Re: [PATCH v8 15/26] pwm: jz4740: Add support for the JZ4725B

From: Paul Cercueil <paul@crapouillou.net>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Linus Walleij <linus.walleij@linaro.org>,
	"thierry.reding@gmail.com" <thierry.reding@gmail.com>,
	Rob Herring <robh+dt@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ralf Baechle <ralf@linux-mips.org>,
	paul.burton@mips.com, James Hogan <jhogan@kernel.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Mathieu Malaterre <malat@debian.org>,
	ezequiel@collabora.co.uk, prasannatsmkumar@gmail.com,
	linux-pwm@vger.kernel.org,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
	<devicetree@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	LINUXWATCHDOG <linux-watchdog@vger.kernel.org>,
	linux-mips@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-clk <linux-clk@vger.kernel.org>,
	od@zcrc.me
Subject: Re: [PATCH v8 15/26] pwm: jz4740: Add support for the JZ4725B
Date: Sun, 16 Dec 2018 15:18:52 +0100	[thread overview]
Message-ID: <1544969932.1649.1@crapouillou.net> (raw)
In-Reply-To: <20181214142628.zwi4hadrju53z6f3@pengutronix.de>

Hi,

Le ven. 14 déc. 2018 à 15:26, Uwe Kleine-König 
<u.kleine-koenig@pengutronix.de> a écrit :
> Hello,
> 
> On Fri, Dec 14, 2018 at 02:50:20PM +0100, Linus Walleij wrote:
>>  On Thu, Dec 13, 2018 at 9:42 PM Uwe Kleine-König
>>  <u.kleine-koenig@pengutronix.de> wrote:
>>  > [Adding Linus Walleij to Cc:]
>>  > On Thu, Dec 13, 2018 at 03:03:15PM +0100, Paul Cercueil wrote:
>>  > > Le jeu. 13 déc. 2018 à 10:24, Uwe Kleine-König
>>  > > <u.kleine-koenig@pengutronix.de> a écrit :
>>  > > > On Wed, Dec 12, 2018 at 11:09:10PM +0100, Paul Cercueil wrote:
>>  > > > >  The PWM in the JZ4725B works the same as in the JZ4740, 
>> except that
>>  > > > > it
>>  > > > >  only has 6 channels available instead of 8.
>>  > > >
>>  > > > this driver is probed only from device tree? If yes, it might 
>> be
>>  > > > sensible to specify the number of PWMs there and get it from 
>> there.
>>  > > > There doesn't seem to be a generic binding for that, but 
>> there are
>>  > > > several drivers that could benefit from it. (This is a bigger 
>> project
>>  > > > though and shouldn't stop your patch. Still more as it 
>> already got
>>  > > > Thierry's ack.)
>>  > >
>>  > > I think there needs to be a proper guideline, as there doesn't 
>> seem to be
>>  > > a consensus about this. I learned from emails with Rob and 
>> Linus (Walleij)
>>  > > that I should not have in devicetree what I can deduce from the 
>> compatible
>>  > > string.
>>  >
>>  > I understood them a bit differently. It is ok to deduce things 
>> from the
>>  > compatible string. But if you define a generic property (say) 
>> "num-pwms"
>>  > that is used uniformly in most bindings this is ok, too. (And 
>> then the
>>  > two different devices could use the same compatible.)
>>  >
>>  > An upside of the generic "num-pwms" property is that the pwm core 
>> could
>>  > sanity check pwm phandles before passing them to the hardware 
>> drivers.
>> 
>>  I don't know if this helps, but in GPIO we have "ngpios" which is
>>  used to augment an existing block as to the number of lines actually
>>  used with it.
>> 
>>  The typical case is that an ASIC engineer synthesize a block for
>>  32 GPIOs but only 12 of them are routed to external pads. So
>>  we augment the behaviour of that driver to only use 12 of the
>>  32 lines.
>> 
>>  I guess using the remaining 20 lines "works" in a sense but they
>>  have no practical use and will just bias electrons in the silicon
>>  for no use.
> 
> This looks very similar to the case under discussion.
> 
>>  So if the PWM case is something similar, then by all means add
>>  num-pwms.
> 
> .. or "npwms" to use the same nomenclature as the gpio binding?

If we're going to do something like this, should it be the drivers or
the core (within pwmchip_add) that checks for this "npwms" property?

> Best regards
> Uwe
> 
> --
> Pengutronix e.K.                           | Uwe Kleine-König        
>     |
> Industrial Linux Solutions                 | 
> http://www.pengutronix.de/  |