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 08:32:59 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
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, Oct 25, 2016 at 08:27:37AM +0200, Boris Brezillon wrote:
> 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?

Well, if we don't need this clock, then why not save the power and
disable it?
However, I'll have to ask Philipp about this clock. It was introduced in

commit 7b27c160c68152581c702b9f1fe362338d2a0cad
Author: Philipp Zabel <p.zabel@...gutronix.de>
Date:   Mon Jun 25 16:15:20 2012 +0200

And even back then the additional clock was not enabled for all register
accesses, so handling this seems broken from the start.

Sascha


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