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

[Fushuai Wang <fushuai.wang@linux.dev>]

>> >> 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").

>
>>         [   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
  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. :-)

-- 
Regards,
WANG