Re: [PATCH] serial: 8250_pci: Add support for ASPEED BMC VUARTs over PCIe

[Andy Shevchenko]

On Thu, Oct 8, 2026 at 4:27 PM Phil Rosenthal via B4 Relay
<devnull+phil.phil.gs@xxxxxxxxxx> wrote:
>
> From: Phil Rosenthal <phil@xxxxxxx>
>
> On systems with an ASPEED BMC, the IPMI Serial over LAN (SOL) console
> shows the firmware and the boot loader but goes silent once Linux
> starts. The UART that the BMC bridges to SOL is not a legacy COM port:
> it sits behind a PCIe function that no driver claims.
>
> ASPEED BMCs such as the AST2600 present this "BMC device" function to
> the host (1a03:2402, class 0c0701). Besides KCS, the BMC firmware can
> expose up to two 16550-compatible virtual UARTs through it, and host
> firmware redirects its console to them, for example "IPMI Card SOL/COM2
> (Pci Bus9,Dev1,Func0,Port0)" in the AMI setup of the ASUS Pro WS
> W680-ACE IPMI. Register these UARTs so that the kernel console and a
> getty can be used over SOL.
>
> The UART registers are in BAR1 at the VUART's LPC I/O address shifted
> left by two, one 32-bit register per 4 bytes: VUART0 at 0xfe0
> (0x3f8 << 2) and VUART1 at 0xbe0 (0x2f8 << 2). A VUART that the BMC has
> not enabled reads back as all ones; skip it so that the ports present
> are numbered contiguously. If the BMC exposes no VUART at all, decline
> the device so that it stays available to other drivers such as ipmi_si,
> whose PCI table matches the same class code.
>
> The function has no INTx pin and advertises 32 MSI vectors. On the ASUS
> board (BMC firmware 1.3.13) no interrupt was delivered on MSI vector 16,
> which is the vector ASPEED's own host driver uses for VUART0: transmit
> stalled and nothing was received. Poll the ports instead.
>
> The same function also exposes the BMC's KCS interface in BAR1.
> 8250_pci does not request the BAR, so an ipmi_si interface described by
> firmware (ACPI or SMBIOS) can still use those registers. ipmi_si's PCI
> driver, which matches the function by class code, can no longer bind it
> while a VUART is enabled.
>
> Tested on an ASUS Pro WS W680-ACE IPMI (BMC firmware 1.3.13): the kernel
> console and a getty run on ttyS5 over IPMI SOL, while a non-PCI ipmi_si
> interface uses the KCS registers in the same BAR.
>
> The register layout and MSI vector assignment follow the host driver
> from ASPEED's SDK as posted in 2023, which was not merged.

There is already a driver for this 8250_aspeed_vuart.c. Please, do not
add new quirks to 8250_pci.c.

-- 
With Best Regards,
Andy Shevchenko