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>

hw/i2c/core.c

@@ -88,18 +88,26 @@ int i2c_bus_busy(I2CBus *bus)
     return !QLIST_EMPTY(&bus->current_devs);
 }
 
-/*
- * Returns non-zero if the address is not valid.  If this is called
- * again without an intervening i2c_end_transfer(), like in the SMBus
- * case where the operation is switched from write to read, this
- * function will not rescan the bus and thus cannot fail.
- */
 /* TODO: Make this handle multiple masters.  */
+/*
+ * Start or continue an i2c transaction.  When this is called for the
+ * first time or after an i2c_end_transfer(), if it returns an error
+ * the bus transaction is terminated (or really never started).  If
+ * this is called after another i2c_start_transfer() without an
+ * intervening i2c_end_transfer(), and it returns an error, the
+ * transaction will not be terminated.  The caller must do it.
+ *
+ * This corresponds with the way real hardware works.  The SMBus
+ * protocol uses a start transfer to switch from write to read mode
+ * without releasing the bus.  If that fails, the bus is still
+ * in a transaction.
+ */
 int i2c_start_transfer(I2CBus *bus, uint8_t address, int recv)
 {
     BusChild *kid;
     I2CSlaveClass *sc;
     I2CNode *node;
+    bool bus_scanned = false;
 
     if (address == I2C_BROADCAST) {
         /*
@@ -130,6 +138,7 @@ int i2c_start_transfer(I2CBus *bus, uint8_t address, int recv)
                 }
             }
         }
+        bus_scanned = true;
     }
 
     if (QLIST_EMPTY(&bus->current_devs)) {
@@ -137,11 +146,21 @@ int i2c_start_transfer(I2CBus *bus, uint8_t address, int recv)
     }
 
     QLIST_FOREACH(node, &bus->current_devs, next) {
+        int rv;
+
         sc = I2C_SLAVE_GET_CLASS(node->elt);
         /* If the bus is already busy, assume this is a repeated
            start condition.  */
+
         if (sc->event) {
-            sc->event(node->elt, recv ? I2C_START_RECV : I2C_START_SEND);
+            rv = sc->event(node->elt, recv ? I2C_START_RECV : I2C_START_SEND);
+            if (rv && !bus->broadcast) {
+                if (bus_scanned) {
+                    /* First call, terminate the transfer. */
+                    i2c_end_transfer(bus);
+                }
+                return rv;
+            }
         }
     }
     return 0;

hw/i2c/i2c-ddc.c

@@ -230,13 +230,15 @@ static void i2c_ddc_reset(DeviceState *ds)
     s->reg = 0;
 }
 
-static void i2c_ddc_event(I2CSlave *i2c, enum i2c_event event)
+static int i2c_ddc_event(I2CSlave *i2c, enum i2c_event event)
 {
     I2CDDCState *s = I2CDDC(i2c);
 
     if (event == I2C_START_SEND) {
         s->firstbyte = true;
     }
+
+    return 0;
 }
 
 static int i2c_ddc_rx(I2CSlave *i2c)

hw/i2c/smbus.c

@@ -67,7 +67,7 @@ static void smbus_do_write(SMBusDevice *dev)
     }
 }
 
-static void smbus_i2c_event(I2CSlave *s, enum i2c_event event)
+static int smbus_i2c_event(I2CSlave *s, enum i2c_event event)
 {
     SMBusDevice *dev = SMBUS_DEVICE(s);
 
@@ -148,6 +148,8 @@ static void smbus_i2c_event(I2CSlave *s, enum i2c_event event)
             break;
         }
     }
+
+    return 0;
 }
 
 static int smbus_i2c_recv(I2CSlave *s)
@@ -249,7 +251,8 @@ int smbus_read_byte(I2CBus *bus, uint8_t addr, uint8_t command)
     }
     i2c_send(bus, command);
     if (i2c_start_transfer(bus, addr, 1)) {
-        assert(0);
+        i2c_end_transfer(bus);
+        return -1;
     }
     data = i2c_recv(bus);
     i2c_nack(bus);
@@ -276,7 +279,8 @@ int smbus_read_word(I2CBus *bus, uint8_t addr, uint8_t command)
     }
     i2c_send(bus, command);
     if (i2c_start_transfer(bus, addr, 1)) {
-        assert(0);
+        i2c_end_transfer(bus);
+        return -1;
     }
     data = i2c_recv(bus);
     data |= i2c_recv(bus) << 8;
@@ -307,7 +311,8 @@ int smbus_read_block(I2CBus *bus, uint8_t addr, uint8_t command, uint8_t *data)
     }
     i2c_send(bus, command);
     if (i2c_start_transfer(bus, addr, 1)) {
-        assert(0);
+        i2c_end_transfer(bus);
+        return -1;
     }
     len = i2c_recv(bus);
     if (len > 32) {