Re: [Xen-devel] [PATCH RFC 00/19] arm64 EFI stub

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2014-07-10 at 10:33 -0700, Roy Franz wrote:
> On Thu, Jul 10, 2014 at 12:58 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
> wrote:
> > On Wed, 2014-07-09 at 11:24 -0700, Roy Franz wrote:
> >> >> +    if ( fdtfile->ptr )
> >> >> +        fdt_size = fdt_totalsize(fdtfile->ptr);
> >> >> +    else
> >> >> +    {
> >> >> +        /* RFRANZ_TODO - missing fdt_create_empty_tree() in libfdt. */
> >> >> +        return NULL;
> >> >> +    }
> >> >
> >> >
> >> > I don't think we currently support booting with no FDT at all, and even
> >> > in the ACPI world we would get one. So I think this would be an error.
> >> > IOW no need to worry about it.
> >> >
> >>
> >> I think that for the ACPI-only case, the EFI stub will be the one
> >> responsible for creating the FDT for passing the UEFI/ACPI to the XEN
> >> kernel.  (I had overlooked the ACPI-only
> >> case when considering the no-FDT case - for Linux we support this
> >> since a kernel could be configured such that it needs nothing from the
> >> FDT other than UEFI stuff, so we need to support it.)
> >> So I think we will need this, but only for the ACPI-only case.
> >
> > I think you probably know better than I but I thought that the way the
> > presence of ACPI was communicated to the kernel (whether via zImage or
> > UEFI stub) was via entries in the FDT's /chosen/ node containing the
> > address of the RSDP. Am I wrong about that?
> >
> > Or maybe I'm thinking about the UEFI services tables being referenced
> > via /chosen/?
> >
> > Ian.
> >
> The ACPI tables are all referenced from the EFI system table, which is put
> into the FDT by the EFI stub.  The EFI stub itself is completely ACPI 
> agnostic -
> it just passes along the EFI structures that may or may not contain ACPI 
> tables.
> The kernel will determine if ACPI support is present by looking for
> the ACPI tables
> in the EFI system table.

Some folks at the Linaro ACPI sprint explained this to me. I think I was
confusing the ACPI pointers with the EFI handle, but even then I was
wrong since this can be passed as a formal parameter to the UEFI entry
point without any DTB being required. So you are right ;-)

Ian.


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