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

[Mark Marshall <mark.marshall60@ntlworld.com>]

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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