linux-kernel - Re: [PATCH v5 2/3] pwm: add MediaTek display PWM driver support

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Date:	Fri, 17 Jul 2015 00:44:10 +0800
From:	YH Huang <yh.huang@...iatek.com>
To:	Daniel Kurtz <djkurtz@...omium.org>
CC:	Thierry Reding <thierry.reding@...il.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Mark Rutland <mark.rutland@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>, <linux-pwm@...r.kernel.org>,
	"open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	srv_heupstream <srv_heupstream@...iatek.com>,
	<linux-mediatek@...ts.infradead.org>,
	"Sascha Hauer" <kernel@...gutronix.de>,
	Yingjoe Chen <yingjoe.chen@...iatek.com>,
	<yh.huang@...iatek.com>
Subject: Re: [PATCH v5 2/3] pwm: add MediaTek display PWM driver support

On Thu, 2015-07-16 at 23:21 +0800, Daniel Kurtz wrote:
> On Thu, Jul 16, 2015 at 3:17 PM, YH Huang <yh.huang@...iatek.com> wrote:
> > On Thu, 2015-07-16 at 14:54 +0800, Daniel Kurtz wrote:
> >> On Thu, Jul 16, 2015 at 1:38 PM, YH Huang <yh.huang@...iatek.com> wrote:
> >> > On Wed, 2015-07-15 at 23:59 +0800, YH Huang wrote:
> >> >> On Mon, 2015-07-13 at 18:19 +0800, Daniel Kurtz wrote:
> >> >> > On Mon, Jul 13, 2015 at 5:04 PM, YH Huang <yh.huang@...iatek.com> wrote:
> >> >> > > +#ifdef CONFIG_PM_SLEEP
> >> >> > > +static int mtk_disp_pwm_suspend(struct device *dev)
> >> >> > > +{
> >> >> > > +       struct mtk_disp_pwm *mdp = dev_get_drvdata(dev);
> >> >> > > +
> >> >> > > +       clk_disable_unprepare(mdp->clk_main);
> >> >> > > +       clk_disable_unprepare(mdp->clk_mm);
> >> >> > > +
> >> >> > > +       return 0;
> >> >> > > +}
> >> >> > > +
> >> >> > > +static int mtk_disp_pwm_resume(struct device *dev)
> >> >> > > +{
> >> >> > > +       struct mtk_disp_pwm *mdp = dev_get_drvdata(dev);
> >> >> > > +       int ret;
> >> >> > > +
> >> >> > > +       ret = clk_prepare_enable(mdp->clk_main);
> >> >> > > +       if (ret < 0)
> >> >> > > +               return ret;
> >> >> > > +
> >> >> > > +       ret = clk_prepare_enable(mdp->clk_mm);
> >> >> > > +       if (ret < 0) {
> >> >> > > +               clk_disable_unprepare(mdp->clk_main);
> >> >> > > +               return ret;
> >> >> > > +       }
> >> >> > > +
> >> >> >
> >> >> > Don't you also have to restore the PWM rate and frequency?
> >> >> >
> >> >> > Is it possible to save power at runtime by leaving mdp->clk_mm enabled
> >> >> > (to generate the PWM signal), but disable mdp->clk_main (clock
> >> >> > required to access PWM registers)?
> >> >>
> >> >> The pwm-backlight driver will restore the data.
> >> >>
> >> >> After I try to disable anyone of the two clocks at runtime, the
> >> >> backlight doesn't work well(no immediate update or losing backlight).
> >> >> So we need to keep both clock enabled.
> 
> Do you mean you see backlight glitch because the clocks / backlight
> were *already on* during the first config (Perhaps left on by the
> bootloader)?
> I don't know how to solve that problem.
> Maybe Thierry does.
> 
> In any case, this is a minor issue; we really shouldn't hold up
> landing the driver to optimize when the clocks are enabled/disabled
> :-). I'm happy enough with what you have in this patch.

Sorry for my terrible expression. Let me try again.
1. We want to disable unnecessary clock at runtime. 
But, I get backlight glitch when I disable clk_main or clk_mm in
mtk_disp_pwm_config(). So both clocks are necessary and we don't disable
them at runtime. 

2. Because pwm-backlight driver calls mtk_disp_pwm_config() before
mtk_disp_pwm_enable(), we will lose the first config if clocks are
enabled in mtk_disp_pwm_enable(). I prefer to enable clocks in probe
function. Samsung did the same way in their pwm driver.

Thanks for your kindly suggestions. I will update the patch soon.

Regards,
YH Huang


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