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

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Date:   Tue, 21 Mar 2017 15:50:56 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Ralph Sennhauser <ralph.sennhauser@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        linux-gpio@...r.kernel.org, Imre Kaloz <kaloz@...nwrt.org>,
        Linus Walleij <linus.walleij@...aro.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

> > > *mvebu_gpioreg_blink(struct mvebu_gpio_chip *mvchip) return
> > > mvchip->membase + GPIO_BLINK_EN_OFF; }
> > >  
> > > +static void __iomem *mvebu_gpioreg_blink_select(struct
> > > mvebu_gpio_chip *mvchip) +{
> > > +	return mvchip->membase + GPIO_BLINK_CNT_SELECT_OFF;
> > > +}  
> > 
> > That's a really weird thing to do. Why not just use this expression in
> > your calls to readl() and writel() directly? Seems a lot of additional
> > code for no gain.

This driver is made more complex by the Armada XP SMP handling. Some
GPIO registers are per-cpu, others are global. Registers for
interrupts in particular are per CPU. So there is a general trend in
this driver to have a function which returns the address of a
register, for a given SOC variant. In this case, it is fixed, so could
be collapsed into the actual read/write. But then it would be
different to all others in this driver...

> > > +	pwm->chip.dev = dev;
> > > +	pwm->chip.ops = &mvebu_pwm_ops;
> > > +	pwm->chip.base = mvchip->chip.base;
> > > +	pwm->chip.npwm = mvchip->chip.ngpio;  
> > 
> > Isn't that a lie? The code above suggests you can only ever have a
> > single GPIO turn into a PWM, so I'd expect ".npwm = 1" here.

Well, any of the 32 GPIOs can be a PWM. But only one can be enabled at
a time. What exactly does pwm->chip.npwm mean? If we say 1 here, how
at run time do we say which of the 32 GPIOs is used as a PWM output?
By saying 32, it is simpler, which ever is enabled first is the one to
use.

	Andrew