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

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.

Best regards
Uwe

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