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

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Date:   Sun, 16 Dec 2018 15:18:52 +0100
From:   Paul Cercueil <paul@...pouillou.net>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ralf Baechle <ralf@...ux-mips.org>, paul.burton@...s.com,
        James Hogan <jhogan@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Mathieu Malaterre <malat@...ian.org>, ezequiel@...labora.co.uk,
        prasannatsmkumar@...il.com, linux-pwm@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        LINUXWATCHDOG <linux-watchdog@...r.kernel.org>,
        linux-mips@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-clk <linux-clk@...r.kernel.org>, od@...c.me
Subject: Re: [PATCH v8 15/26] pwm: jz4740: Add support for the JZ4725B

Hi,

Le ven. 14 déc. 2018 à 15:26, Uwe Kleine-König 
<u.kleine-koenig@...gutronix.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@...gutronix.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@...gutronix.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/  |