Re: ACPI vs DT at runtime

On Tue, Nov 19, 2013 at 11:30:15AM +0000, Mark Rutland wrote:

On Fri, Nov 15, 2013 at 05:52:41PM +0000, Olof Johansson wrote:

quoted

On Fri, Nov 15, 2013 at 09:57:17AM +0000, Mark Rutland wrote:

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On Fri, Nov 15, 2013 at 01:44:10AM +0000, Olof Johansson wrote:

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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.

If we don't know what quality standards we require for ACPI, then I
would rather see ACPI delayed until we are more comfortable with it than
to enforce an arbitrary set of rules required to enable mapping it to
device tree.

That's essentially what we're saying here: ACPI isn't ready, let's use DT while
it's sorted out and if someone wants to try to require ACPI at some level
today, they should figure out how to translate it into DT. In reality, it might
be easiest to ship a static DT file while ACPI is being developed and sorted
out.

I think we're getting bogged down by the hypothetical AML-in-DT case. We won't
know if we want/need it until we see what kind of stuff vendors think they will
need to do in AML. On x86 it's mostly used to abstract out per-board
differences, not whole SoC behavior. It also depends on how much of their
hardware differences the silicon vendors decide to punt to software to manage
-- that's going to work a lot less well in this type of environment than it
does on 32-bit today.

quoted

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.

I'm not sure it's entirely reasonable to assume that Microsoft will
swoop in and develop standards that are useful to us or even applicable
to the vast majority of the systems that are likely to exist. If they
do, then we will (by the expectation that Linux should be able to run
wherever another OS can) have to support whatever standards they may
create.

Regardless of whether we place support for it into Linux, we should
certainly be spending time now attempting to understand ACPI, what it
does and does not provide, and how it can be moved towards something
that fulfils our needs and we can support long-term. We should certainly
not be taking a back seat approach.

Outside of the ARM Linux community there is work ongoing to change ACPI
to better suit the level of variation we seem in the ARM space (see
Darren Hart's presentation from ELCE [1]). We need to be involved now in
order to make sure that this is actually generally applicable.

I agree that there should be engagement, but not at the expense of forward
progress using the technologies we're finally sorting out how to use
successfully. In other words, we should continue enabling DT on these
platforms while ACPI happens in the background for now. People who
will longer-term depend on ACPI should of course get engaged in that,
but things aren't going to work well if they abandon working on the
short-to-medium term solutions of continued DT usage.

quoted

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.

Until ACPI is able to provide all the necessary information, and has
suitable standard mechanisms for describing the horrible portions we are
only just figuring out how to describe in DT, then ACPI is eitehr no
better or worse than DT, and should not be used.

However, we should be looking into it so that when portions of it
eventually appear we have enough of an understanding to cull the
obviously broken parts.

I think we're in full agreement on this.

I would also expect that _any_ ACPI support we would accept would not
rely on any board-specific support code whatsoever -- either everything
comes from ACPI or the platform doesn't boot. If we allow board files
for a transitionary period to ACPI we'd be making the same mistake we
did with DT.

I don't think anyone is arguing for board files here, that's a dead end. :-)
There might however be quirks needed for drivers here and there, at least for
new platforms (chipsets), errata workarounds, etc. That's expected, and it's
something that they need on x86 today as well.

quoted

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.

To me, reworking the AML code and drivers to support AML + DT feels like
dragging the whole kernel through it. I would rather see ACPI in full or
no ACPI than a gigantic ARM-specific set of changes to general
infrastructure that we'd expect to deprecate in future once we
understand (the future state of) ACPI.

I think we're getting bogged down in details here, unfortunately. Let's see
what people have in mind to implement in AML before we go too far down this
path. In theory it could be a nightmare to deal with it given the feature set
and complexity of AML, but in reality chances are we can do quite well with
just device-tree (as Arnd and others have been arguing as well).

So I think we can postpone much of this debate until we actually have examples
of what people will use in the real world.


[pruning out some double quotes and more AML-related arguments here]

quoted

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.

This common place is going to end up handling arbitrarily different
idioms in each format (e.g. how GPIOs are represented), and is almost
certainly going to become a sprawling mess. Also having "all the fixups"
in there makes this sound like an every-single-board file, which is
something I think everyone would like to avoid.

Yeah, it would need to be generic enough to not require per-board changes.
Again, let's see how things turn out in reality before we go too far down that
path, doing it on theoretical needs is going to overly complicate things.

[...]

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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.

This may be true. I worry that by working on this assumption we will
lead people to do the wrong thing by focusing on short-term gain (i.e.
placing board-specific hacks all other drivers and not handling the
general case), rather than getting a long term solution together.

There's going to be need for both, and we can't sacrifice work on the
short-to-medium term solution because I really don't want things to
stumble right away. As Jon is saying, lots of companies are spending
significant resources on this, and we need to make sure things are as
successful as possible right out the door.

That means not restricting short-term solutions with promises/wishes/hopes
that things will magically get sorted out by themselves down the
road. Given the initial output from the ACPI side of things, that's way
too scary a bet to make.

quoted

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.

If a vendor does this, with a DTB that correctly describes their
hardware then I am not against it (and would prefer this case to mapping
from ACPI to DT). For that case we will also require a nailed-down boot
protocol that allows for either DTB or ACPI (only one at a time).

UEFI already allows this, as far as I know? Even if both are passed, we can
easily make DT take precedence over ACPI, just like DT overrides ATAGs today.

quoted

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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.

This is still sounding awfully complicated.

Yes, but so is doing a native ACPI implementation. Anyway, let's hold off until
we know what we're actually going to need.

quoted

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.

There are many people in the Linux community who have ACPI experience.
They may not be active on the ARM side, but it's not fair to say there
are no known best practices. I will agree that for the level of
variation we're likely to expect we are pushing the boundaries.

Right -- as we've seen with DT where it was easy-peasy on PowerPC since there
were only 2-4 real vendors involved, things get substantially more complicated
when you increase that by an order of magnitude. In this case, we'd be
increasing it with an order of magnitude _and_ bring in their firmware
engineers to be involved too (since we would no longer have control over what
gets put into the tables).

While there are people who have a lot of ACPI experience today, the environment
around it (community/participants and technical diversity) _and_ the new need
for more complex things is new to everybody. As we've seen with DT, we can't
always rely on the existing people to have capacity for the onslaught of
activity that will come down the pipe.

quoted

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.

Again, I'm not sure that we should rely on Windows as our saviour from
insanity. A lot of the issues we are going to encounter are going to be
in vendor-specific code (i.e. drivers), and I do not believe that is
solved by having a single entity in charge of the general frameworks
those are built upon.

As mentioned in other replies in the thread, I was a bit careless in how
I phrased this. We can't just wait until they're done, but we also shouldn't
do it all ourselves. And we definitely shouldn't bet our house on it resulting
in a useable solution at this point.

quoted

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.

I agree that today a DTB is preferable to an ACPI implementation.

I think that mapping from ACPI => DT is less sane than building support
for ACPI, and is not going to help us in the long term.

I think it'll depend on what will actually be needed on real systems. And
I'm not making many assumptions on long term here -- we can't let that
distract us from getting the first few years sorted out first.

I think that we need to be involved in ACPI from today if we have any
hope of having something sane in future.

We agree more than we disagree on this whole discussion, I believe.


-Olof
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