sio0: port may not be enabled

Jeremy Chadwick koitsu at
Fri Apr 20 15:23:30 UTC 2007

On Fri, Apr 20, 2007 at 06:03:22PM +0300, Stefan Lambrev wrote:
> Hi list,
> I have some problems with one of my servers and need serial to 
> investigate more,
> unfortunately I have and problems with serial port too.
> from dmesg:
> sio0: configured irq 4 not in bitmap of probed irqs 0
> sio0: port may not be enabled
> sio0: <16550A-compatible COM port> port 0x3f8-0x3ff irq 4 flags 0x10 on 
> acpi0
> sio0: type 16550A
> I thought it can be a problem because of shared IRQs .. but vmstat -i 
> shows this:
> % vmstat -i
> interrupt                          total       rate
> irq4: sio0                             7          0
> irq15: ata1                           69          0
> irq20: atapci0                      3783         15
> irq21: uhci0 uhci*                     2          0
> cpu0: timer                       491698       1990
> Total                             495559       2006
> from /boot/device.hints :
> hint.sio.0.port="0x3F8"
> hint.sio.0.flags="0x10"
> hint.sio.0.irq="4"
> uname -srm
> FreeBSD 6.2-STABLE amd64
> I checked my BIOS - both irq 3&4 are reserved for ISA,
> and com port 1 is setup to use irq4 port 3F8.
> What else can I check?

There's nothing wrong with your BIOS configuration, nor is it related
to shared IRQs  The issue really isn't your fault.

Look closely at the dmesg line, note what device sio0 is claiming to be
associated with (acpi0, not isa0):

> sio0: <16550A-compatible COM port> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0

This is one of the drawbacks to using ACPI.

Some systems apparently tie the serial port to ACPI functionality in a
different way.  For example, I have a couple boxes which have sio0
attached to acpi0 that work fine.  In some other cases, I have ones
which result in a non-working serial port unless I disable ACPI (thus
sio0 shows up as being attached to isa0).

FreeBSD has iffy support for sio attached to ACPI.  I don't know if this
has changed much, but it's been on the to-do list for quite some time,
and I don't know why it hasn't gotten more attention (no offence
intended); serial console is an absolute pre-requisite in this day and

The only way to get around this is to disable ACPI entirely, either in
your BIOS or in FreeBSD.

| Jeremy Chadwick                                    jdc at |
| Parodius Networking                  |
| UNIX Systems Administrator                      Mountain View, CA, USA |
| Making life hard for others since 1977.                  PGP: 4BD6C0CB |

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