linux-kernel - RE: [PATCH] clk:mmp: clk-mix.c fix divide-by-zero

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Message-ID: <E490CD805F7529488761C40FD9D26EF12AAC0143@DGGEMM527-MBS.china.huawei.com>
Date:   Sat, 30 Mar 2019 02:20:27 +0000
From:   Nixiaoming <nixiaoming@...wei.com>
To:     Stephen Boyd <sboyd@...nel.org>,
        "chao.xie@...vell.com" <chao.xie@...vell.com>,
        "mturquette@...libre.com" <mturquette@...libre.com>
CC:     "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] clk:mmp: clk-mix.c fix divide-by-zero

On 3/30/2019 6:48 AM Stephen Boyd wrote:
>Quoting nixiaoming (2019-03-29 04:46:00)
>> The _get_div function has a branch with a return value of 0
>> Add a check on the return value of _get_div to avoid divide-by-zero
>> 
>> Signed-off-by: nixiaoming <nixiaoming@...wei.com>
>
>Similar questions apply here as they do on the generic divider patch you
>sent.
>
_get_div() in both files is a different function, with a divide-by-zero problem
I will organize it into a patch set later.

>> ---
>>  drivers/clk/mmp/clk-mix.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>> 
>> diff --git a/drivers/clk/mmp/clk-mix.c b/drivers/clk/mmp/clk-mix.c
>> index 90814b2..9d152c2 100644
>> --- a/drivers/clk/mmp/clk-mix.c
>> +++ b/drivers/clk/mmp/clk-mix.c
>> @@ -245,6 +245,8 @@ static int mmp_clk_mix_determine_rate(struct clk_hw *hw,
>>                         div_val_max = _get_maxdiv(mix);
>>                         for (j = 0; j < div_val_max; j++) {
>>                                 div = _get_div(mix, j);
>> +                               if (!div) /* avoid divide-by-zero */
>
>Why can't we return 1 for the divider value here?

I personally understand that an exception or skip should be thrown after dividing by 0.
Directly modified to other values, I am not sure whether it affects the logic
My logical understanding of this code is not clear enough, I still need your guidance.
>
>> +                                       continue;
>>                                 mix_rate = parent_rate / div;
>>                                 gap = abs(mix_rate - req->rate);
>>                                 if (!parent_best || gap < gap_best) {
>> @@ -341,6 +343,8 @@ static unsigned long mmp_clk_mix_recalc_rate(struct clk_hw *hw,
>>         shift = mix->reg_info.shift_div;
>>  
>>         div = _get_div(mix, MMP_CLK_BITS_GET_VAL(mux_div, width, shift));
>> +       if (!div) /* avoid divide-by-zero */
>
>Same question.
I personally understand that an exception or skip should be thrown after dividing by 0.
Directly modified to other values, I am not sure whether it affects the logic
My logical understanding of this code is not clear enough, I still need your guidance.
>
>> +               return -EINVAL;
>>  
>>         return parent_rate / div;
>>  }
>