Re: [PATCH v3 2/7] dt-bindings: serial: 8250: aspeed: add aspeed,vuart-over-pci bool prop

[Grégoire Layet]

Hi Krzysztof,

> > It's a Virtual UART. Internally, it's two FIFOs accessible via
> > 8250-compatible register sets on both ends.
>
> I do not know what is Virtual UART...

Yes the name of ASPEED is a bit weird, it means that it is not a real
UART. It is two 8250-compatible register sets, and the data path is
handled by two hardware FIFOs (one per direction).

> > There is 4 Virtuals UARTs on the LPC bus of the AST2600 and 2 of them
> > are bridged over the PCI bus.
> > So, from the host, you can access the 8250 register set on the PCI bus.
>
> You mean these appear (or are) as PCI devices?

Yes but not as a UART devices.
The two VUART are accessible under the same PCIe BMC device, at the
VUART addresses configured on the BMC. They are accessible on the BAR1
of the PCIe BMC device.
There is a 2 bit left shift between the addresses of the registers on
the BMC LPC bus and the addresses accessible over the PCI device.
0x3F8 on the BMC is accessible at BAR1 0xFE0 over the PCI device.

The PCIe BMC Device is one PCI device where you can access several
peripherals. It's not one PCI device per VUART.

For example, you can also enable the KCS channel 4 and it will be also
accessible via the BAR1 of the PCI BMC device. KCS is a protocol used
for IPMI.

Only these two peripherals can be accessed over the PCI BMC device and
they need to be enabled.
The host cannot access everything on the LPC bus of the BMC.

Best regards,
Grégoire