Re: [PATCH 19/19] Documentation: ACPI for ARM64

From: Arnd Bergmann
Date: Thu Aug 14 2014 - 16:53:42 EST

On Thursday 14 August 2014 11:27:23 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).

I would expect that Juno is a superset of what servers need. If this
ACPI patch set is sufficient to support every device present on Juno
with an ACPI firmware, what would be a strong indication that server
platforms work as well.

OTOH, if ACPI on Juno only supports half the features that the hardware
has, that doesn't tell us much about the suitability of ACPI for any
real-world system, server or not.

> > 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.
> Given the answer to (a) and what other features are needed, we may or
> may not mandate (b). We were pretty clear few months ago that (b) is
> still required but at the time we were only openly talking about ACPI
> 5.0 which was lacking many features. I think we need to revisit that
> position based on how usable ACPI 5.1 for ARM (and current kernel
> implementation) is. Would you mind elaborating what an ACPI-only
> platform miss?
> I would expect a new server platform designed with ACPI in mind to
> require minimal SoC specific code, so we may only see a few patches
> under drivers/ for such platforms adding ACPI-specific probing (possibly
> new drivers as well if it's a new component).

That is at least the hope, but I have no way of knowing whether it
works that way or not, other than seeing the actual patches.

It is rather annoying that we first have to wait for hardware
to become available to actually see that code, but I guess that
means that those hardware vendors no longer see ACPI as something
on their critical path, so we definitely shouldn't hurry things
until we understand the reason for the endless delay of platform
support patches.

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