linux-kernel - Re: [PATCH 7/9] pwm i.MX: fix clock lookup

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Message-ID: <20120907074138.GN26594@pengutronix.de>
Date:	Fri, 7 Sep 2012 09:41:38 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Benoît Thébaudeau 
	<benoit.thebaudeau@...ansee.com>
Cc:	HACHIMI Samir <shachimi@...neo-embedded.com>,
	shawn guo <shawn.guo@...aro.org>,
	thierry reding <thierry.reding@...onic-design.de>,
	linux-kernel@...r.kernel.org, kernel@...gutronix.de,
	Philipp Zabel <p.zabel@...gutronix.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 7/9] pwm i.MX: fix clock lookup

On Thu, Sep 06, 2012 at 09:09:42PM +0200, Benoît Thébaudeau wrote:
> On Thursday, September 6, 2012 8:42:56 PM, Sascha Hauer wrote:
> > On Thu, Sep 06, 2012 at 08:31:58PM +0200, Benoît Thébaudeau wrote:
> > > On Thursday, September 6, 2012 2:48:13 PM, Sascha Hauer wrote:
> > > > 
> > > > +	int ret;
> > > > +
> > > > +	ret = clk_prepare_enable(imx->clk_ipg);
> > > > +	if (ret)
> > > > +		return ret;
> > > >  
> > > > -	return imx->config(chip, pwm, duty_ns, period_ns);
> > > > +	ret = imx->config(chip, pwm, duty_ns, period_ns);
> > > > +
> > > > +	clk_disable_unprepare(imx->clk_ipg);
> > > > +
> > > > +	return ret;
> > > >  }
> > > >  
> > > >  static int imx_pwm_enable(struct pwm_chip *chip, struct
> > > >  pwm_device
> > > >  *pwm)
> > > > @@ -169,7 +179,7 @@ static int imx_pwm_enable(struct pwm_chip
> > > > *chip,
> > > > struct pwm_device *pwm)
> > > >  	struct imx_chip *imx = to_imx_chip(chip);
> > > >  	int ret;
> > > >  
> > > > -	ret = clk_prepare_enable(imx->clk);
> > > > +	ret = clk_prepare_enable(imx->clk_per);
> > > >  	if (ret)
> > > >  		return ret;
> > > 
> > > Have you tested that this actually works on i.MX53?
> > > 
> > > I have tested it successfully on i.MX35 (with a few additions to
> > > platform code).
> > > But i.MX35 has a single bit controlling both PWM IPG and PER clock
> > > gates.
> > > 
> > > On i.MX53, there are 2 separate control bits for these. So, if ipg
> > > clk is
> > > strictly required to access PWM registers, even if per clk is
> > > enabled, this code
> > > should not work without adding
> > 
> > I tested this on i.MX53, but you're right, this seems to be wrong.
> > I'll
> > recheck tomorrow.
> 
> I've performed a few more tests with bare register accesses in the bootloader to
> see how the PWM IP behaves.
> 
> On i.MX25, i.MX35 and i.MX51, read accesses always work, whatever the state of
> the IPG and PER clock gates.
> 
> On i.MX25 and i.MX51, write accesses always work, whatever the state of the IPG
> and PER clock gates.
> 
> On i.MX35, write accesses work if and only if the IPG and PER clocks are not
> gated off (single control bit for both).

Ok, I tested on i.MX53 and i.MX27.

On i.MX53 the registers are always accessible. I also measured with an
oscilloscope that the ipg/per clock gates en/disable the PWM when it's
configured for the corresponding clock.

On i.MX27 register accesses also work regardless of the clock gates.
Here we have a single clock gate which only gates the ipg clock.

btw the i.MX6 has a single gate per pwm which are described as "PWMx clocks"

So it seems while not 100% correct the current code seems to be safe.

Sascha

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