+ if (is_real_interrupt(up->port.irq)) {
+ /*
+ * Test for UARTs that do not reassert THRE when the
+ * transmitter is idle and the interrupt has already
+ * been cleared. Real 16550s should always reassert
+ * this interrupt whenever the transmitter is idle and
+ * the interrupt is enabled. Delays are necessary to
+ * allow register changes to become visible.
+ */
+ spin_lock_irqsave(&up->port.lock, flags);
+
+ wait_for_xmitr(up, UART_LSR_THRE);
+ serial_out_sync(up, UART_IER, UART_IER_THRI);
+ udelay(1); /* allow THRE to set */
+ serial_in(up, UART_IIR);
+ serial_out(up, UART_IER, 0);
+ serial_out_sync(up, UART_IER, UART_IER_THRI);
+ udelay(1); /* allow a working UART time to re-assert THRE */
+ iir = serial_in(up, UART_IIR);
+ serial_out(up, UART_IER, 0);
+
+ spin_unlock_irqrestore(&up->port.lock, flags);
+
+ /*
+ * If the interrupt is not reasserted, setup a timer to
+ * kick the UART on a regular basis.
+ */
+ if (iir & UART_IIR_NO_INT) {
+ pr_debug("ttyS%d - using backup timer\n", port->line);
+ up->timer.function = serial8250_backup_timeout;
+ up->timer.data = (unsigned long)up;
+ mod_timer(&up->timer, jiffies +
+ poll_timeout(up->port.timeout) + HZ/5);
+ }
+ }
+