linux-kernel - Re: [PATCH] clk: divider: fix rate calculation for fractional rates

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Message-ID: <52FA3132.2030400@ti.com>
Date:	Tue, 11 Feb 2014 16:18:26 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Mike Turquette <mturquette@...aro.org>
CC:	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clk: divider: fix rate calculation for fractional rates

Mike, ping.

 Tomi

On 06/11/13 13:06, Tomi Valkeinen wrote:
> clk-divider.c does not calculate the rates consistently at the moment.
> 
> As an example, on OMAP3 we have a clock divider with a source clock of
> 864000000 Hz. With dividers 6, 7 and 8 the theoretical rates are:
> 
> 6: 144000000
> 7: 123428571.428571...
> 8: 108000000
> 
> Calling clk_round_rate() with the rate in the first column will give the
> rate in the second column:
> 
> 144000000 -> 144000000
> 143999999 -> 123428571
> 123428572 -> 123428571
> 123428571 -> 108000000
> 
> Note how clk_round_rate() returns 123428571 for rates from 123428572 to
> 143999999, which is mathematically correct, but when clk_round_rate() is
> called with 123428571, the returned value is surprisingly 108000000.
> 
> This means that the following code works a bit oddly:
> 
> rate = clk_round_rate(clk, 123428572);
> clk_set_rate(clk, rate);
> 
> As clk_set_rate() also does clock rate rounding, the result is that the
> clock is set to the rate of 108000000, not 123428571 returned by the
> clk_round_rate.
> 
> This patch changes the clk-divider.c to use DIV_ROUND_UP when
> calculating the rate. This gives the following behavior which fixes the
> inconsistency:
> 
> 144000000 -> 144000000
> 143999999 -> 123428572
> 123428572 -> 123428572
> 123428571 -> 108000000
> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...com>
> Cc: Mike Turquette <mturquette@...aro.org>
> ---
>  drivers/clk/clk-divider.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> index 8d3009e..43348f3 100644
> --- a/drivers/clk/clk-divider.c
> +++ b/drivers/clk/clk-divider.c
> @@ -24,7 +24,7 @@
>   * Traits of this clock:
>   * prepare - clk_prepare only ensures that parents are prepared
>   * enable - clk_enable only ensures that parents are enabled
> - * rate - rate is adjustable.  clk->rate = parent->rate / divisor
> + * rate - rate is adjustable.  clk->rate = DIV_ROUND_UP(parent->rate / divisor)
>   * parent - fixed parent.  No clk_set_parent support
>   */
>  
> @@ -115,7 +115,7 @@ static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
>  		return parent_rate;
>  	}
>  
> -	return parent_rate / div;
> +	return DIV_ROUND_UP(parent_rate, div);
>  }
>  
>  /*
> @@ -185,7 +185,7 @@ static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate,
>  		}
>  		parent_rate = __clk_round_rate(__clk_get_parent(hw->clk),
>  				MULT_ROUND_UP(rate, i));
> -		now = parent_rate / i;
> +		now = DIV_ROUND_UP(parent_rate, i);
>  		if (now <= rate && now > best) {
>  			bestdiv = i;
>  			best = now;
> @@ -207,7 +207,7 @@ static long clk_divider_round_rate(struct clk_hw *hw, unsigned long rate,
>  	int div;
>  	div = clk_divider_bestdiv(hw, rate, prate);
>  
> -	return *prate / div;
> +	return DIV_ROUND_UP(*prate, div);
>  }
>  
>  static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
> @@ -218,7 +218,7 @@ static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
>  	unsigned long flags = 0;
>  	u32 val;
>  
> -	div = parent_rate / rate;
> +	div = DIV_ROUND_UP(parent_rate, rate);
>  	value = _get_val(divider, div);
>  
>  	if (value > div_mask(divider))
> 



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