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

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Date:   Mon, 20 Mar 2017 14:49:21 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Ralph Sennhauser <ralph.sennhauser@...il.com>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        Imre Kaloz <kaloz@...nwrt.org>,
        Alexandre Courbot <gnurou@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        "open list:PWM SUBSYSTEM" <linux-pwm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] gpio: mvebu: Add limited PWM support

On Sat, Mar 18, 2017 at 04:37:53PM +0100, Andrew Lunn wrote:
> > > +static void mvebu_pwm_free(struct pwm_chip *chip, struct pwm_device *pwmd)
> > > +{
> > > +       struct mvebu_pwm *pwm = to_mvebu_pwm(chip);
> > > +       struct gpio_desc *desc = gpio_to_desc(pwmd->pwm);
> > > +       unsigned long flags;
> > > +
> > > +       spin_lock_irqsave(&pwm->lock, flags);
> > > +       gpiod_free(desc);
> > > +       pwm->used = false;
> > > +       spin_unlock_irqrestore(&pwm->lock, flags);
> > > +}
> > 
> > No need to set the output value to zero or something here?
> > And turn off blinking? Or is that done some other way?
> 
> Hi Linus
> 
> The disable op will turn of blinking. I've not checked, but i assume
> the PWM core will not allow you to free an enabled PWM?

Actually it will. It's probably a good idea to add a WARN_ON() to the
PWM core if that situation arises. I don't think going as far as
prohibiting it will do any good, though. It's not like drivers will
have much of a choice if pwm_put() fails. Typically they'd do that in
their ->remove() call, at which point failure is difficult to deal with.

> > I think both of these need to be tagged __maybe_unused to not give
> > noise in randconfig builds.
> 
> I've not seen any 0-day patch emails giving warnings. So i suspect it
> is O.K.

Linus was probably referring to !PM configurations. I'm not sure how
often they'll get run, but as long as it doesn't make it into linux-next
the chances aren't very high (I don't think the 0-day builder executes
randconfig builds).

Thierry

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