From: Billy Tsai <billy_tsai@aspeedtech.com> To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de> 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 06:23:06 +0000 [thread overview] Message-ID: <C451B628-C0CC-47E9-84EF-42DB8518FE1E@aspeedtech.com> (raw) In-Reply-To: <20210517060615.3hyifoebyrddsrta@pengutronix.de> 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); div_h = order_base_2(div_h); if (div_h > 0xf) div_h = 0xf; div_l = rate * state->period; div_l >>= (__fls(PWM_ASPEED_FIXED_PERIOD + 1) + div_h); div_l = DIV_ROUND_DOWN_ULL(div_l, NSEC_PER_SEC); How about this change of the driver? Thanks
WARNING: multiple messages have this Message-ID (diff)
From: Billy Tsai <billy_tsai@aspeedtech.com> To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de> 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 06:23:06 +0000 [thread overview] Message-ID: <C451B628-C0CC-47E9-84EF-42DB8518FE1E@aspeedtech.com> (raw) In-Reply-To: <20210517060615.3hyifoebyrddsrta@pengutronix.de> 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); div_h = order_base_2(div_h); if (div_h > 0xf) div_h = 0xf; div_l = rate * state->period; div_l >>= (__fls(PWM_ASPEED_FIXED_PERIOD + 1) + div_h); div_l = DIV_ROUND_DOWN_ULL(div_l, NSEC_PER_SEC); How about this change of the driver? Thanks _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2021-05-17 6:23 UTC|newest] Thread overview: 20+ 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 ` 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 ` Billy Tsai 2021-05-14 2:48 ` [v5 2/2] pwm: Add Aspeed ast2600 PWM support Billy Tsai 2021-05-14 2:48 ` Billy Tsai 2021-05-15 15:18 ` Uwe Kleine-König 2021-05-15 15:18 ` Uwe Kleine-König 2021-05-17 2:53 ` Billy Tsai 2021-05-17 2:53 ` Billy Tsai 2021-05-17 6:06 ` Uwe Kleine-König 2021-05-17 6:06 ` Uwe Kleine-König 2021-05-17 6:23 ` Billy Tsai [this message] 2021-05-17 6:23 ` Billy Tsai 2021-05-17 6:35 ` Uwe Kleine-König 2021-05-17 6:35 ` Uwe Kleine-König 2021-05-17 7:12 ` Billy Tsai 2021-05-17 7:12 ` Billy Tsai 2021-05-17 17:10 ` Uwe Kleine-König 2021-05-17 17:10 ` Uwe Kleine-König
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