ACPI vs DT at runtime

[Olof Johansson]

On Fri, Nov 15, 2013 at 09:57:17AM +0000, 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.
> 
> We are certainly under a lot of pressure with the device tree migration,
> and I agree that adding another information source is going to be a
> source of pain. However, I'd argue that we're going to encounter pain
> regardless of which approach we take.
> 
> I'm of the opinion that the only way we should support ACPI is as a
> first-class citizen. 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.

It's obvious from the very first submission, from a vendor that has worked
closely with "the community" (i.e. one enterprise distro at least), that this
is completely new territory for _everyone_ involved. There's no way that we can
enforce quality standards for ACPI yet -- we don't know what they look like!
Nobody knows how to design a good ACPI implementation or binding.

Oh wait, there's people who have been doing this for years. Microsoft. They
should be the ones driving this and taking the pain for it. Once the platform
is enabled for their needs, we'll sort it out at our end. After all, that has
worked reasonably well for x86 platforms.

If we knew exactly what we wanted, it'd be a different story. _We
don't_. We're into year FOUR of the device tree conversion and we're just
now reaching a point where we think we know what a good solution looks
like. The first two years were easy -- they were the trivial devices we're
now talking about on ACPI. After that it got harder. Going through all
of that again with ACPI? No. No way. Microsoft gets to do it and while
they're busy sorting it out, we'll boot with device tree.

Once they're done, we'll figure out how to enable new hardware. Sure, someone
needs to keep them sane and participate in the standardization process, but we
don't have to drag the whole kernel through it.

> 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.

It's not about seeing value, it's about trying to shoehorn an existing
implementation into ACPI right now. People don't program with the ACPI
mindset since that's not what they're used to. So everybody needs to
re-learn everything all over again.

The clock issues still have to be dealt with in ACPI _somewhere_,
and they likely have to be dealt with by the same software team at the
vendors that do Linux kernel work. So by forcing them to figure out how
to do it in ACPI, we're distracting from their efforts to support their
hardware better in the kernel in the first place. That's not win-win,
it's lose-lose. Or rather, it's lose for all of us, and win for Microsoft
since they are DoS:ing us.

> > 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...

We can definitely modify device tree contents at runtime, it's just that
nobody besides the POWER server guys are doing that today. So that's
not a strong argument in ACPI's favor. We should fix device-tree where
needed instead.

Also, we can either have some of our better people sort out the ACPI-to-DT
translation and management, and get it done right, or we can rely on all the
vendors to get ACPI right in all their drivers. While some of them will,
I suspect we'll be more successful driving this from a common place. It
also gives us a place to stick all the fixups for broken firmware,
since the first generations of ARM servers are bound to have them.

And best of all, it allows us to continue focus on device tree and
getting that done right, than splitting all efforts into two. We can't
affort that right now.

> > Just like PowerPC scrapes the OpenFirmware client interface to build a
> > flat device tree, we should add a pre-boot stage that scrapes
> > ACPI/UEFI data and constructs an appropriate device-tree. We can still
> > bring over ACPI methods and represent those in the DT, but we should
> > _not_ have two native interfaces.
> 
> I'm not sure the two cases are comparable. The format of the FDT was
> designed to encode the data structure used by OpenFirmware, and thus the
> two map to each other pretty well. I do not believe that mapping from
> ACPI tables to an FDT blob is anywhere near as simple, and as I mention
> above I do not believe that we can just copy over the ACPI methods in
> isolation.

Sorry, I wasn't implying that there's a one-time trivial conversion
to be made in the generic case, but it can still be done in a similar
manner even though it will be more complex.

Nobody is expecting there to be zero work for new drivers with ACPI;
after all, the driver itself still has to be written. We're talking differences
from board to board and system to system here, which we can definitely handle
through a translator as well.

And, as you say, if the first platforms are going to be trivial and easy to
implement with ACPI, then doing a translation for them will be simple too.

Shipping a firmware with ACPI is really no different from shipping a firmware
with a flashed device tree. Whatever bugs or strange things the vendor chooses
to do in there, we'll have to live with forever.

We all know DT considerably better to a point where I would recommend
that they flash a DTB in their UEFI firmware instead of go with ACPI. For
simple hardware and basic devices we've got most bindings sorted out by
now, and we've decided on backwards compatibility from here on out.

> > It might be done via having a skeleton/framework DT for the vendor
> > platform that is updated via UEFI/ACPI data, or it might be
> > constructed entirely out of tables coming from firmware. I don't care
> > about the methods for how it is done, but I do feel strongly that we
> > should _not_ introduce a second API for everything. I can't think of a
> > single good reason to do it.
> 
> Where does this skeleton/framework come from? Within the kernel?

Since there might need to be runtime modifications done to the tree based on
ACPI events, it seems to make sense to do it in the kernel, so that translation
state and such can be kept around for use by the runtime modification parts.

Having it extractable out of the kernel tree for testing purposes would be
greatly appreciated, so it can be ran through valgrind, add testcases, etc.

> That
> sounds like an arcane board file equivalent, and is counter to the
> entire reason for using UEFI and ACPI -- having a well-defined
> (excluding particular driver bindings, and I'm not arguing well-defined
> means nice) stable standard that allows the kernel to boot on an
> arbitrary platform without requiring arbitrary platform-specific code
> everywhere in the boot path.
> 
> It might not be pretty, and it will certainly require a lot of work, but
> I'd prefer it at least for a semblance of uniformity.

That is a red herring -- that booting standard has _nothing_ to do with
ACPI. Once you've got a standard that is well-defined enough like that,
you no longer need the runtime portions of ACPI to deal with it. You
can just hardcode it. You need a way to probe _which_ type of standard
is used, but from there on out you can assume that things are the same
way.

> I think that trying to shoe-horn ACPI-derived information into device
> tree is fundamentally the wrong approach. I don't think it encourages
> best practices, and I don't think it's beneficial to the long term
> health of Linux or the ecosystem as a whole.

There are no known best practices with ACPI. It's a complete fumbling
around in the dark right now. We're complaining about reviewer bandwith
on DT -- do we have a single reviewer for ACPI on ARM that actually
knows what a good solution looks like? I don't think so.

So, until there's strong evidence that ACPI is actually going to be
a sane solution, in other words, until someone has shipped a system
that uses it (with Windows) and does it successfully without being
a hot mess, we shouldn't litter the kernel with it.

Vendors can standardise of UEFI if they want, but I would much rather
see them bundle DTB images with their firmware today, than rely on early
buggy and still-early-on-the-learning-curve ACPI bindings that we then
have to live with forever.


-Olof