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

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Message-ID: <16ae9733-3e22-db7f-56c4-47c1a55cf23c@quicinc.com>
Date:   Wed, 30 Aug 2023 14:10:08 +0530
From:   Devi Priya <quic_devipriy@...cinc.com>
To:     Marijn Suijten <marijn.suijten@...ainline.org>
CC:     <andersson@...nel.org>, <agross@...nel.org>,
        <konrad.dybcio@...aro.org>, <mturquette@...libre.com>,
        <sboyd@...nel.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-clk@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <quic_saahtoma@...cinc.com>
Subject: Re: [PATCH] clk: qcom: clk-rcg2: Fix wrong RCG clock rate for high
 parent frequencies



On 7/21/2023 12:37 AM, Marijn Suijten wrote:
> Can you retitle this to state "overflow" rather than just "wrong"?
> That's more descriptive.
> 
> E.g "Fix clockrate overflow for high parent frequencies"
Sure okay
> 
> On 2023-07-20 14:03:04, 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. So, use
>> unsigned long long for rate computations to avoid overflow.
>>
>> Signed-off-by: Devi Priya <quic_devipriy@...cinc.com>
>> ---
>>   drivers/clk/qcom/clk-rcg2.c | 12 ++++++------
>>   1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/clk/qcom/clk-rcg2.c b/drivers/clk/qcom/clk-rcg2.c
>> index e22baf3a7112..42d00b134975 100644
>> --- a/drivers/clk/qcom/clk-rcg2.c
>> +++ b/drivers/clk/qcom/clk-rcg2.c
>> @@ -156,18 +156,18 @@ static int clk_rcg2_set_parent(struct clk_hw *hw, u8 index)
>>    *            hid_div       n
>>    */
>>   static unsigned long
>> -calc_rate(unsigned long rate, u32 m, u32 n, u32 mode, u32 hid_div)
>> +calc_rate(unsigned long parent_rate, u32 m, u32 n, u32 mode, u32 hid_div)
>>   {
>> +	u64 rate = parent_rate;
>> +
>>   	if (hid_div) {
>>   		rate *= 2;
>> -		rate /= hid_div + 1;
>> +		do_div(rate, hid_div + 1);
> 
> I'm pretty sure mult_frac() could have solved this as well, without
> temporarily going to u64?
> 
>      mult_frac(rate, 2, hid_div + 1)

Yes, sure will update
> 
>>   	}
>>   
>>   	if (mode) {
>> -		u64 tmp = rate;
>> -		tmp *= m;
>> -		do_div(tmp, n);
>> -		rate = tmp;
>> +		rate *= m;
>> +		do_div(rate, n);
> 
>      mult_frac(rate, m, n)

Will update in V2

Thanks,
Devi Priya
> 
> Or am I totally wrong?
> 
> - Marijn
> 
>>   	}
>>   
>>   	return rate;
>> -- 
>> 2.17.1
>>