 0 for success. */
     int (*send)(I2CSlave *s, uint8_t data);
 
-    /* Slave to master.  */
+    /*
+     * Slave to master.  This cannot fail, the device should always
+     * return something here.  Negative values probably result in 0xff
+     * and a possible log from the driver, and shouldn't be used.
+     */
     int (*recv)(I2CSlave *s);
 
-    /* Notify the slave of a bus state change.  */
-    void (*event)(I2CSlave *s, enum i2c_event event);
+    /*
+     * Notify the slave of a bus state change.  For start event,
+     * returns non-zero to NAK an operation.  For other events the
+     * return code is not used and should be zero.
+     */
+    int (*event)(I2CSlave *s, enum i2c_event event);
 } I2CSlaveClass;
 
 struct I2CSlave