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

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Date:	Mon, 29 Jun 2015 11:27:27 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Yingjoe Chen <yingjoe.chen@...iatek.com>
Cc:	YH Huang <yh.huang@...iatek.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, Jun 18, 2015 at 09:58:59PM +0800, Yingjoe Chen wrote:
> 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.

Good point. I think every compiler should be able to optimize this, but
the shift isn't any worse than a divide and if we can proactively avoid
portability issues, let's go with the shift.

Thierry

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