On Fri, 10 Jun 2016, Thierry Reding wrote:

On Fri, Jun 10, 2016 at 03:06:35PM +0100, Lee Jones wrote:

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On Fri, 10 Jun 2016, Thierry Reding wrote:

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On Thu, Jun 09, 2016 at 12:41:07PM +0100, Lee Jones wrote:

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On Wed, 08 Jun 2016, Rob Herring wrote:

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On Wed, Jun 08, 2016 at 10:21:35AM +0100, Lee Jones wrote:

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We're renaming the 'st,pwm-num-chan' binding to 'st,pwm-num-devs' to
be more inline with the naming conventions of the subsystem.  Where
we used to treat each line as a channel, the PWM convention is to
describe them as devices.

That's all linux implementation details and you are breaking 
compatibility.

Normally I'd agree with you, but I happen to know that a) this IP is
currently unused and b) up until this point (and probably beyond), ST
always ship the DTB with the kernel, so there will be no breakage.

Heh... I've long given up on trying to make that argument go anywhere.
The general rule is that once we support a binding in a kernel release
we have to support it indefinitely. If you really want to go ahead with
this change (I don't think you should), you'd at least have to document
both properties and support st,pwm-num-chan in the driver for backwards
compatibility.

I understand what the *general* rule is, but we have to remember why
this rule was put into place and apply some common sense.  In some
Enterprise user-cases where DTBs are written into ROM or where they
are difficult /impossible to update, I can completely understand the
requirement to support previous incarnations.  However in this, the
real world, DTBs are shipped with their corresponding kernels.  We
would lack a great deal of functionality if they weren't.  It is
therefor, foolhardy and inappropriate to stick to this rule just
'cos.

I used to advocate the very same point of view a couple of years ago but
was repeatedly told that it's irrelevant, and everybody was to be held
to the same standards, irrespective of how easy it is to update the DTB
in lockstep with the kernel.

Part of me still wishes that the rules were a little less strict, but a
decision was made on this years ago, so I'm just repeating this here in
an attempt to save you from wasting your time arguing.

Then again, there have been occasions where decisions were undone, so
you might have better luck nowadays if you're willing to take this to
the DT bindings and ARM-SoC maintainers.

Bottom line: NAK on the backwards-incompatible DT binding change from me
based on earlier decisions made on this topic. But I may be swayed if
everyone else thinks ABI stability is no longer something we consider
important for DT.

I've seen properties come and go.  Both using the deprecation process
and otherwise.  By NAKing these types of changes, you're just
exacerbating the situation.  If you know it's the right
(non-foolhardy) thing to do, just do it.

Although, I feel the point is moot for this particular driver now,
what with all of the pdata being moved into the driver.

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The second documentation adaption entails adding support for PWM
capture devices.  A new clock is required as well as an IRQ line.
We're also adding a new property similar to the one described
above, but for capture channels.  Typically, there will be less
capture channels than PWM-out, since all channels have the latter
capability, but only some have capture support.

Humm, sounds like all of this should be implied from compatible strings.

You mean have a bunch of of_machine_is_compatibles() scattered around?

I don't understand why you need this at all. Quite frankly I don't even
know why st,pwm-num-devs exists. I probably missed it back at the time.
Usually, like Rob suggests, this should be inferred from the compatible
string. One commonly used way to avoid scattering explicit checks for
the compatible string is to add this information to the of_device_id
table. See a bunch of existing drivers for reference.

Yes, I am aware of the strategy, and happy to oblige if this is your
suggestion.  I'll move all platform data into the driver and eradicate
the DT properties.

Great!

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Also, why make a separation of output vs. capture channels at this
point? Could you not simply obtain the total number of PWM channels,
preferably from SoC data associated with the compatible string, and
check at ->capture() time whether or not the particular PWM supports
this?

As-is, you imply that you have n (output) + m (capture) channels, and
that 0..n-1 are output and n..n+m-1 are capture channels. What if that
no longer holds true, but 0 and 2 are the only ones that support
capture?

We do?  What makes you think that?

Because there's nothing saying otherwise? The DT binding is completely
silent on the matter, and the above is the most logical interpretation
that I came up with.

But you made it up. :)

It's not documented because it's irrelevant how channels are numbered.

Actually, looking at the docs, the IP doesn't change from one channel
to another, so in theory they all support capture.  The only issue is
whether they are plumbed in or not.  I'll take a  closer look at all
the platforms we support when I move the pdata.

Looking at the driver it seems like it's actually the other way around
and you have the first m channels that support both output and capture
functionality. But the issue remains that this isn't documented in the
DT binding documentation. And the current properties are also not very
flexible because they allow for only a single scheme of assigning the
capability.

If you move all of that knowledge into the driver, things become a lot
easier, in my opinion.

Sure.

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If you check for this at runtime you can avoid complicated DT parsing
code, but still get the safety check which should be enough to encourage
people to use the right channels in DT.

I'm pretty sure I can move all the data into the driver.  I did want
to avoid having lots of different compatible strings, but if that's
what you're suggesting, I can introduce one per supported platform.

That sounds like the simplest solution to me.

Okay.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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