UARTs not working on a Supermicro A2SAV / Linux works ;-(

Andre Albsmeier andre at
Tue Mar 13 17:14:34 UTC 2018

The UARTs on the brand new Supermicro A2SAV mainboard
are detected on 11.1-STABLE as

uart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 on acpi0

which is consistent with the BIOS settings.

Everything seems to work when it comes to setting of parameters and even
the DTR and RTS lines change state when opening cuau0 or cuau1, e.g. with

But sending or receiving characters fails -- to be exact, no single
character will be received and only the very first one will be sent to
the remote device.

We remember this behaviour from the good old times when we had to jumper
base port addresses and IRQs on ISA cards: If we got the IRQ wrong the
same effect happened: The first char could be sent because the TX register
was empty but the IRQ telling the kernel that it can send the next char
never arrived.

When using debug.uart_force_poll=1 in loader.conf it works (at least if
we type in characters slowly, of course).

So it really seems to have to do with interrupts again but I have no
idea where to look.

The A2SAV manual talks about the Super I/O being a Nuvoton NCT5523D. Can
it be that we miss some support for this? But if the devices really behave
like standard 16550s we shouldn't need any specific support at all, I think.

I disabled the corresponding ACPI functions by using

debug.acpi.avoid="\_SB_.PCI0.SBRG.UAR1 \_SB_.PCI0.SBRG.UAR2"

and the devices were detected properly as

uart0: <16550 or compatible> at port 0x3f8 irq 4 flags 0x10 on isa0
uart1: <16550 or compatible> at port 0x2f8 irq 3 on isa0

but this didn't help (same behaviour).

The really bad news are: Using Linux (Ubuntu 16.04) it works. Devices are
detected here as:
[    0.261209] pnp 00:01: [dma 0 disabled]
[    0.261295] pnp 00:01: Plug and Play ACPI device, IDs PNP0501 (active)
[    0.261774] pnp 00:02: [dma 0 disabled]
[    0.261868] pnp 00:02: Plug and Play ACPI device, IDs PNP0501 (active)
[    1.479082] Serial: 8250/16550 driver, 32 ports, IRQ sharing enabled
[    1.499962] 00:01: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
[    1.520943] 00:02: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A

and we can send and receive characters without problems.

So it really seems FreeBSD does something wrong on the A2SAV board when
it comes to interrupts of the serial ports.

Any ideas how to start?

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