Re: [Linaro-acpi] [PATCH v5 18/18] Documentation: ACPI for ARM64

[Arnd Bergmann]

On Tuesday 06 January 2015 23:55:58 Jon Masters wrote:
> On 01/06/2015 05:06 PM, Jon Masters wrote:
> > Hi Arnd,
> > 
> > Happy New Year!
> > 
> > On 01/06/2015 02:21 PM, Arnd Bergmann wrote:
> >> On Tuesday 06 January 2015 11:24:43 Jon Masters wrote:
> >>> On 01/06/2015 06:20 AM, Catalin Marinas wrote:
> >>>
> >>>> Now, what's preventing a vendor firmware from providing only ACPI
> >>>> tables? Do we enforce it in some way (arm-acpi.txt, kernel warning etc.)
> >>>> that both DT and ACPI are supported, or at least that dts files are
> >>>> merged in the kernel first?
> >>>
> >>> I know of some (server) firmware that will only provide ACPI in the
> >>> medium term, so this is coming.
> >>
> >> Medium term is fine, as long as they are not expecting their hardware
> >> to be supported by Linux before ACPI support is stable enough for
> >> general consumption.
> > 
> > To be clear, I think that's reasonable for upstream. I may love ACPI,
> > but vendors can always ship kernels with a config supporting ACPI only
> > platforms in the interim period if they have a commercial justification
> > and that doesn't have to be supported in terms of the upstream default.

I would hope that none of the ACPI-only machines are meant to run Linux
as a primary operating system, that would be very sad.

Vendors that are interested in Linux support should instead work on getting
their hardware supported upstream so they don't need a private kernel to
match their private firmware.

> > But, perhaps there's a way to have it both ways, you could consider also
> > a CONFIG_EXPERT option that would allow you to build a kernel with ACPI
> > only support in the medium term. That way, if someone is running a
> > vendor kernel, but wants to track upstream development more closely,
> > they can do so on such hardware by enabling the expert config bit.

I don't see how this helps. The main point of requiring users to add
the option is to ensure that everyone understands the support is
experimental and not guaranteed to work across firmware releases or
kernel versions, until we have stopped making incompatible changes.

If someone is tracking the upstream kernel, they should know that they
have to get working DT support in first, and test both ways with the
same kernel anyway.

> Clarification: I'm suggesting that in the medium term the dependency
> upon CONFIG_EXPERT either goes away or is replaced with requiring ACPI
> and DTB in the non "expert" case

Sure, no debate on that.

> and requiring "expert" selected to allow a kernel that will boot with
> ACPI only. But only a suggestion.

This is a separate issue. I personally think we shouldn't bother with
this, as such a configuration would just break a lot of assumptions
we make today and removes hardware support, but it's something we
can discuss after we get to the point of having ACPI enabled by default.

	Arnd
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