linux-kernel - Re: [pwm] question about potential division by zero

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Message-ID: <20170517174829.Horde.T6_ImT2RRkpgldJZDNKUSr5@gator4166.hostgator.com>
Date:   Wed, 17 May 2017 17:48:29 -0500
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Matthias Brugger <matthias.bgg@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [pwm] question about potential division by zero

Hi Matthias,

Quoting Matthias Brugger <matthias.bgg@...il.com>:

> On 16/05/17 23:56, Gustavo A. R. Silva wrote:
>>
>> Hello everybody,
>>
>> While looking into Coverity ID 1408721 I ran into the following  
>> piece of code at /drivers/pwm/pwm-mediatek.c:77:
>>
>>  77static int mtk_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>>  78                          int duty_ns, int period_ns)
>>  79{
>>  80        struct mtk_pwm_chip *pc = to_mtk_pwm_chip(chip);
>>  81        struct clk *clk = pc->clks[MTK_CLK_PWM1 + pwm->hwpwm];
>>  82        u32 resolution, clkdiv = 0;
>>  83
>>  84        resolution = NSEC_PER_SEC / clk_get_rate(clk);
>>  85
>>  86        while (period_ns / resolution > 8191) {
>>  87                resolution *= 2;
>>  88                clkdiv++;
>>  89        }
>>  90
>>  91        if (clkdiv > 7)
>>  92                return -EINVAL;
>>  93
>>  94        mtk_pwm_writel(pc, pwm->hwpwm, PWMCON, BIT(15) | BIT(3)  
>> | clkdiv);
>>  95        mtk_pwm_writel(pc, pwm->hwpwm, PWMDWIDTH, period_ns /  
>> resolution);
>>  96        mtk_pwm_writel(pc, pwm->hwpwm, PWMTHRES, duty_ns / resolution);
>>  97
>>  98        return 0;
>>  99}
>>
>> The issue here is that in case _clk_ is null, function  
>> clk_get_rate() at line 84 will return zero and a division by zero  
>> will occur.
>>
>> So my question here is if there is any chance for _clk_ to be null  
>> at line 81, hence ending up triggering a division by zero at line 84?
>
> No it can't, in the probe function will error out if one of the  
> seven clocks are not found.
>
> for (i = 0; i < MTK_CLK_MAX; i++) {
> 	pc->clks[i] = devm_clk_get(&pdev->dev, mtk_pwm_clk_name[i]);
> 	if (IS_ERR(pc->clks[i]))
> 		return PTR_ERR(pc->clks[i]);
> }
>
> It registers a pwm chip with five PWMs. When the config function is  
> called one of the five PWMs is identified (with a value from 0-4)  
> which correspondents to the MTK_CLK_PWM[1-5], so no bug here neither.
>

I get it.

Thank you very much for the clarification.
--
Gustavo A. R. Silva