linux-kernel - RE: [v3 2/3] pwm: Add Aspeed ast2600 PWM support

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Date:   Mon, 7 Nov 2022 09:31:00 +0000
From:   Biju Das <biju.das.jz@...renesas.com>
To:     Billy Tsai <billy_tsai@...eedtech.com>,
        "jdelvare@...e.com" <jdelvare@...e.com>,
        "linux@...ck-us.net" <linux@...ck-us.net>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "joel@....id.au" <joel@....id.au>,
        "andrew@...id.au" <andrew@...id.au>,
        "lee.jones@...aro.org" <lee.jones@...aro.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "u.kleine-koenig@...gutronix.de" <u.kleine-koenig@...gutronix.de>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        BMC-SW <BMC-SW@...eedtech.com>,
        "garnermic@...a.com" <garnermic@...a.com>
CC:     kernel test robot <lkp@...el.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>
Subject: RE: [v3 2/3] pwm: Add Aspeed ast2600 PWM support

Hi Billy Tsai,

> -----Original Message-----
> From: Billy Tsai <billy_tsai@...eedtech.com>
> Sent: 07 November 2022 09:26
> To: Biju Das <biju.das.jz@...renesas.com>; jdelvare@...e.com; linux@...ck-
> us.net; robh+dt@...nel.org; joel@....id.au; andrew@...id.au;
> lee.jones@...aro.org; thierry.reding@...il.com; u.kleine-
> koenig@...gutronix.de; p.zabel@...gutronix.de; linux-hwmon@...r.kernel.org;
> devicetree@...r.kernel.org; linux-arm-kernel@...ts.infradead.org; linux-
> aspeed@...ts.ozlabs.org; linux-kernel@...r.kernel.org; linux-
> pwm@...r.kernel.org; BMC-SW <BMC-SW@...eedtech.com>; garnermic@...a.com
> Cc: kernel test robot <lkp@...el.com>; Geert Uytterhoeven
> <geert+renesas@...der.be>
> Subject: Re: [v3 2/3] pwm: Add Aspeed ast2600 PWM support
> 
> On 2022/11/7, 4:55 PM, "Biju Das" <biju.das.jz@...renesas.com> wrote:
> 
>     > > -----Original Message-----
>     > > From: Billy Tsai <billy_tsai@...eedtech.com>
>     > > Sent: 07 November 2022 08:48
>     > > To: Biju Das <biju.das.jz@...renesas.com>; jdelvare@...e.com;
> linux@...ck-
>     > > us.net; robh+dt@...nel.org; joel@....id.au; andrew@...id.au;
>     > > lee.jones@...aro.org; thierry.reding@...il.com; u.kleine-
>     > > koenig@...gutronix.de; p.zabel@...gutronix.de; linux-
> hwmon@...r.kernel.org;
>     > > devicetree@...r.kernel.org; linux-arm-kernel@...ts.infradead.org;
> linux-
>     > > aspeed@...ts.ozlabs.org; linux-kernel@...r.kernel.org; linux-
>     > > pwm@...r.kernel.org; BMC-SW <BMC-SW@...eedtech.com>;
> garnermic@...a.com
>     > > Cc: kernel test robot <lkp@...el.com>
>     > > Subject: Re: [v3 2/3] pwm: Add Aspeed ast2600 PWM support
>     > >
>     > > On 2022/11/2, 11:56 PM, "Biju Das" <biju.das.jz@...renesas.com>
> wrote:
>     > >
>     > >     > > +	parent_dev = of_find_device_by_node(np);
>     > >     > > +	priv->clk = devm_clk_get_enabled(&parent_dev->dev, NULL);
>     > >     > > +	if (IS_ERR(priv->clk))
>     > >     > > +		return dev_err_probe(dev, PTR_ERR(priv->clk),
>     > >     > > +				     "Couldn't get clock\n");
>     > >
>     > >     > What is the use case?
>     > >
>     > >     > Is pwm configured by boot loader initially ?
>     > >
>     > >     > Or
>     > >
>     > >     > pwm configured by Linux, not by the bootloader initially?
>     > >
>     > >     > Or
>     > >
>     > >     > Driver needs to handle both cases?
>     > >
>     > >     > Just asking, because you are turning on the clock unnecessarily
> here,
>     > >     > If you need to address all the use cases. If it is just first
> one, then
>     > >     > It is ok.
>     > >
>     > > Hi Biju,
>     > >
>     > > The driver want to handle all of the use cases. Can you tell me why
> turning
>     > > on the clock is unnecessarily here?
> 
>     > For the use case, "pwm configured by Linux, not by the bootloader
> initially",
> 
>     > You are unnecessarily turning on the clocks. You could
> 
>     > enable it during pwm_enable
>     > and disable it during pwm_disable.
> 
>     > For configuring registers, while pwm is in disable state,
>     > you could just turn on the clock and do the register configurations and
> turn it off.
> 
>     > By this way you are saving power.
> 
> Hi Biju,
> 
> This clock is the source clock for the pwm controller (include the accessing
> for the register).

OK, But the driver has only one clock (priv->clk = devm_clk_get_enabled(&parent_dev->dev, NULL)).
You are always turning it on during probe.


> For the output of the PWM (pwm_enable/pwm_disable), the controller have
> another register to control it.

That is enable/disable register for controlling the output of PWM.

Cheers,
Biju