Linux guests wait ~30 seconds when closing the emulated /dev/ttyUSB0.
During that time, the kernel driver is sending many control URBs
requesting GetModemStat (5).  Real hardware returns a status with
FTDI_THRE (Transmitter Holding Register) and FTDI_TEMT (Transmitter
Empty) set.  QEMU leaves them clear, and it seems Linux is waiting for
FTDI_TEMT to be set to indicate the tx queue is empty before closing.

Set the bits when responding to a GetModemStat query and avoid the
shutdown delay.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

---
Looking at a USB dump for a real FTDI USB adapter, I see these bits set
in all the bulk URBs where QEMU currently has them clear.
---
 hw/usb/dev-serial.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/usb/dev-serial.c b/hw/usb/dev-serial.c
index 77b964588b..d2c03681b7 100644
--- a/hw/usb/dev-serial.c
+++ b/hw/usb/dev-serial.c
@@ -332,7 +332,7 @@ static void usb_serial_handle_control(USBDevice *dev, USBPacket *p,
         break;
     case DeviceInVendor | FTDI_GET_MDM_ST:
         data[0] = usb_get_modem_lines(s) | 1;
-        data[1] = 0;
+        data[1] = FTDI_THRE | FTDI_TEMT;
         p->actual_length = 2;
         break;
     case DeviceOutVendor | FTDI_SET_EVENT_CHR:
-- 
2.24.1