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>

2 files changed, 6 insertions, 2 deletions

diff --git a/hw/timer/ds1338.c b/hw/timer/ds1338.c
index f5d04dd5d7..3849b74a68 100644
--- a/hw/timer/ds1338.c
+++ b/hw/timer/ds1338.c
@@ -94,7 +94,7 @@ static void inc_regptr(DS1338State *s)
     }
 }
 
-static void ds1338_event(I2CSlave *i2c, enum i2c_event event)
+static int ds1338_event(I2CSlave *i2c, enum i2c_event event)
 {
     DS1338State *s = DS1338(i2c);
 
@@ -113,6 +113,8 @@ static void ds1338_event(I2CSlave *i2c, enum i2c_event event)
     default:
         break;
     }
+
+    return 0;
 }
 
 static int ds1338_recv(I2CSlave *i2c)
diff --git a/hw/timer/twl92230.c b/hw/timer/twl92230.c
index 7ba4e9a7c9..b8d914e49b 100644
--- a/hw/timer/twl92230.c
+++ b/hw/timer/twl92230.c
@@ -713,12 +713,14 @@ static void menelaus_write(void *opaque, uint8_t addr, uint8_t value)
     }
 }
 
-static void menelaus_event(I2CSlave *i2c, enum i2c_event event)
+static int menelaus_event(I2CSlave *i2c, enum i2c_event event)
 {
     MenelausState *s = TWL92230(i2c);
 
     if (event == I2C_START_SEND)
         s->firstbyte = 1;
+
+    return 0;
 }
 
 static int menelaus_tx(I2CSlave *i2c, uint8_t data)