linux-kernel - Re: [PATCH 3/6] pwm: sun4i: Add a quirk for bus clock

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Date:   Mon, 29 Jul 2019 18:45:16 +0200
From:   Maxime Ripard <mripard@...nel.org>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Jernej Škrabec <jernej.skrabec@...l.net>,
        mark.rutland@....com, linux-pwm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-sunxi@...glegroups.com,
        linux-kernel@...r.kernel.org, wens@...e.org, robh+dt@...nel.org,
        thierry.reding@...il.com, kernel@...gutronix.de,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/6] pwm: sun4i: Add a quirk for bus clock

On Mon, Jul 29, 2019 at 06:14:35PM +0200, Uwe Kleine-König wrote:
> Hello Jernej,
>
> On Mon, Jul 29, 2019 at 05:48:36PM +0200, Jernej Škrabec wrote:
> > Dne ponedeljek, 29. julij 2019 ob 08:38:25 CEST je Uwe Kleine-König
> > napisal(a):
> > > Hello,
> > >
> > > On Fri, Jul 26, 2019 at 08:40:42PM +0200, Jernej Skrabec wrote:
> > > > H6 PWM core needs bus clock to be enabled in order to work.
> > > >
> > > > Add a quirk for it.
> > > >
> > > > Signed-off-by: Jernej Skrabec <jernej.skrabec@...l.net>
> > > > ---
> > > >
> > > >  drivers/pwm/pwm-sun4i.c | 15 +++++++++++++++
> > > >  1 file changed, 15 insertions(+)
> > > >
> > > > diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> > > > index 1b7be8fbde86..7d3ac3f2dc3f 100644
> > > > --- a/drivers/pwm/pwm-sun4i.c
> > > > +++ b/drivers/pwm/pwm-sun4i.c
> > > > @@ -72,6 +72,7 @@ static const u32 prescaler_table[] = {
> > > >
> > > >  };
> > > >
> > > >  struct sun4i_pwm_data {
> > > >
> > > > +	bool has_bus_clock;
> > > >
> > > >  	bool has_prescaler_bypass;
> > > >  	bool has_reset;
> > > >  	unsigned int npwm;
> > > >
> > > > @@ -79,6 +80,7 @@ struct sun4i_pwm_data {
> > > >
> > > >  struct sun4i_pwm_chip {
> > > >
> > > >  	struct pwm_chip chip;
> > > >
> > > > +	struct clk *bus_clk;
> > > >
> > > >  	struct clk *clk;
> > > >  	struct reset_control *rst;
> > > >  	void __iomem *base;
> > > >
> > > > @@ -382,6 +384,16 @@ static int sun4i_pwm_probe(struct platform_device
> > > > *pdev)>
> > > >  		reset_control_deassert(pwm->rst);
> > > >
> > > >  	}
> > > >
> > > > +	if (pwm->data->has_bus_clock) {
> > > > +		pwm->bus_clk = devm_clk_get(&pdev->dev, "bus");
> > >
> > > Similar to my suggestion in patch 2: I'd use devm_clk_get_optional() and
> > > drop struct sun4i_pwm_data::has_bus_clock.
> >
> > This one is not so simple. This patch has incorrect logic. Correct logic would
> > be to use "devm_clk_get(&pdev->dev, NULL)" for variants without bus clock as
> > it is done already and "devm_clk_get(&pdev->dev, "bus")" and
> > "devm_clk_get(&pdev->dev, "mod")" for variants with bus clock.
>
> Then maybe something like the following?:
>
> 	busclk = devm_clk_get_optional(..., "bus");
> 	modclk = devm_clk_get_optional(..., "mod");
>
> 	/*
> 	 * old dtbs might have a single clock but no clock names. Fall
> 	 * back to this for compatibility reasons.
> 	 */
> 	if (!modclk) {
> 		modclk = devm_clk_get(..., NULL);
> 	}

Again, there's nothing optional about these clocks. You need a
particular set of clocks for a given generation, and a separate set of
them on another generation of SoCs.

It really isn't about DT validation. We're really making sure that the
device can be operational. It's as much of a validation step than
making sure we have mapped registers (reg), or an interrupt if we had
any.

Maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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