Re: [PATCH] serial: 8250: Clear CON_PRINTBUFFER on port re-registration

[Andy Shevchenko]

On Thu, Apr 16, 2026 at 2:32 PM Fushuai Wang <fushuai.wang@xxxxxxxxx> wrote:

> >> >> When two PnP devices map to the same physical port, the serial8250 driver
> >> >> removes and re-registers the console structure for the same port.
> >> >
> >> > Is it a real device out of there? Can you share what that is?
> >>
> >> Yes, it's a real device.
> >> In my Intel(R) Xeon(R) 6971P-C machine, the boot log shows:
> >>         [   17.242984] 00:04: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
> >>         [   17.251352] printk: console [ttyS0] disabled
> >
> >>         [   17.257934] serial 00:04: Runtime PM usage count underflow!
> >
> > This is strange, what kernel version is this?
>
> Please ingore this. I just test the patch in kernel 6.6. And this log
> has nothing to do with this problem. It was fixed in patch ed2761958ad7
> ("tty: serial: 8250: Fix another runtime PM usage counter underflow").

Thanks for clarification.

> >>         [   17.258360] 00:05: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
> >>         [   17.258516] printk: console [ttyS0] enabled
> >>         [   29.643013] serial8250: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A
> >>
> >> The issue occurs when BIOS "Serial Device" option is set to BMC:
> >>           Setup Question  = Serial Device
> >>           Help String     = Sets the Serial Device used to output bios serial log
> >>           Token   = 9003   // Do NOT change this line
> >>           Offset  = 2F8
> >
> > Is it an IO port?
>
> No, Offset = 2F8 is the offset in BIOS configuration space, not the I/O port.
>
> >>           Width   = 01
> >>           BIOS Default =[01]BMC
> >>           Options = *[01]BMC       // Move "*" to desired Option
> >>                     [02]S3M
> >>
> >> So 00:05 may be the BMC serial port device. From ACPI paths:
> >>         /sys/bus/pnp/devices/00:04/firmware_node/path: \_SB_.LPC0.UAR1
> >>         /sys/bus/pnp/devices/00:05/firmware_node/path: \_SB_.UAR1
> >
> > Do I understand correctly that both devices refer to the same physical
> > device?! How on earth is it supposed to work?
>
> Not exactly the same physical device, but they map to the same I/O port (0x3f8).
> They are different PnP devices with different ACPI paths:
>
>   00:04: \_SB_.LPC0.UAR1  (LPC COM1)
>   00:05: \_SB_.UAR1       (BMC serial port)
>
> When BIOS "Serial Device" option is set to BMC, both 00:04 and 00:05 are mapped to
> the same physical I/O port 0x3f8. And then:
>
>   1.00:04 is probed first and registers the console for port 0x3f8
>   2.00:05 is detected and also needs to use port 0x3f8

This is simply wrong. Do we ever support such a FW configuration?
Why on earth are there two devices for the same resource exposed to
the OS? It smells like a bug in BIOS.

>   3.Since port 0x3f8 is already in use, the driver must remove 00:04 first before
>     adding 00:05
>   4.This process involves console re-registration, which causes the entire
>     log buffer to be reprinted
>
> Sorry if my explanation is not clear - I don't work with serial subsystem often. :-)

Now it's elaborated well enough.

-- 
With Best Regards,
Andy Shevchenko