Re: more about serial ports: do they even work?

[Michael Tokarev <mjt@tls.msk.ru>]

Michael Tokarev ?????:
> After some debugging and debugging, with a help
> Hollis Blanchard on #kvm@freenode, I discovered
> that kvm (or, rather, qemu) does not work correctly
> with serial ports, at least on linux.  One problem
> report has already here, author Cc'd -- see e.g.
> http://marc.info/?l=kvm&m=122995568009533&w=2 .
> 
> Here's what's going on.
> 
> When opening a host's port, kvm resets the status
> lines, doing this:
> 
> ioctl(13, TIOCMGET, [TIOCM_DTR|TIOCM_RTS|TIOCM_CTS|TIOCM_DSR|0x4000])
> ioctl(13, TIOCMSET, [TIOCM_DTR|TIOCM_RTS])
> 
> which results in the following set
> 
> ioctl(13, TIOCMGET, [TIOCM_DTR|TIOCM_RTS|TIOCM_CTS|TIOCM_DSR])

Here's the possible solution (NotAPAtch(tm)):

In kvm-xx/qemu/qemu-char.c:

    case CHR_IOCTL_SERIAL_SET_TIOCM:
        {
            int sarg = *(int *)arg;
            int targ = 0;        <==== change this 0 to 0x4000
            if (sarg | CHR_TIOCM_DTR)
                targ |= TIOCM_DTR;
            if (sarg | CHR_TIOCM_RTS)
                targ |= TIOCM_RTS;
            ioctl(s->fd_in, TIOCMSET, &targ);
        }
        break;

This is obviously a hack, esp. since this bit is not
always present even on linux (after reading 8250.c
driver).

Real fix will be, I guess, to read the full set
first, and combine it with DTR|RTS received from
guest, something like this:

    case CHR_IOCTL_SERIAL_SET_TIOCM:
        {
            int sarg = *(int *)arg;
            int targ = 0;
            ioctl(s->fd_in, TIOCMGET, &targ);
            if (!(sarg | CHR_TIOCM_DTR))
                targ &= ~TIOCM_DTR;
            if (!(sarg | CHR_TIOCM_RTS))
                targ ~= ~TIOCM_RTS;
            ioctl(s->fd_in, TIOCMSET, &targ);
        }
        break;

I.e., to always keep all the other bits, but
allow changing DTR and RTS.

Again, I don't know how it's linux-specific, but
it seems the solution above should work on other
platforms just fine.

/mjt