linux-kernel - Re: [PATCH v2 2/2] pwm: add MediaTek display PWM driver support

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Date:	Thu, 18 Jun 2015 21:58:59 +0800
From:	Yingjoe Chen <yingjoe.chen@...iatek.com>
To:	YH Huang <yh.huang@...iatek.com>
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>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<srv_heupstream@...iatek.com>,
	<linux-mediatek@...ts.infradead.org>,
	"Sascha Hauer" <kernel@...gutronix.de>
Subject: Re: [PATCH v2 2/2] pwm: add MediaTek display PWM driver support

On Thu, 2015-06-18 at 18:19 +0800, YH Huang wrote:
> On Fri, 2015-06-12 at 12:20 +0200, Thierry Reding wrote:
> > > +/* Shift log2(PWM_PERIOD_MAX + 1) as divisor */
> > > +#define PWM_PERIOD_BIT_SHIFT	12
> > 
> > I wasn't very clear about this in my earlier review, so let me try to
> > explain why I think this is confusing. You use this as a divisor, but
> > you encode it as a shift. It's also PWM_PERIOD_MAX + 1, so I think it
> > would make more sense to drop this, keep PWM_PERIOD_MAX as above and
> > then replace the
> > 
> > 	>> PWM_PERIOD_BIT_SHIFT
> > 	
> > below by
> > 
> > 	/ (PWM_PERIOD_MAX + 1)
> > 
> 
> Maybe I can change in this way:
> Remove this: #define PWM_PERIOD_MAX		0x00000fff
> Using ">> PWM_PERIOD_BIT_SHIFT" is faster than "/ (PWM_PERIOD_MAX + 1)"
> Is this right?


The place which use this shift is:

	clk_div = div_u64(rate * period_ns, NSEC_PER_SEC) >> 
			  PWM_PERIOD_BIT_SHIFT;

div_u64 return u64. If we change >> to /, and somehow compiler didn't
optimize that div into shift, it will cause build error.

Joe.C


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