linux-kernel - Re: [v5 2/2] pwm: Add Aspeed ast2600 PWM support

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Message-ID: <52C0ED72-65A7-4C39-820F-D4368191878B@aspeedtech.com>
Date:   Mon, 17 May 2021 07:12:53 +0000
From:   Billy Tsai <billy_tsai@...eedtech.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
CC:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "andrew@...id.au" <andrew@...id.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "joel@....id.au" <joel@....id.au>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        BMC-SW <BMC-SW@...eedtech.com>,
        "lee.jones@...aro.org" <lee.jones@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [v5 2/2] pwm: Add Aspeed ast2600 PWM support

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?

Thanks