linux-kernel - Re: [PATCH v3 20/20] dt-bindings: pwm: sti: Update DT bindings with recent changes

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Date:	Fri, 10 Jun 2016 15:18:55 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	Rob Herring <robh@...nel.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kernel@...inux.com, maxime.coquelin@...com, patrice.chotard@...com,
	linux-pwm@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v3 20/20] dt-bindings: pwm: sti: Update DT bindings with
 recent changes

On Thu, Jun 09, 2016 at 12:41:07PM +0100, Lee Jones wrote:
> On Wed, 08 Jun 2016, Rob Herring wrote:
> 
> > On Wed, Jun 08, 2016 at 10:21:35AM +0100, Lee Jones wrote:
> > > 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.

> > > 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.

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?

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.

Thierry

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