hw/char: a question about watch callback function in serial

[LIU Zhiwei]

Hi folks,

I see many UART implementations have a G_IO_OUT | G_IO_HUP  callback 
function.

In hw/serial.c, it is serial_watch_cb, setting by the following code,

  s->watch_tag = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP,

                      serial_watch_cb, s);

In hw/candence_uart.c, it is cadence_uart_xmit, setting by the following 
code,

        guint r = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP,

                                        cadence_uart_xmit, s);



I tried to call it with booting a Linux, but the interface will never be 
called.

Can someone give a reasonable answer why needs this interface, or a way 
to call it.

Best Regards,
Zhiwei