[PATCH 19/19] Documentation: ACPI for ARM64

[Olof Johansson]

On Mon, Jul 28, 2014 at 11:12:29AM +0100, Mark Rutland wrote:
> On Mon, Jul 28, 2014 at 10:07:50AM +0100, Arnd Bergmann wrote:
> > On Saturday 26 July 2014 19:34:48 Olof Johansson wrote:
> > > On Thu, Jul 24, 2014 at 6:00 AM, Hanjun Guo <hanjun.guo at linaro.org> wrote:
> > > > +Relationship with Device Tree
> > > > +-----------------------------
> > > > +
> > > > +ACPI support in drivers and subsystems for ARMv8 should never be mutually
> > > > +exclusive with DT support at compile time.
> > > > +
> > > > +At boot time the kernel will only use one description method depending on
> > > > +parameters passed from the bootloader.
> > > 
> > > Possibly overriden by kernel bootargs. And as debated for quite a
> > > while earlier this year, acpi should still default to off -- if a DT
> > > and ACPI are both passed in, DT should at this time be given priority.
> > 
> > I think this would be harder to do with the way that ACPI is passed in
> > to the kernel. IIRC, you always have a minimal DT information based on
> > the ARM64 boot protocol, but in the case of ACPI, this contains pointers
> > to the ACPI tables, which are then used for populating the Linux platform
> > devices (unless acpi=disabled is set), while the other contents of the
> > DTB may be present but we skip the of_platform_populate state.
> > 
> > If this is correct, then replacing the firmware-generated dtb with a
> > user-provided on would implicitly remove the ACPI tables from visibility,
> > which is exactly what we want.
> 
> That's not quite true.
> 
> There might not be any DTB, or the user might have provided a DTB with
> only /chosen/bootargs. Trying to distinguish the many cases of possible
> DTB is broken as a concept.
> 
> The EFI stub will attempt to get a DTB from somewhere (provided by
> filename or as a system table with a magic UUID), and if it can't find
> one will generate an empty stub DTB.
> 
> Then it will go and perform some EFI memory setup, placing some
> properties in the DTB (which might be a stub) describing the EFI memory
> map.
> 
> Then we boot the kernel proper, which sees the EFI pointers and looks
> for an ACPI table. If they are found, ACPI is used. Otherwise we attempt
> to use the DTB (which might be a stub).
> 
> Unless ACPI is explcitly disabled, ACPI will be used if present.

Ah, I saw this after I replied to Arnd's email.

The above sounds more like how I envisioned things working, so based on that,
the default just needs to be flipped and we should be fine (i.e. ACPI needs to
be explicitly enabled).


-Olof