char/cadence_uart: Fix can_receive logic

The can_receive logic was only taking into account the RxFIFO
occupancy. RxFIFO population is only used for the echo and normal modes
however. Improve the logic to correctly return the true number of
receivable characters based on the current mode:

Normal mode: RxFIFO vacancy.
Remote loopback: TxFIFO vacancy.
Echo mode: The min of the TxFIFO and RxFIFO vacancies.
Local Loopback: Return non-zero (to implement droppage)
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 36a58440c9ca5080151e95765c2c81342de8a8df.1388626249.git.peter.crosthwaite@xilinx.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/char/cadence_uart.c

@@ -233,8 +233,16 @@ static void uart_parameters_setup(UartState *s)
 static int uart_can_receive(void *opaque)
 {
     UartState *s = (UartState *)opaque;
+    int ret = MAX(RX_FIFO_SIZE, TX_FIFO_SIZE);
+    uint32_t ch_mode = s->r[R_MR] & UART_MR_CHMODE;
 
-    return RX_FIFO_SIZE - s->rx_count;
+    if (ch_mode == NORMAL_MODE || ch_mode == ECHO_MODE) {
+        ret = MIN(ret, RX_FIFO_SIZE - s->rx_count);
+    }
+    if (ch_mode == REMOTE_LOOPBACK || ch_mode == ECHO_MODE) {
+        ret = MIN(ret, TX_FIFO_SIZE - s->tx_count);
+    }
+    return ret;
 }
 
 static void uart_ctrl_update(UartState *s)