RE: [Xen-devel] Re: [Xen-changelog] If Xen is told to use a serial console via a com1= or com2= directive

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> I wrote drivers/char/hvsi.c, so I've been here before, and I'd 
> definitely recommend not trying to hijack serial (ttyS) 
> nodes. It just 
> doesn't make sense: what does hardware flow control mean to a virtual 
> console? What about baud rate, parity, stop bits, IO port? 
> Even worse, 
> all those ioctls like TIOCSERGETLSR ("get line status register") -- 
> which ioctls can you return an error for, and which can you fake, and 
> what will you break when you fake it? Example from anaconda 
> (Red Hat's 
> installer):
>      if (major(sb.st_rdev) != 3 && major(sb.st_rdev) != 136 &&
>          (virtpcon != NULL)){
>          if ((ioctl (0, TIOCLINUX, &twelve) < 0) &&
>              (ioctl(0, TIOCGSERIAL, &si) != -1))
>              flags |= LOADER_FLAGS_SERIAL;
>      }
> Wouldn't want to break that. I wonder if we want LOADER_FLAGS_SERIAL 
> set or not. I wonder what other installers do...
> 
> I haven't looked at hijacking the tty driver, but I'm wary 
> for the same 
> reasons. I'd suggest not trying to take over anybody else's 
> major/minor.

If you have a COM port configured in Xen the default behaviour has been
for xencons to grab ttyS0. This has been the case for some time, and as
far as I'm aware this works smoothly with anaconda and other installers
-- at least I haven't heard of any complaints.

The problem I was alluding to in my checkin comments was when you have
both xencons and an 8250 driver. It's not possible for xencons to
register as ttyS0 in this instance, though registering as a high number
ttyS seems to work OK.

I considered registering a xencons specific major/minor e.g. ttyX0, but
that requires support in the distros, udev, devfs etc. Since the
intention is to loosely emulate a serial driver, I don't think what
we're doing is too evil. It seems to work and is good from a user
experience POV (excepting the 8250 and xencons case).

Ian








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