From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Billy Tsai <billy_tsai@aspeedtech.com>
Cc: "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
"kernel@pengutronix.de" <kernel@pengutronix.de>,
"linux-aspeed@lists.ozlabs.org" <linux-aspeed@lists.ozlabs.org>,
"linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>,
"andrew@aj.id.au" <andrew@aj.id.au>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"robh+dt@kernel.org" <robh+dt@kernel.org>,
"thierry.reding@gmail.com" <thierry.reding@gmail.com>,
"joel@jms.id.au" <joel@jms.id.au>,
"p.zabel@pengutronix.de" <p.zabel@pengutronix.de>,
BMC-SW <BMC-SW@aspeedtech.com>,
"lee.jones@linaro.org" <lee.jones@linaro.org>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>
Subject: Re: [v5 2/2] pwm: Add Aspeed ast2600 PWM support
Date: Mon, 17 May 2021 08:06:15 +0200 [thread overview]
Message-ID: <20210517060615.3hyifoebyrddsrta@pengutronix.de> (raw)
In-Reply-To: <7A439233-C5FF-4BCA-8A5C-945EB847F487@aspeedtech.com>
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Hello Billy,
On Mon, May 17, 2021 at 02:53:44AM +0000, Billy Tsai wrote:
> On 2021/5/15, 11:57 PM,Uwe Kleine-Königwrote:
>
> > > + div_h = DIV_ROUND_DOWN_ULL(div_h,
> > > + (FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1));
> > > + div_h = DIV_ROUND_DOWN_ULL(div_h, NSEC_PER_SEC);
>
> > As a division is an expensive operation you can better first multiply
> > NSEC_PER_SEC and FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1 and divide by
> > the result.
>
> When I multiply NSEC_PER_SEC and FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1 the result will overflow
> for 32-bits and the divisor type of do_div is 32-bits so I need to do div twice to avoid the issue.
> Can you give me some suggests?
Hmm, you're right. There doesn't seem to be a div64_64, I thought there
was one. Anyhow, while looking at the various divide functions I saw
that dividing by a constant shouldn't be that expensive, so I think the
sane way is to keep the two divisions and add a comment describing the
problem.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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next prev parent reply other threads:[~2021-05-17 6:10 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-14 2:48 [v5 0/2] Support pwm driver for aspeed ast26xx Billy Tsai
2021-05-14 2:48 ` [v5 1/2] dt-bindings: Add bindings for aspeed pwm-tach Billy Tsai
2021-05-14 2:48 ` [v5 2/2] pwm: Add Aspeed ast2600 PWM support Billy Tsai
2021-05-15 15:18 ` Uwe Kleine-König
2021-05-17 2:53 ` Billy Tsai
2021-05-17 6:06 ` Uwe Kleine-König [this message]
2021-05-17 6:23 ` Billy Tsai
2021-05-17 6:35 ` Uwe Kleine-König
2021-05-17 7:12 ` Billy Tsai
2021-05-17 17:10 ` Uwe Kleine-König
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