add polling support for ns16550

diff -r 378e1c58bcd2 xen/drivers/char/ns16550.c
@@ -30,6 +30,8 @@
     unsigned long io_base;   /* I/O port or memory-mapped I/O address. */
     char *remapped_io_base;  /* Remapped virtual address of mmap I/O.  */ 
     struct irqaction irqaction;
+    struct ac_timer timer;
+    unsigned int timeout;
 } ns16550_com[2] = { { 0 } };
 
 /* Register offsets */

@@ -121,6 +123,34 @@
     }
 }
 
+static void ns16550_timeout(void *data)
+{
+    struct serial_port *port = (struct serial_port *)data;
+    struct ns16550 *uart = port->uart;
+    struct cpu_user_regs *regs = guest_cpu_user_regs();
+    char lsr, ier;
+    int rx_cnt = port->tx_fifo_size;
+
+    /* interrupts must be disabled, this is likely to generate some */
+    ier = ns_read_reg(uart, IER);
+    if ( ier )
+        ns_write_reg(uart, IER, 0);
+
+    lsr = ns_read_reg(uart, LSR);
+
+    while ( (lsr & LSR_DR) && rx_cnt-- ) {
+        serial_rx_interrupt(port, regs);
+        lsr = ns_read_reg(uart, LSR);
+    }
+    if ( lsr & LSR_THRE )
+        serial_tx_interrupt(port, regs);
+
+    if ( ier )
+        ns_write_reg(uart, IER, ier);
+
+    set_ac_timer(&uart->timer, NOW() + MILLISECS(uart->timeout));
+}
+
 static int ns16550_tx_empty(struct serial_port *port)
 {
     struct ns16550 *uart = port->uart;

@@ -178,15 +208,38 @@
         port->tx_fifo_size = 16;
 }
 
+static void ns16550_set_timeout(struct serial_port *port)
+{
+    struct ns16550 *uart = port->uart;
+    unsigned int bits;
+
+    bits = uart->data_bits + uart->stop_bits;
+
+    if ( uart->parity != PARITY_NONE )
+        bits++;
+
+    bits = bits * port->tx_fifo_size;
+
+    /* uart timeout in ms */
+    uart->timeout = (1000 * bits) / uart->baud;
+}
+
 static void ns16550_init_postirq(struct serial_port *port)
 {
     struct ns16550 *uart = port->uart;
     int rc;
 
-    if ( uart->irq <= 0 )
+    if ( uart->irq < 0 )
         return;
 
     serial_async_transmit(port);
+
+    if ( uart->irq == 0 ) {
+        ns16550_set_timeout(port);
+        init_ac_timer(&uart->timer, ns16550_timeout, port, smp_processor_id());
+        set_ac_timer(&uart->timer, NOW() + MILLISECS(uart->timeout));
+        return;
+    }
 
     uart->irqaction.handler = ns16550_interrupt;
     uart->irqaction.name    = "ns16550";