linux-kernel - Re: [PATCH] clk: tegra: Fix an infinite loop when clock rate is zero

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Message-ID: <20180924080631.GE7636@tbergstrom-lnx.Nvidia.com>
Date:   Mon, 24 Sep 2018 11:06:31 +0300
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     ryang <decatf@...il.com>
CC:     Prashant Gaikwad <pgaikwad@...dia.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        <linux-clk@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clk: tegra: Fix an infinite loop when clock rate is zero

On Fri, Sep 21, 2018 at 06:00:37PM -0400, ryang wrote:
> Calling clk_set_rate or clk_round_rate will lock up the kernel when the
> rate is zero. This avoids the infinite loop and uses a slightly more
> optimized p divider calculation.
> 

Acked-By: Peter De Schrijver <pdeschrijver@...dia.com>

At some point we should also limit pdiv to its maximum possible value, but
that's not so obvious as we need to take into account PLLs where pdiv is
non-linear.

Peter.

> Signed-off-by: ryang <decatf@...il.com>
> ---
>  drivers/clk/tegra/clk-pll.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/clk/tegra/clk-pll.c b/drivers/clk/tegra/clk-pll.c
> index 830d1c87fa7c..17a058c3bbc1 100644
> --- a/drivers/clk/tegra/clk-pll.c
> +++ b/drivers/clk/tegra/clk-pll.c
> @@ -582,9 +582,8 @@ static int _calc_rate(struct clk_hw *hw, struct tegra_clk_pll_freq_table *cfg,
>  	}
>  
>  	/* Raise VCO to guarantee 0.5% accuracy */
> -	for (cfg->output_rate = rate; cfg->output_rate < 200 * cfreq;
> -	     cfg->output_rate <<= 1)
> -		p_div++;
> +	p_div = rate ? fls((200 * cfreq) / rate) : 0;
> +	cfg->output_rate = rate << p_div;
>  
>  	cfg->m = parent_rate / cfreq;
>  	cfg->n = cfg->output_rate / cfreq;
> -- 
> 2.17.1
>