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

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Message-Id: <20251222-duty_cycle_precision-v1-1-b0da8e9fdab7@oss.qualcomm.com>
Date: Mon, 22 Dec 2025 22:38:14 +0530
From: Taniya Das <taniya.das@....qualcomm.com>
To: Bjorn Andersson <andersson@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, Taniya Das <quic_tdas@...cinc.com>
Cc: 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, Taniya Das <taniya.das@....qualcomm.com>
Subject: [PATCH] clk: qcom: rcg2: compute 2d using duty fraction directly

From: Taniya Das <quic_tdas@...cinc.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);
 
 	/*
 	 * 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>