Re: [PATCH 0/4] xen/arm: Unbreak ACPI

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 12 Oct 2020, Elliott Mitchell wrote:
> On Mon, Oct 12, 2020 at 12:02:14PM -0700, Stefano Stabellini wrote:
> > On Sat, 10 Oct 2020, Julien Grall wrote:
> > > Therefore, I think the code should not try to find the STAO. Instead, it
> > > should check whether the SPCR table is present.
> > 
> > Yes, that makes sense, but that brings me to the next question.
> > 
> > SPCR seems to be required by SBBR, however, Masami wrote that he could
> > boot on a system without SPCR, which gets me very confused for two
> > reasons:
> > 
> > 1) Why there is no SPCR? Isn't it supposed to be mandatory? Is it
> > because there no UART on Masami's system?
> 
> I'm on different hardware, but some folks have setup Tianocore for it.
> According to Documentation/arm64/acpi_object_usage.rst,
> "Required: DSDT, FADT, GTDT, MADT, MCFG, RSDP, SPCR, XSDT".  Yet when
> booting a Linux kernel directly on the hardware it lists APIC, BGRT,
> CSRT, DSDT, DBG2, FACP, GTDT, PPTT, RSDP, and XSDT.
> 
> I don't know whether Linux's ACPI code omits mention of some required
> tables and merely panics if they're absent.  Yet I'm speculating the list
> of required tables has shrunk, SPCR is no longer required, and the
> documentation is out of date.  Perhaps SPCR was required in early Linux
> ACPI implementations, but more recent ones removed that requirement?

I have just checked and SPCR is still a mandatory table in the latest
SBBR specification. It is probably one of those cases where the firmware
claims to be SBBR compliant, but it is not, and it happens to work with
Linux.