linux-kernel - Re: [PATCH v5 1/4] gpio: mvebu: Add limited PWM support

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Message-ID: <20170421111952.54978e80@free-electrons.com>
Date:   Fri, 21 Apr 2017 11:19:52 +0200
From:   Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Ralph Sennhauser <ralph.sennhauser@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Jason Cooper <jason@...edaemon.net>,
        Alexandre Courbot <gnurou@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Russell King <linux@...linux.org.uk>,
        linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Gregory Clement <gregory.clement@...e-electrons.com>,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        linux-gpio@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Subject: Re: [PATCH v5 1/4] gpio: mvebu: Add limited PWM support

Hello,

On Wed, 12 Apr 2017 17:19:32 +0200, Andrew Lunn wrote:

> Yep. It was a compromise. By adding a new binding for the GPIO driver,
> this might be possible. But it did not seem worth such a major change.
> 
> The prime use of this feature is for controlling a fan. So far, i've
> not seen any hardware with more than one fan, i.e. needs more than one
> PWM. Nor have i seen any hardware with the GPIO for the fan being on
> the third bank. A hardware manufacture could add multiple fans, but i
> doubt it, they make noise and fail. And if a manufacture does place a
> fan on the third bank, it can still be controlled as a plain GPIO fan,
> as we have been doing for the last few years.

Right.

> So i personally think it is an O.K. compromise.

I clearly don't want to block this, but I believe this is a very good
illustration of why stable DT bindings simply don't work. We are
realizing here that having each GPIO bank represented as a separate DT
node doesn't work, because this blinking functionality is not per GPIO
bank, but global to all GPIO banks.

I am totally fine with compromise, and having things simple first, and
extend them later if needed. But this stable DT binding rule makes this
quite impossible: what is a compromise today might put you in big
troubles tomorrow.

Anyway, it's fine for me, I don't think it's worth the effort making a
much more complicated solution/change.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com