ACPI vs DT at runtime

[Rob Herring]

On Sun, Nov 17, 2013 at 11:19 PM, Jon Masters <jonathan at jonmasters.org> wrote:
> Olof, thanks for starting this thread. Mark, thanks for the followup.
>
> Comments on both inline, below. But before I reply to the specific
> points, let me add that this is clearly an emotional topic for many.
> There are a great many companies involved in ARMv8 on servers, and a
> great many have yet to say anything in public. I am trying to strike a
> balance constantly by being fair to those who have announced and those
> who have yet to do so. But regardless, we have one chance here to make a
> good server platform that can challenge the incumbent architectures. If
> I weren't utterly convinced of that, and of the need for such standards
> as UEFI and ACPI, then I wouldn't be so vocal about it. Given who is
> involved in this space, regardless of a decision to adopt ACPI now or
> later, it is coming. It can be done right now, or not.
>
> I (and others) pushed 3 years ago for the adoption of ACPI. Dong, and
> others instigated the legal processes that resulted in the movement of
> ACPI under UEFI Forum recently, to become a fully open standard that can
> be shaped - both by the Linux community, and by others. ACPI.next will
> benefit from the same development process that has shaped UEFI standards
> over the past few years, and most people here can easily get involved in
> shaping that standard - as they can on x86 as well now.
>
> I am pushing for a few other things to become public that will help to
> explain why ACPI is being adopted and provide a standardized description
> of the ways in which it will be used/consumed.
>
> On 11/15/2013 04:57 AM, Mark Rutland wrote:
>> On Fri, Nov 15, 2013 at 01:44:10AM +0000, Olof Johansson wrote:
>>> The more I start to see early UEFI/ACPI code, the more I am certain
>>> that we want none of that crap in the kernel. It's making things
>>> considerably messier, while we're already very busy trying to convert
>>> everything over and enable DT -- we'll be preempting that effort just
>>> to add even more boilerplate everywhere and total progress will be
>>> hurt.
>
> Firstly, I would note that I don't expect DeviceTree to go away. Only on
> server class systems. I expect all server class ARMv8 systems in the
> Enterprise/Cloud environment to boot using UEFI and ACPI. This is
> certainly the case of most future design starts already underway. These
> can either be supported properly, or not, but ignoring the impending
> ACPI systems isn't practical. Translation won't work reliably either.
>
> For the record, I did suggest to Applied that the first posting of that
> SATA driver not have the ACPI bits in (since we know it needs cleaning
> up to use the key/value approaches already discussed, and so on), but
> after chatting with Loc about it, it seemed reasonable to use the
> opportunity to start a discussion - which seems to be on cue here.

That and the Exynos SATA support are great examples of how ACPI won't
help solve anything. Both are "standard AHCI" yet require lots of code
at runtime that ACPI is simply not going abstract away. Even we had to
do modifications for highbank due to an errata. What's to keep the
next gen chips from changing because they will. New process technology
will mean new phy which will no doubt have different programming or
quirks.


>> I'm of the opinion that the only way we should support ACPI is as a
>> first-class citizen.
>
> There really isn't another way to do it in my opinion.
>
>> We don't need to modify every driver and subsystem
>> to support ACPI, only those necessary to support the minimal set of
>> platforms using ACPI. ACPI is new in the arm space, and we can enforce
>> quality standards on ACPI _now_ above what we're allowing for DT, and
>> avoid future problems.
>
> This is key. It's not going to be ACPI everywhere. It's going to be ACPI
> on server class systems. And later, maybe some client systems. But the
> big push is from the server crowd. We need to build systems that in 5-10
> years time can still boot an older kernel. This can't be done without a
> standardized/versioned enumeration/discovery mechanism like ACPI that
> has an API enshrined in stone as far as compatibility. Device Tree is
> wonderful, anyone can make a binding and use it. Or change the binding
> in the next kernel release. Or...this doesn't work in the server space.
> Server platforms aren't vertically welded shut like in the embedded
> space, where DeviceTree has brought all kinds of benefits for those
> building with a single kernel for many different targets, but has also
> seen a huge amount of churn from one kernel to the next. If I counted
> the number of times I've been told "just update your dtb"...then I would
> be shivering in the corner a sadden wreck. You can't "just update your
> dtb" on a server class system. You shouldn't.

I think people are being misled that ACPI somehow solves the problem
of new systems just work. It doesn't. It is having a single platform,
tons of legacy h/w or emulation of the h/w, and the ability to trap
into the firmware. That all takes upfront design and verification
requirements to achieve. This is not how the ARM world works. Yes, we
have secret documents attempting to standardize things, but they are
hardly sufficient. Using AHCI for SATA is specified, but the above
example shows why this is insufficient. Granted, those chips were not
designed to target any standardization doc, but I doubt anyone reads
the doc and believes they are not already compliant. When it comes
down to changing the kernel or spinning the Si, vendors are going to
do the former.

> But anyway, emotional plea aside, a very large number of ACPI systems
> are coming. Yes, I've been pushing to get existing players to switch,
> but that's because I know what is coming. And if you want certain other
> players to appear in this space, you'll need to have ACPI for them, too.

Well, first a large number of DT systems are coming because ACPI is
simply not ready. Yes, some of the core ACPI support is getting in
place, but until how to deal with it in drivers is worked out drivers
are going to use DT. As this thread shows, there is currently nothing
more than ideas thrown around of how to deal with both.

>> There may even be things which we don't have to deal with at all on ACPI
>> systems as used in servers (e.g. clock management), but perhaps we will
>> if people see value in those elements.
>>
>>> The server guys really want UEFI for their boot protocols,
>>> installation managers, etc, etc. That's fine, let them do that, but
>>> that doesn't mean we need to bring the same APIs all the way into the
>>> kernel.
>>>
>>> So, I'm strongly urging that whatever the server guys try to do, it
>>> will in the end result in the ACPI data being translated into DT
>>> equivalents, such that the kernel _only_ needs to handle data via DT.
>>
>> I'm not sure that translating ACPI tables to dt makes any sense. If AML
>> is used (even sparingly), I do not believe that we can do any sensible
>> conversion to device tree. My understanding is that AML includes
>> functionality for modifying ACPI tables, and I don't see how we can
>> possibly support that without having to add a lot of boilerplate to all
>> the code handling AML to add a device tree backend...
>
> AML includes code that is runtime interpreted, for things like power
> button emulation and the like. You can't just translate this. This comes
> up every few years - it's impractical. You'll end up having to ship both
> DTB and ACPI tables for a system. Which means two tables for a platform
> vendor to get right. You know what will happen - only one table with be
> correct. Perhaps it seems that it will be the DTB that is more correct,
> and this might be true this week, but by 2016 I *guarantee* you that the
> ACPI table will be the one winning.

Perhaps we spend so much pointless effort on ACPI conversion rather
than areas that really benefit end users that we win the battle and
lose the war.

Rob