char/cadence_uart: Use the TX fifo for transmission

Populate the TxFIFO with the Tx data before sending. Prepares
support for proper Tx flow control implementation.
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: bdf7f8af2ef02839bea18665701bc2612f7baa6f.1388626249.git.peter.crosthwaite@xilinx.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/char/cadence_uart.c

@@ -292,7 +292,22 @@ static void uart_write_tx_fifo(UartState *s, const uint8_t *buf, int size)
         return;
     }
 
-    qemu_chr_fe_write_all(s->chr, buf, size);
+    if (size > TX_FIFO_SIZE - s->tx_count) {
+        size = TX_FIFO_SIZE - s->tx_count;
+        /*
+         * This can only be a guest error via a bad tx fifo register push,
+         * as can_receive() should stop remote loop and echo modes ever getting
+         * us to here.
+         */
+        qemu_log_mask(LOG_GUEST_ERROR, "cadence_uart: TxFIFO overflow");
+        s->r[R_CISR] |= UART_INTR_ROVR;
+    }
+
+    memcpy(s->tx_fifo + s->tx_count, buf, size);
+    s->tx_count += size;
+
+    qemu_chr_fe_write_all(s->chr, s->tx_fifo, s->tx_count);
+    s->tx_count = 0;
 }
 
 static void uart_receive(void *opaque, const uint8_t *buf, int size)