linux-kernel - Re: [PATCH 1/6] pwm: imx: Rewrite imx_pwm_*_v1 code to facilitate switch to atomic pwm operation

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Date:   Tue, 25 Oct 2016 08:27:37 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Sascha Hauer <s.hauer@...gutronix.de>
Cc:     Lukasz Majewski <l.majewski@...ess.pl>,
        Thierry Reding <thierry.reding@...il.com>,
        Stefan Agner <stefan@...er.ch>, linux-pwm@...r.kernel.org,
        Bhuvanchandra DV <bhuvanchandra.dv@...adex.com>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        Fabio Estevam <fabio.estevam@....com>,
        Fabio Estevam <festevam@...il.com>,
        Lothar Wassmann <LW@...o-electronics.de>
Subject: Re: [PATCH 1/6] pwm: imx: Rewrite imx_pwm_*_v1 code to facilitate
 switch to atomic pwm operation

On Tue, 25 Oct 2016 07:54:54 +0200
Sascha Hauer <s.hauer@...gutronix.de> wrote:

> On Sun, Oct 23, 2016 at 11:45:41PM +0200, Lukasz Majewski wrote:
> > The code has been rewritten to remove "generic" calls to
> > imx_pwm_{enable|disable|config}.
> > 
> > Such approach would facilitate switch to atomic PWM (a.k.a ->apply())
> > implementation.
> > 
> > Suggested-by: Stefan Agner <stefan@...er.ch>
> > Suggested-by: Boris Brezillon <boris.brezillon@...e-electrons.com>
> > Signed-off-by: Lukasz Majewski <l.majewski@...ess.pl>
> > ---
> >  drivers/pwm/pwm-imx.c | 46 ++++++++++++++++++++++++++++++++++------------
> >  1 file changed, 34 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/pwm/pwm-imx.c b/drivers/pwm/pwm-imx.c
> > index c37d223..83e43d5 100644
> > --- a/drivers/pwm/pwm-imx.c
> > +++ b/drivers/pwm/pwm-imx.c
> > @@ -66,8 +66,6 @@ struct imx_chip {
> >  static int imx_pwm_config_v1(struct pwm_chip *chip,
> >  		struct pwm_device *pwm, int duty_ns, int period_ns)
> >  {
> > -	struct imx_chip *imx = to_imx_chip(chip);
> > -
> >  	/*
> >  	 * The PWM subsystem allows for exact frequencies. However,
> >  	 * I cannot connect a scope on my device to the PWM line and
> > @@ -85,26 +83,52 @@ static int imx_pwm_config_v1(struct pwm_chip *chip,
> >  	 * both the prescaler (/1 .. /128) and then by CLKSEL
> >  	 * (/2 .. /16).
> >  	 */
> > +	struct imx_chip *imx = to_imx_chip(chip);
> >  	u32 max = readl(imx->mmio_base + MX1_PWMP);
> >  	u32 p = max * duty_ns / period_ns;
> > +	int ret;
> > +
> > +	ret = clk_prepare_enable(imx->clk_ipg);
> > +	if (ret)
> > +		return ret;
> > +
> >  	writel(max - p, imx->mmio_base + MX1_PWMS);
> >  
> > +	clk_disable_unprepare(imx->clk_ipg);
> > +
> >  	return 0;
> >  }
> >  
> > -static void imx_pwm_set_enable_v1(struct pwm_chip *chip, bool enable)
> > +static int imx_pwm_enable_v1(struct pwm_chip *chip, struct pwm_device *pwm)
> >  {
> >  	struct imx_chip *imx = to_imx_chip(chip);
> > +	int ret;
> >  	u32 val;
> >  
> > +	ret = clk_prepare_enable(imx->clk_ipg);
> > +	if (ret)
> > +		return ret;
> > +
> >  	val = readl(imx->mmio_base + MX1_PWMC);
> > +	val |= MX1_PWMC_EN;
> > +	writel(val, imx->mmio_base + MX1_PWMC);
> >  
> > -	if (enable)
> > -		val |= MX1_PWMC_EN;
> > -	else
> > -		val &= ~MX1_PWMC_EN;
> > +	clk_disable_unprepare(imx->clk_per);  
> 
> This looks wrong. You start by enabling clk_ipg which is needed for
> register access, but then end with disabling clk_per which is needed for
> driving the actual PWM output. This function should probably enable
> clk_per on entry and enable clk_ipg while accessing registers.

Oh, I didn't notice there was 2 different clocks here. This probably
means you have to enable clk_ipg when manipulating the registers in
->disable().

One question, if there's a separate clk for the registers, why don't we
leave this clock enabled, and disable it in ->suspend() or ->remove()
instead of enabling/disabling it each time we access the registers?