linux-kernel - Re: [PATCH] clk: qcom: rcg2: compute 2d using duty fraction directly

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Message-ID: <emy273nvnbzznvufe6fmbysrln6d7lm4xi5rwsuwnj4kjlalvx@7j4dxyd2f25l>
Date: Mon, 22 Dec 2025 13:09:54 -0600
From: Bjorn Andersson <andersson@...nel.org>
To: Taniya Das <taniya.das@....qualcomm.com>
Cc: Michael Turquette <mturquette@...libre.com>, 
	Stephen Boyd <sboyd@...nel.org>, Taniya Das <quic_tdas@...cinc.com>, 
	Ajit Pandey <ajit.pandey@....qualcomm.com>, Imran Shaik <imran.shaik@....qualcomm.com>, 
	Jagadeesh Kona <jagadeesh.kona@....qualcomm.com>, linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: qcom: rcg2: compute 2d using duty fraction directly

On Mon, Dec 22, 2025 at 10:38:14PM +0530, Taniya Das wrote:
> From: Taniya Das <quic_tdas@...cinc.com>

Please use oss.qualcomm.com.

> 
> The duty-cycle calculation in clk_rcg2_set_duty_cycle() currently
> derives an intermediate percentage `duty_per = (num * 100) / den` and
> then computes:
> 
>     d = DIV_ROUND_CLOSEST(n * duty_per * 2, 100);
> 
> This introduces integer truncation at the percentage step (division by
> `den`) and a redundant scaling by 100, which can reduce precision for
> large `den` and skew the final rounding.
> 
> Compute `2d` directly from the duty fraction to preserve precision and
> avoid the unnecessary scaling:
> 
>     d = DIV_ROUND_CLOSEST(n * duty->num * 2, duty->den);
> 
> This keeps the intended formula `d ≈ n * 2 * (num/den)` while performing
> a single, final rounded division, improving accuracy especially for small
> duty cycles or large denominators. It also removes the unused `duty_per`
> variable, simplifying the code.
> 
> There is no functional changes beyond improved numerical accuracy.
> 
> Fixes: 7f891faf596ed ("clk: qcom: clk-rcg2: Add support for duty-cycle for RCG")
> Signed-off-by: Taniya Das <quic_tdas@...cinc.com>
> ---
> Signed-off-by: Taniya Das <taniya.das@....qualcomm.com>
> ---
>  drivers/clk/qcom/clk-rcg2.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/clk/qcom/clk-rcg2.c b/drivers/clk/qcom/clk-rcg2.c
> index e18cb8807d73534c6437c08aeb524353a2eab06f..2838d4cb2d58ea1e351d6a5599045c72f4dc3801 100644
> --- a/drivers/clk/qcom/clk-rcg2.c
> +++ b/drivers/clk/qcom/clk-rcg2.c
> @@ -755,7 +755,7 @@ static int clk_rcg2_get_duty_cycle(struct clk_hw *hw, struct clk_duty *duty)
>  static int clk_rcg2_set_duty_cycle(struct clk_hw *hw, struct clk_duty *duty)
>  {
>  	struct clk_rcg2 *rcg = to_clk_rcg2(hw);
> -	u32 notn_m, n, m, d, not2d, mask, duty_per, cfg;
> +	u32 notn_m, n, m, d, not2d, mask, cfg;
>  	int ret;
>  
>  	/* Duty-cycle cannot be modified for non-MND RCGs */
> @@ -774,10 +774,8 @@ static int clk_rcg2_set_duty_cycle(struct clk_hw *hw, struct clk_duty *duty)
>  
>  	n = (~(notn_m) + m) & mask;
>  
> -	duty_per = (duty->num * 100) / duty->den;
> -
>  	/* Calculate 2d value */
> -	d = DIV_ROUND_CLOSEST(n * duty_per * 2, 100);
> +	d = DIV_ROUND_CLOSEST(n * duty->num * 2, duty->den);

This looks better/cleaner. But for my understanding, can you share some
example numbers that shows the problem?

Regards,
Bjorn

>  
>  	/*
>  	 * Check bit widths of 2d. If D is too big reduce duty cycle.
> 
> ---
> base-commit: cc3aa43b44bdb43dfbac0fcb51c56594a11338a8
> change-id: 20251222-duty_cycle_precision-796542baecab
> 
> Best regards,
> -- 
> Taniya Das <taniya.das@....qualcomm.com>
>