[PATCH 19/19] Documentation: ACPI for ARM64

[Mark Rutland]

On Mon, Aug 18, 2014 at 10:29:26AM +0100, Hanjun Guo wrote:
> On 2014-8-15 18:01, Catalin Marinas wrote:
> > Hanjun,
> 
> Hi Catalin,
> 
> > 
> > On Fri, Aug 15, 2014 at 10:09:42AM +0100, Hanjun Guo wrote:
> >> On 2014-8-14 18:27, Catalin Marinas wrote:
> >>> On Thu, Aug 14, 2014 at 04:21:25AM +0100, Hanjun Guo wrote:
> >>>> On 2014-8-14 7:41, Rafael J. Wysocki wrote:
> >>>>> On Tuesday, August 12, 2014 07:23:47 PM Catalin Marinas wrote:
> >>>>>> If we consider ACPI unusable on ARM but we still want to start merging
> >>>>>> patches, we should rather make the config option depend on BROKEN
> >>>>>> (though if it is that unusable that no real platform can use it, I would
> >>>>>> rather not merge it at all at this stage).
> >>>>>
> >>>>> I agree here.
> >>>>>
> >>>>> I would recommend creating a separate branch for that living outside of the
> >>>>> mainline kernel and merging it when there are real users.
> >>>>
> >>>> Real users will coming soon, we already tested this patch set on real hardware
> >>>> (ARM64 Juno platform),
> >>>
> >>> I don't consider Juno a server platform ;) (but it's good enough for
> >>> development).
> >>>
> >>>> and I think ARM64 server chips and platforms will show up before 3.18
> >>>> is released.
> >>>
> >>> That's what I've heard/seen. The questions I have are (a) whether
> >>> current ACPI patchset is enough to successfully run Linux on such
> >>> _hardware_ platform (maybe not fully optimised, for example just WFI
> >>> cpuidle) and (b) whether we still want to mandate a DT in the kernel for
> >>> such platforms.
> >>
> >> For (a), this patch set is only for ARM64 core, not including platform
> >> specific device drivers, it will be covered by the binding of _DSD or
> >> explicit definition of PNP ID/ACPI ID(s).
> > 
> > So we go back to the discussions we had few months ago in Macau. I'm not
> > concerned about the core ARM and architected peripherals covered by ACPI
> > 5.1 (as long as the current patches get positive technical review). But
> > I'm concerned about the additional bits needed for a real SoC like _DSD
> > definitions, how they get reviewed/accepted (or is it just the vendor's
> > problem?).
> 
> As the _DSD patch set sent out by Intel folks, _DSD definitions are just
> DT definitions. To use _DSD or not, I think it depends on OEM use cases,
> we can bring up Juno without _DSD (Graeme is working on that, still need
> some time to clean up the code).

Let's not confuse matters by saying that _DSD is DT. DSD allows for
key-value pairs, and has a reference mechanism roughly equivalent to
phandles. That does not make them the same thing.

Not having any guidelines for vendors is an extremely bad idea. The DT
bindings we get a chance to review often have major issues. I do not
believe that vendors will do things sanely without good guidance and
strong incentives.

[...]

> >> For ACPI 5.1, it fixes many problems for ARM:
> >> - weak definition for GIC, so we introduce visualization, v2m and
> >>   part of GICv3/4 (redistributors) support.
> >> - No support for PSCI. Fix it to support PSCI 0.2+;
> >> - Not support for Always-on timer and SBSA-L1 watchdog.
> > 
> > These are all good, that's why we shouldn't even talk about ACPI 5.0 in
> > the ARM context.
> > 
> >> - How to describe device properties, so _DSD is introduced for
> >>   device probe.
> > 
> > For the last bullet, is there any review process (at least like what we
> > have for DT bindings)? On top of such process, do we have guidelines and
> > example code on how the Linux support should be implemented. As Olof
> > mentioned, should we see how the DT and ACPI probing paths work
> > together? I really think we should be very clear here and not let
> > vendors invent their own independent methods.
> 
> As said above, Intel folks provided some good examples for that, and
> clarified a lot of things:
> 
> https://lkml.org/lkml/2014/8/17/10

Quite frankly, the examples provided in the _DSD series are atrocious.
They constitute a trivial mapping of some existing DT bindings to ACPI
which do not appear to have gone through any sort of review w.r.t.
remaining idiomatic.

Thanks,
Mark.