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

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Message-ID: <20150629092145.GF5431@ulmo>
Date:	Mon, 29 Jun 2015 11:21:47 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	YH Huang <yh.huang@...iatek.com>
Cc:	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>, yingjoe.chen@...iatek.com
Subject: Re: [PATCH v2 2/2] pwm: add MediaTek display PWM driver support

On Thu, Jun 18, 2015 at 06:19:44PM +0800, YH Huang wrote:
> On Fri, 2015-06-12 at 12:20 +0200, Thierry Reding wrote:
> > On Thu, May 21, 2015 at 09:29:24PM +0800, YH Huang wrote:
[...]
> > > +#define DISP_PWM_CON_1		0x14
> > > +#define PWM_PERIOD_MASK		0xfff
> > > +#define PWM_PERIOD_MAX		0x00000fff
> > 
> > Same here. PWM_PERIOD_MAX isn't actually used anywhere, so perhaps just
> > drop it altogether. But see also below...
> > 
> > > +/* 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 compiler should be able to optimize the division to a shift, so both
will likely result in the same code. But I don't mind much either way as
long as we don't define two symbols for essentially the same value.

Thierry

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