linux-kernel - [PATCH V2] clk: qcom: clk-rcg2: Fix clock rate overflow for high parent frequencies

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Message-ID: <20230901073640.4973-1-quic_devipriy@quicinc.com>
Date:   Fri, 1 Sep 2023 13:06:40 +0530
From:   Devi Priya <quic_devipriy@...cinc.com>
To:     <andersson@...nel.org>, <agross@...nel.org>,
        <konrad.dybcio@...aro.org>, <mturquette@...libre.com>,
        <sboyd@...nel.org>, <mturquette@...aro.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-clk@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC:     <quic_devipriy@...cinc.com>, <quic_saahtoma@...cinc.com>
Subject: [PATCH V2] clk: qcom: clk-rcg2: Fix clock rate overflow for high parent frequencies

If the parent clock rate is greater than unsigned long max/2 then
integer overflow happens when calculating the clock rate on 32-bit systems.
As RCG2 uses half integer dividers, the clock rate is first being
multiplied by 2 which will overflow the unsigned long max value.
Hence, replace the common pattern of doing 64-bit multiplication
and then a do_div() call with simpler mult_frac call.

Fixes: bcd61c0f535a ("clk: qcom: Add support for root clock generators (RCGs)")
Signed-off-by: Devi Priya <quic_devipriy@...cinc.com>
---
 Changes in V2:
	- Replaced 64-bit multiplication & a do_div call with mult_frac
	  call as suggested by Marijn Suijten.
	- Updated the subject title
	- Added Fixes tag
	- Did not pick up the R-b tag due to the above changes.

 drivers/clk/qcom/clk-rcg2.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/drivers/clk/qcom/clk-rcg2.c b/drivers/clk/qcom/clk-rcg2.c
index a42f661550ca..f64d69164547 100644
--- a/drivers/clk/qcom/clk-rcg2.c
+++ b/drivers/clk/qcom/clk-rcg2.c
@@ -159,15 +159,11 @@ static unsigned long
 calc_rate(unsigned long rate, u32 m, u32 n, u32 mode, u32 hid_div)
 {
 	if (hid_div) {
-		rate *= 2;
-		rate /= hid_div + 1;
+		rate = mult_frac(rate, 2, hid_div + 1);
 	}
 
 	if (mode) {
-		u64 tmp = rate;
-		tmp *= m;
-		do_div(tmp, n);
-		rate = tmp;
+		rate = mult_frac(rate, m, n);
 	}
 
 	return rate;
-- 
2.34.1