linux-kernel - Re: [PATCH v2] clk:mxs: Fix bug on frequency divider

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Date:	Fri, 18 Sep 2015 09:21:17 +0200
From:	Stefan Wahren <stefan.wahren@...e.com>
To:	Victorien Vedrine <victorien.vedrine@...rys.net>,
	linux-kernel@...r.kernel.org
Cc:	linux-clk@...r.kernel.org, sboyd@...eaurora.org,
	mturquette@...libre.com, shawn.guo@...aro.org,
	"Fabio.Estevam@...escale.com" <Fabio.Estevam@...escale.com>,
	Marek Vasut <marex@...x.de>, Shawn Guo <shawnguo@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] clk:mxs: Fix bug on frequency divider

+Shawn's new address
+linux-arm-kernel

> On drivers/clk/mxs/clk-frac.c, the function clk_frac_round_rate returned a bad
> result. The division before multiplication computes a wrong value ; the
> calculation is inverted to fix the problem. The second issue is that the exact
> rate have decimals and they are truncate. The consequence is that the function
> clk_frac_set_rate (which use the result of clk_frac_round_rate) computes a
> wrong value for the register (the rate generated can be closer to the desired
> rate). The correction is : if there is decimal to the result, it is rounded to
> the next larger integer.
> On drivers/clk/mxs/clk-frac.c, the function clk_frac_recalc_rate returned
> a bad result. The multiplication is made before the division to compute a
> correct value.
>
> Signed-off-by: Victorien Vedrine <victorien.vedrine@...rys.net>
> ---
>  drivers/clk/mxs/clk-frac.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/clk/mxs/clk-frac.c b/drivers/clk/mxs/clk-frac.c
> index e6aa6b5..b3fa7fa 100644
> --- a/drivers/clk/mxs/clk-frac.c
> +++ b/drivers/clk/mxs/clk-frac.c
> @@ -42,11 +42,13 @@ static unsigned long clk_frac_recalc_rate(struct clk_hw *hw,
>  {
>  	struct clk_frac *frac = to_clk_frac(hw);
>  	u32 div;
> +	u64 tmp_rate;
>  
>  	div = readl_relaxed(frac->reg) >> frac->shift;
>  	div &= (1 << frac->width) - 1;
>  
> -	return (parent_rate >> frac->width) * div;
> +	tmp_rate = (u64)parent_rate * div;
> +	return tmp_rate >> frac->width;
>  }
>  
>  static long clk_frac_round_rate(struct clk_hw *hw, unsigned long rate,
> @@ -55,7 +57,7 @@ static long clk_frac_round_rate(struct clk_hw *hw, unsigned long rate,
>  	struct clk_frac *frac = to_clk_frac(hw);
>  	unsigned long parent_rate = *prate;
>  	u32 div;
> -	u64 tmp;
> +	u64 tmp, tmp_rate, result;
>  
>  	if (rate > parent_rate)
>  		return -EINVAL;
> @@ -68,7 +70,11 @@ static long clk_frac_round_rate(struct clk_hw *hw, unsigned long rate,
>  	if (!div)
>  		return -EINVAL;
>  
> -	return (parent_rate >> frac->width) * div;
> +	tmp_rate = (u64)parent_rate * div;
> +	result = tmp_rate >> frac->width;
> +	if ((result << frac->width) < tmp_rate)
> +		result += 1;
> +	return result;
>  }
>  
>  static int clk_frac_set_rate(struct clk_hw *hw, unsigned long rate,

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