[PATCH 6.19 515/781] clk: qcom: rcg2: compute 2d using duty fraction directly

Greg Kroah-Hartman
2026-02-25T01:36:33Z

From: Taniya Das <taniya.das@oss.qualcomm.com>

[ Upstream commit d6205a1878dd4cc9664c4b4829b68a29c0426efc ]

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 <taniya.das@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260105-duty_cycle_precision-v2-1-d1d466a6330a@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 e18cb8807d735..2838d4cb2d58e 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.
--
2.51.0