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

[Nate Lawson]

Bruce Evans wrote:
> On Thu, 3 Aug 2006, Bruce Evans wrote:
>> 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.
> 
> Bruce

Please see the thread titled "acpi on msi-9218 (-current) swaps sio0 and 
sio1" on the freebsd-acpi@ list.  John Baldwin's approach is the one I 
like, where hints can be used to wire device unit numbers to a 
particular device with a bus-specific string.  For instance, 
hint.fxp.0.location="0:4:0" would mean a bus/slot/function to PCI and be 
meaningless to other busses.  Another example: 
hint.sio.0.location="COMA" would mean a Device node in ACPI named "COMA" 
would be wired to sio0 and meaningless to other devices.

As marcel@ pointed out, for serial ports in particular, you want the 
resource of I/O port as the primary identifier.  So the default hint 
would be something like hint.sio.0.location="io=0x3f8".

Other than some minor logic to allocate device units in newbus, the only 
change would be to decide on location naming schemes for the relevant 
bus types.  Something like "io=0x3f8" might always match resources via 
RIDs and be bus-independent.  I haven't thought much about the best 
naming scheme but I like the general idea of attaching a named ID to a 
unit to wire it to a known device, where the name means something to 
only a particular bus.

-- 
Nate