[PATCH 19/19] Documentation: ACPI for ARM64

[Hanjun Guo]

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).

> 
> I think SBSA is too vague to guarantee a kernel image running on a
> compliant platform without additional (vendor-specific) tweaks. So what
> I asked for is (1) a document (guide) to define the strict set of ACPI
> features and bindings needed for a real SoC and (2) proof that the
> guidelines are enough for real hardware. I think we have (1) under
> review with some good feedback so far. As for (2), we can probably only
> discuss Juno openly. I think you could share the additional Juno patches
> on this list so that reviewers can assess the suitability. If we deem
> ACPI not (yet) suitable for Juno, is there other platform we could see
> patches for?

Ok, we will send out all the patches for Juno in next version for review,
as mentioned above, we still need more time to clean up the code.

> 
>>> 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?
>>
>> Do you mean something still missing? We still miss some features for
>> ARM in ACPI, but I think they are not critical, here is the list I can
>> remember:
>> - ITS for GICv3/4;
>> - SMMU support;
>> - CPU idle control.
> 
> I agree, these are not critical at this stage. But they only refer to
> architected peripherals. Is there anything else missing for an SoC? Do
> we need to define clocks?

No, I prefer not. As we discussed in this thread before, we don't need
clock definition if we use SBSA compatible UART on Juno.

> 
>> 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

> 
>>> 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).
>>>
>>>> For this patch set, DT is the first class citizen at now:
>>>>
>>>> a) We can always set CONFIG_ACPI as off in Kconfig, and use DT only;
>>>
>>> Not just off but, based on maturity, depend on EXPERT.
>>
>> Ok. And don't set ACPI default off (pass acpi=on to enable it)?
> 
> That's my view, just make it clear ACPI is experimental at the Kconfig
> level because longer term we won't mandate SoCs to provide both DT and
> ACPI tables.

I agree with you that if we set ACPI default off, firmware will always
pass acpi=on if they want to use ACPI, so I think it would be better
to depend on EXPERT instead.

Olof, is it ok to you too?

Thanks
Hanjun