From: Thierry Reding <thierry.reding@gmail.com>
To: Yingjoe Chen <yingjoe.chen@mediatek.com>
Cc: YH Huang <yh.huang@mediatek.com>,
Matthias Brugger <matthias.bgg@gmail.com>,
Mark Rutland <mark.rutland@arm.com>,
Rob Herring <robh+dt@kernel.org>, Pawel Moll <pawel.moll@arm.com>,
linux-pwm@vger.kernel.org, devicetree@vger.kernel.org,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
srv_heupstream@mediatek.com, linux-mediatek@lists.infradead.org,
Sascha Hauer <kernel@pengutronix.de>
Subject: Re: [PATCH v2 2/2] pwm: add MediaTek display PWM driver support
Date: Mon, 29 Jun 2015 11:27:27 +0200 [thread overview]
Message-ID: <20150629092726.GG5431@ulmo> (raw)
In-Reply-To: <1434635939.22029.7.camel@mtksdaap41>
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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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WARNING: multiple messages have this Message-ID (diff)
From: thierry.reding@gmail.com (Thierry Reding)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH v2 2/2] pwm: add MediaTek display PWM driver support
Date: Mon, 29 Jun 2015 11:27:27 +0200 [thread overview]
Message-ID: <20150629092726.GG5431@ulmo> (raw)
In-Reply-To: <1434635939.22029.7.camel@mtksdaap41>
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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next prev parent reply other threads:[~2015-06-29 9:28 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-21 13:29 [PATCH v2 0/2] Add MediaTek display PWM driver YH Huang
2015-05-21 13:29 ` YH Huang
2015-05-21 13:29 ` YH Huang
2015-05-21 13:29 ` [PATCH v2 1/2] dt-bindings: pwm: add MediaTek display PWM bindings YH Huang
2015-05-21 13:29 ` YH Huang
2015-05-21 13:29 ` YH Huang
2015-06-12 10:23 ` Thierry Reding
2015-06-12 10:23 ` Thierry Reding
2015-06-18 9:00 ` YH Huang
2015-06-18 9:00 ` YH Huang
2015-06-18 9:00 ` YH Huang
2015-05-21 13:29 ` [PATCH v2 2/2] pwm: add MediaTek display PWM driver support YH Huang
2015-05-21 13:29 ` YH Huang
2015-05-21 13:29 ` YH Huang
2015-06-12 10:20 ` Thierry Reding
2015-06-12 10:20 ` Thierry Reding
2015-06-18 10:19 ` YH Huang
2015-06-18 10:19 ` YH Huang
2015-06-18 10:19 ` YH Huang
2015-06-18 13:58 ` Yingjoe Chen
2015-06-18 13:58 ` Yingjoe Chen
2015-06-18 13:58 ` Yingjoe Chen
2015-06-29 9:27 ` Thierry Reding [this message]
2015-06-29 9:27 ` Thierry Reding
2015-06-29 9:21 ` Thierry Reding
2015-06-29 9:21 ` Thierry Reding
2015-06-12 10:25 ` Thierry Reding
2015-06-12 10:25 ` Thierry Reding
2015-06-18 9:02 ` YH Huang
2015-06-18 9:02 ` YH Huang
2015-06-18 9:02 ` YH Huang
2015-05-25 2:14 ` [PATCH v2 0/2] Add MediaTek display PWM driver Yingjoe Chen
2015-05-25 2:14 ` Yingjoe Chen
2015-05-25 2:14 ` Yingjoe Chen
2015-06-05 6:15 ` YH Huang
2015-06-05 6:15 ` YH Huang
2015-06-05 8:39 ` Matthias Brugger
2015-06-05 8:39 ` Matthias Brugger
2015-06-05 10:45 ` YH Huang
2015-06-05 10:45 ` YH Huang
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