i386/100831: sio ignores BIOS information about serial ports - bounty offered

Bruce Evans bde at zeta.org.au
Wed Aug 2 19:54:02 UTC 2006

On Thu, 3 Aug 2006, Bruce Evans wrote:

> On Wed, 2 Aug 2006, Jo Rhett wrote:
>>>> 4. There should be no ill effect from doing this.  Console will never 
>>>> break
>>>> and go to the wrong port.
>>> Yes, "should be".  The wiring should be fairly deterministic once you
>>> get it to work.  I think it will continue to work even if someone fixes
>>> (?) ACPI to prefer the hints order to the ACPI order.  However, it would
>>> break if someone fixes (?) ACPI to prefer the BIOS order to both the hints
>>> order and the ACPI order (I think ACPI is using its own order and doesn't
>>> know that you've swapped the order in the BIOS).
>> I'm not sure I understand the difference.  The ACPI order is the order that 
>> the BIOS presents them in when querying it via ACPI, yes?
> I don't completely understand this either.  I think there is a non-ACPI part
> of the BIOS that FreeBSD (or all OS's doesn't see).  As I understand your
> configuration, you start with COM1 and COM2 at the usual places but don't
> want to use COM1 so you change the BIOS settings for COM2 to the usual ones
> for COM1 and maybe vice versa.  This changes soft jumpers or whatever is
> needed for FreeBSD to see it.  Then you use a BIOS option to swap the ports
> so that everything is supposed to see COM2 as COM1.  The boot loader sees
> this but ACPI in FreeBSD doesn't.  I think this changes is only made in
> some BIOS table that ACPI in FreeBSD doesn't know about.  In my test, I
> only swapped the settings of COM1 and COM2 in the BIOS, since I couldn't
> find a BIOS option to swap the unit number assignments, so it was not
> completely incorrect for ACPI in FreeBSD to swap the settings.

More details on my test:
- after swapping the settings of COM1 and COM2, both FreeBSD-ACPI and WinXP
    see only the settings swapped.  WinXP didn't report any changes to the
    devices.  I now think this is completely correct.  Swapping like this
    requires swapping in device.hints to keep the same assignment of physical
    devices to unit numbers.
- after swapping the settings and then disabling COM1, FreeBSD-ACPI moves
    COM2 down to COM1 (sio0) and sio1 goes away, while WinXP doesn't change
    unit numbers and COM1 goes away.  I think the WinXP behaviour is correct
    and FreeBSD-ACPI just allocates unit numbers starting at 0.


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