Re: [PATCH] ACPI / ARM64: Remove EXPERT dependency for ACPI on ARM64

[Will Deacon <will.deacon@arm.com>]

On Thu, Mar 31, 2016 at 03:20:03PM +0200, Ard Biesheuvel wrote:
> On 31 March 2016 at 14:36, Will Deacon <will.deacon@arm.com> wrote:
> > On Thu, Mar 31, 2016 at 02:04:05PM +0200, Rafael J. Wysocki wrote:
> >> On Thu, Mar 31, 2016 at 5:44 AM, Hanjun Guo <guohanjun@huawei.com> wrote:
> >> > On 2016/3/31 1:58, Mark Brown wrote:
> >> >> When ACPI was originally merged for arm64 it had only been tested on
> >> >> emulators and not on real physical platforms and no platforms were
> >> >> relying on it.  This meant that there were concerns that there might be
> >> >> serious issues attempting to use it on practical systems so it had a
> >> >> dependency on EXPERT added to warn people that it was in an early stage
> >> >> of development with very little practical testing.  Since then things
> >> >> have moved on a bit.  We have seen people testing on real hardware and
> >> >> now have people starting to produce some platforms (the most prominent
> >> >> being the 96boards Cello) which only have ACPI support and which build
> >> >> and run to some useful extent with mainline.
> >> >>
> >> >> This is not to say that ACPI support or support for these systems is
> >> >> completely done, there are still areas being worked on such as PCI, but
> >> >> at this point it seems that we can be reasonably sure that ACPI will be
> >> >> viable for use on ARM64 and that the already merged support works for
> >> >> the cases it handles.  For the AMD Seattle based platforms support
> >> >> outside of PCI has been fairly complete in mainline a few releases now.
> >> >>
> >> >> This is also not to say that we don't have vendors working with ACPI who
> >> >> are trying do things that we would not consider optimal but it does not
> >> >> appear that the EXPERT dependency is having a substantial impact on
> >> >> these vendors.
> >> >>
> >> >> Given all this it seems that at this point the EXPERT dependency mainly
> >> >> creates inconvenience for users with systems that are doing the right
> >> >> thing and gets in the way of including the ACPI code in the testing that
> >> >> people are doing on mainline.  Removing it should help our ongoing
> >> >> testing cover those platforms with only ACPI support and help ensure
> >> >> that when ACPI code is merged any problems it causes for other users are
> >> >> more easily discovered.
> >> >>
> >> >> Signed-off-by: Mark Brown <broonie@kernel.org>
> >> >
> >> > Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
> >> >
> >> >> ---
> >> >>  drivers/acpi/Kconfig | 2 +-
> >> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> >>
> >> >> diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
> >> >> index 82b96ee8624c..bf5dc1ac3446 100644
> >> >> --- a/drivers/acpi/Kconfig
> >> >> +++ b/drivers/acpi/Kconfig
> >> >> @@ -5,7 +5,7 @@
> >> >>  menuconfig ACPI
> >> >>       bool "ACPI (Advanced Configuration and Power Interface) Support"
> >> >>       depends on !IA64_HP_SIM
> >> >> -     depends on IA64 || X86 || (ARM64 && EXPERT)
> >> >> +     depends on IA64 || X86 || ARM64
> >> >>       depends on PCI
> >> >>       select PNP
> >> >>       default y
> >>
> >> OK
> >>
> >> What do the ARM64 maintainers think?
> >
> > I'm unsure about whether or not we want 'default y' here, but I'd like
> > to wait for Catalin to come back from his 2-week holiday before going
> > anywhere with this patch.
> 
> Ack to that, but ...
> 
> > It's only fair that his opinion should be
> > taken into account, and there's not a huge rush for this. I do consider
> > "ACPI-only platforms" as simply "platforms without a .dtb" (modulo
> > any significant AML code) and therefore don't view this as a blocking
> > issue.
> >
> 
> ... this patch will only affect those systems. DT-based systems, even
> if they provide an ACPI description as well, will not invoke the ACPI
> code unless you go out of your way to either boot without a DT or pass
> 'acpi=force' on the command line. On the other hand, it will make
> generic kernels (defconfig, etc) bootable on ACPI only systems, which
> currently require special kernel builds. Another important reason for
> this change is to get more build testing coverage for the arm64 ACPI
> code that we had so much fuss about, e.g, by the kbuild test robot.

I'd really like to get away from the concept of ACPI-only systems. Would
we reject a .dtb contribution for such a machine?

> > We should also take into account the large amount of ongoing ACPI work
> > (e.g. the DSD and PRP0001 saga, PCI and IORT support, virtualisation work,
> > etc), and decide whether or not that's currently in a state where we want
> > to encourage people to start using this in their arm64 kernels.
> >
> 
> The question is not about using it, the question is about
> incorporating it into the default build. The runtime opt-in is not
> going away with this patch.

I understand that, but I still think that removing the dependency on
EXPERT is indicative of saying "this stuff is good to be used by the
masses", irrespective of a cmdline option. Maybe that's true, but it's
not immediately obvious to me, with all the patches in flight.

Will