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

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Date:   Mon, 4 Sep 2023 22:36:33 +0200
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     Devi Priya <quic_devipriy@...cinc.com>
Cc:     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, quic_saahtoma@...cinc.com
Subject: Re: [PATCH V2] clk: qcom: clk-rcg2: Fix clock rate overflow for high
 parent frequencies

On 2023-09-01 13:06:40, Devi Priya wrote:
> 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

Wasn't it doing 32-bit multiplication on 32-bit systems?  Glad to see the u64
mul and div cleaned up in the if (mode) block either way though.

> and then a do_div() call with simpler mult_frac call.

mul_frac()

> 
> 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.

Don't forget to add reviewers to CC on followup revisions :)

Reviewed-by: Marijn Suijten <marijn.suijten@...ainline.org>

> 	- 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
>