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

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Message-ID: <C451B628-C0CC-47E9-84EF-42DB8518FE1E@aspeedtech.com>
Date:   Mon, 17 May 2021 06:23:06 +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>,
        "kernel@...gutronix.de" <kernel@...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>,
        "p.zabel@...gutronix.de" <p.zabel@...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, 2:06 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