linux-kernel - Re: [PATCH v3 2/6] pwm: mediatek: fix pwm source clock selection

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Message-ID: <1499323435.22478.26.camel@mhfsdcap03>
Date:   Thu, 6 Jul 2017 14:43:55 +0800
From:   Zhi Mao <zhi.mao@...iatek.com>
To:     Matthias Brugger <matthias.bgg@...il.com>
CC:     <john@...ozen.org>, Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        <linux-pwm@...r.kernel.org>, <srv_heupstream@...iatek.com>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>, <yingjoe.chen@...iatek.com>,
        <yt.shen@...iatek.com>, <sean.wang@...iatek.com>,
        <zhenbao.liu@...iatek.com>
Subject: Re: [PATCH v3 2/6] pwm: mediatek: fix pwm source clock selection

On Thu, 2017-07-06 at 14:16 +0800, Zhi Mao wrote:
> On Wed, 2017-07-05 at 13:09 +0200, Matthias Brugger wrote:
> > 
> > On 06/30/2017 08:05 AM, Zhi Mao wrote:
> > > In original code, the pwm output frequency is not correct
> > > when set bit<3>=1 to PWMCON register.
> > > 
> > > Signed-off-by: Zhi Mao <zhi.mao@...iatek.com>
> > > ---
> > >   drivers/pwm/pwm-mediatek.c |    2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/pwm/pwm-mediatek.c b/drivers/pwm/pwm-mediatek.c
> > > index 5c11bc7..d08b5b3 100644
> > > --- a/drivers/pwm/pwm-mediatek.c
> > > +++ b/drivers/pwm/pwm-mediatek.c
> > > @@ -91,7 +91,7 @@ static int mtk_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > >   	if (clkdiv > 7)
> > >   		return -EINVAL;
> > >   
> > > -	mtk_pwm_writel(pc, pwm->hwpwm, PWMCON, BIT(15) | BIT(3) | clkdiv);
> > > +	mtk_pwm_writel(pc, pwm->hwpwm, PWMCON, BIT(15) | clkdiv);
> > 
> > Just for clarification, BIT(15) enables old PWM mode, which ignores 
> > CLKSEL (BIT(3)). Therefore setting BIT(3) does not have any effect and 
> > can be discarded.
> > 
> > Am I correct? I took mt7623n datasheet for reference.
> > 
> > Regards,
> > Matthias
> > 
> Yes, remove setting bit<3> will not take any effect. 
> PWMCON bit<3> is pwm source clock selecting register.
> You can check the datasheet of MT7623 for details.
> 
> Regards
> Zhi
> 
Hi Mattias,

Ignore the above reply, I explain this bit<3> issue for you.
In the data sheet of MT7623: 
PWMCON bit<3> is PWM source clock selecting register
0: CLK=CLKSRC
1: CLK=CLKSRC/1625
for example,
bit<3>=0, PWM clk source is 26M
bit<3>=1, PWM clk source is 16K
The frequency of PWM output will based on this clock source
if set bit<3>=1, it will cause the frequency of PWM output is not
correct. I also use the oscilloscope device to measure the output,     
set bit<3>=0, the output meet expectation. 
 
Regards
Zhi
> > >   	mtk_pwm_writel(pc, pwm->hwpwm, PWMDWIDTH, period_ns / resolution);
> > >   	mtk_pwm_writel(pc, pwm->hwpwm, PWMTHRES, duty_ns / resolution);
> > >   
> > > 
>