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

[Elliott Mitchell <ehem+xen@xxxxxxx>]

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documentation is out of date.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?

> 2) If there is no SPCR, how did Masami manage to boot Xen?
> I take without any serial output? Just with the framebuffer?

On my board the provided tables are sufficient to let Linux identify
ttyAMA0.  Linux's efifb driver finds the framebuffer and apparently has
it usable.


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