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

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Date:   Tue, 16 May 2017 16:56:53 -0500
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Matthias Brugger <matthias.bgg@...il.com>
Cc:     linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: [pwm] question about potential division by zero


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?

I'm trying to figure out if this is a false positive or something that  
needs to be fixed.

I'd really appreciate any comment on this.

Thank you
--
Gustavo A. R. Silva