[Qemu-devel] Re: more about serial ports: do they even work?

[Mark Marshall]

Michael Tokarev wrote:
> David S. Ahern wrote:
> > Michael Tokarev wrote:
> > > David S. Ahern wrote:
> > > > I don't recall seeing a followup to this post.
> > > >
> > > > To put Michael's second suggestion into patch form, the following fixes
> > > > the problem for me:
> > > >
> > > > --- kvm-81.orig/qemu/qemu-char.c        2008-12-14 06:16:27.000000000 -0700
> > > > +++ kvm-81/qemu/qemu-char.c     2009-02-02 14:12:20.000000000 -0700
> > > > @@ -1078,20 +1078,21 @@
> > > >              if (sarg | TIOCM_DTR)
> > > >                  *targ |= CHR_TIOCM_DTR;
> > > >              if (sarg | TIOCM_RTS)
> > > >                  *targ |= CHR_TIOCM_RTS;
> > > >          }
> > > >          break;
> > > >      case CHR_IOCTL_SERIAL_SET_TIOCM:
> > > >          {
> > > >              int sarg = *(int *)arg;
> > > >              int targ = 0;
> > > > +            ioctl(s->fd_in, TIOCMGET, &targ);
> > > here, one more operation is necessary:
> > >                targ &= ~(TIOCM_DTR|TIOCM_RTS);
> > Interesting. that change was not needed to fix my problem.
>
> It just means you (or, rather, your guests) never really needed to
> DROP those signal lines, only to raise them.
>
> > > >              if (sarg | CHR_TIOCM_DTR)
> > > >                  targ |= TIOCM_DTR;
> > > >              if (sarg | CHR_TIOCM_RTS)
> > > >                  targ |= TIOCM_RTS;
Is this code really correct.  If it is is there a comment somewhere 
describing why it's correct?  I would have expected the lines above to be:

              if (sarg & CHR_TIOCM_DTR)
                  targ |= TIOCM_DTR;
              if (sarg & CHR_TIOCM_RTS)
                  targ |= TIOCM_RTS;

(Just an observation as this went past).

MM

> Without that line above, the code never drops the two bits, once
> set they can't be "removed" anymore.
>
> By the way, this is upstream qemu issue, not kvm one, and has to be
> pushed as such.  Good you CC'd qemu list.
>
> /mjt
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