Re: [PATCH 1/3] dt-bindings: arm/marvell: ABI unstability warning about Marvell 7K/8K

Hi,

On Thu, Feb 25, 2016 at 04:16:47PM +0000, Mark Rutland wrote:

On Thu, Feb 25, 2016 at 09:09:27AM +0100, Thomas Petazzoni wrote:

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Hello Rob,

On Wed, 24 Feb 2016 16:07:04 -0600, Rob Herring wrote:

quoted

You should fix the incomplete information problem. I pushed back on
this, you got more information and already the binding is closer to
reality.

"You should fix the incomplete information problem". You seem to think
that it is a simple thing that can be fixed with a magic wand. But it's
not.

Either because the internal processes are complicated, or simply
because the Linux kernel support is done without cooperation from the
HW vendor (it's not the case of this Marvell platform, but it's the
case of many other platforms).

Yes, this is a problem in some cases, and that should be considered in
those cases. There are always shades of grey.

Per the above, that isn't relevant in this case. This is a pretty
black-and-white stand against the usual rules.

Unless your stance, at least on the sunxi discussion was pretty much
black-and-white too. You basically said that we were supposed to
maintain a stable ABI starting right now, even though we know for a
fact that we have bindings that are simply not working.

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Mark didn't say don't submit. First, there is value in submitting even
if not accepted immediately and you have to carry some patches for
some time. It was also suggested that you err on the side of less
information in DT if things are uncertain and you have not done that.

Submitting without merging is useless. The point of submitting is to
get the code merged, to reduce the amount of out-of-tree patches we
have to carry, and to allow users of the platform to simply run
mainline on their platform.

Submitting prototypes and RFCs is the usual way we get things reviewed
early, and to allow maintainers and others to get a feel for things
earlier. Submitting patches _for merging_ when you're not sure about
things and don't want to agree to support them is what's being pushed
back on.

Just to make sure we're on the same page, are you saying that we must
not merge code unless we're 100% sure of how it works, in its
entirety?

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So this proposal really doesn't make any sense. Just like Mark initial
statement of not submitting code so early.

As what I said was evidently ambiguous, I'm on about code being _merged_
prior to being ready. Code and bindings should certainly be posted for
review as soon as possible. However, it should be recognised when things
aren't quite ready for mainline.

Even if something's unclear about a device, you can highlight more
general issues (e.g. problematic edge cases in common subsystem code),
and that's possible to have merged even if the binding isn't ready.

If you're unsure about something, but still want it merged, then you
have to commit to maintaining that as far as reasonably possible, even
if it turns out to not be quite right.

Except that *you* are forcing us to maintain code to deal with a use
case that simply doesn't happen, which is the definition of dead code.

Would you be ok to be forced to maintain dead code?

You've been talking about theorical use-cases, do you have any real
world one where someone actually made a case for this DT as an ABI
non-sense?

I mean, we've had the U-Boot sunxi maintainer saying they didn't care,
and actually arguing against it.

As it turns out, he's also involved in the Fedora port. If it was a
pain point for them, don't you think he would have raised it?

The only ones that seem to have a problem with it are the one not
involved in it in any way.

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So please get to an agreement between DT binding maintainers. And stop
saying this ridiculous statement that HW vendors should stop submitting
their code to the mainline.

You want us to agree, then you won't like the answer. Bindings must be
stable. I'll allow exceptions to that, but not without push back.

In general, if there is disagreement about whether stability is
required, then it is required. See the recent sunxi discussion. That's
more between users and platform maintainers though.

Do you realize that this all DT binding stuff is today the *biggest* to
getting HW support in the Linux kernel? It has become more complicated
to merge a 4 properties DT binding than to merge multiple thousands of
lines of driver code. 

As times have changed, pain points have moved around.

To some extent that is unavoidable; more up-front effort is required
where crutches we previously relied on are not applicable.

Elsewhere we can certainly do better.

Throwing your hands up and stating "this is unstable, it might change"
is a crutch. It prevents any real solution to the pain points you
encounter, and creates pain points for others. It only provides the
_illusion_ of support.

https://kernelci.org/soc/mvebu/
https://kernelci.org/soc/sunxi/

I guess we should become magicians.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com