Re: [PATCH] set altix preferred console

[Mark Maule <maule@sgi.com>]

On Tue, Oct 04, 2005 at 01:07:14PM -0600, Bjorn Helgaas wrote:
> > Our prom currently does not build up ACPI tables describing PCI (or any
> > IO)
> > devices.  We are working toward that goal, but it is still
> > work in progress and will be some time yet before this is done.
> >
> > It is because of this shortcoming that we have to rely on the PCDP table
> > to provide the io addresses needed for VGA console.
> >
> > Outside of PCDP, our earliest opportunity to get information about VGA
> > devices is through the call sequence initiated with
> > subsys_initcall(sn_pci_init), which would seem way too late for early boot
> > messages.
> 
> It's the same for us.  We don't find out about our built-in serial
> ports until the 8250 driver initializes, which is pretty late.  So
> we have "8250_early" that uses information from the PCDP to talk
> to the serial port before the normal 8250 driver discovers it.
> The important thing is that the PCDP doesn't influence the naming
> of the device (and there isn't really a "name" for VGA devices,
> AFAIK).

Ok.  I was assuming that tty0 implied a vga device, but I guess tty0 is
really the VT driver which drives whatever conswitchp is pointing to ...

> 
> > Out of curiousity, what are the mmio_tra/ioport_tra fields in the PCDP PCI
> > interface structure intended for if not for this purpose?
> 
> You *can* use the _tra fields in the PCDP -- that's what they're
> there for.  But the PCDP should be *optional*, and I think it's
> not for your current setup.
> 
> If we don't build pcdp.o on an HP box, the console still works
> just fine; it just doesn't work until the 8250 driver discovers
> the port, which is fairly late.
> 
> So I think all you need to do is add some SN-specific code
> somewhere that discovers the VGA device.  It sounds like the
> only way to do that right now is to get the information from
> the PCDP, so I guess that's what you need to do.  But having
> this duplicated in the SN code means that you won't need the
> change to pcdp.c.
> 

Ok.  One other hitch is that the sn prom doesn't guarantee that all
VGA cards will have their firmware executed.  The only vga cards guaranteed
to be init'd are the ones present in the pcdp table which could be a
subset of the vga cards in a system.  Were it not for this, we could
just pick one (say lowest segment/bus number or some such thing) during
sn pci discovery.  But we'll have to scan the pcdp in order to see what
cards are available (unless there's some other standard way to tell if a
vga card is initialized).

Mark