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 19:10:28 +0200 [thread overview]
Message-ID: <20210517171028.qioxg53dpzlq4izu@pengutronix.de> (raw)
In-Reply-To: <52C0ED72-65A7-4C39-820F-D4368191878B@aspeedtech.com>
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On Mon, May 17, 2021 at 07:12:53AM +0000, Billy Tsai wrote:
> Hi,
>
> > On 2021/5/17, 2:35 PM,Uwe Kleine-Königwrote:
>
> > On Mon, May 17, 2021 at 06:23:06AM +0000, Billy Tsai wrote:
> > > Hi,
> > > On 2021/5/17, 2:06 PM,Uwe Kleine-Königwrote:
> > >
> > > 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.
> > > According to our fixed value, I think that I can use bit shift to reduce one divide function:
> > >
> > > rate = clk_get_rate(priv->clk);
> > > /* Get the smallest value for div_h */
> > > div_h = rate * state->period;
> > > div_h >>= (__fls(PWM_ASPEED_FIXED_PERIOD + 1) +
> > > __fls(FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1));
> > > div_h = DIV_ROUND_DOWN_ULL(div_h, NSEC_PER_SEC);
>
> > Did you check how this is compiled to code? I'd expect that it doesn't
> > result in better code than writing it as a division. Given that a
> > division is easier to understand for a human reader, I'd stick to that.
>
> I found that I can use div64_64 through #include <linux/math64.h> and use "div64_u64":
>
> u64 div_h, div_l, divisor;
> u32 index = pwm->hwpwm;
>
> rate = clk_get_rate(priv->clk);
> /* Get the smallest value for div_h */
> div_h = rate * state->period;
> divisor = (u64)NSEC_PER_SEC * (PWM_ASPEED_FIXED_PERIOD + 1) *
> (FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1);
> div_h = div64_u64(div_h, divisor);
> div_h = order_base_2(div_h);
> if (div_h > 0xf)
> div_h = 0xf;
>
> div_l = rate * state->period;
> divisor = (u64)NSEC_PER_SEC * (PWM_ASPEED_FIXED_PERIOD + 1) *
> BIT(div_h);
> div_l = div64_u64(div_l, divisor);
>
> Can I use this one?
Looks good to me. If you want to improve further you can expand the
comment about div_h to somethink like:
/*
* Pick a small value for div_h so that div_l can be big which
* results in a finer resolution near the target period value.
*/
Another detail I don't like much is that the name div_h is only
justified after the last assignment. I don't have a good suggestion here
though.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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prev parent reply other threads:[~2021-05-17 17:12 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
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 [this message]
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