Re: [RFC PATCH 2/5] clk: Introduce 'clk_round_rate_nearest()'

[Uwe Kleine-König]

Hello,

it's great you pick that up.

On Wed, May 14, 2014 at 03:30:52PM -0700, Soren Brinkmann wrote:
> Introduce a new API function to round a rate to the closest possible
> rate the HW clock can generate.
> In contrast to 'clk_round_rate()' which works similar, but always returns
> a frequency <= its input rate.
> 
> The code comes from Uwe and was copied from this LKML thread:
> https://lkml.org/lkml/2013/3/21/115
> 
> Signed-off-by: Soren Brinkmann <soren.brinkmann@xxxxxxxxxx>
> ---
> 
>  drivers/clk/clk.c   | 26 ++++++++++++++++++++++++--
>  include/linux/clk.h | 14 ++++++++++++--
>  2 files changed, 36 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index dff0373f53c1..b715f5a9826c 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -1011,8 +1011,9 @@ unsigned long __clk_round_rate(struct clk *clk, unsigned long rate)
>   * @rate: the rate which is to be rounded
>   *
>   * Takes in a rate as input and rounds it to a rate that the clk can actually
> - * use which is then returned.  If clk doesn't support round_rate operation
> - * then the parent rate is returned.
> + * use and does not exceed the requested frequency, which is then returned.
> + * If clk doesn't support round_rate operation then the parent rate
> + * is returned.
>   */
>  long clk_round_rate(struct clk *clk, unsigned long rate)
>  {
> @@ -1027,6 +1028,27 @@ long clk_round_rate(struct clk *clk, unsigned long rate)
>  EXPORT_SYMBOL_GPL(clk_round_rate);
>  
>  /**
> + * clk_round_rate_nearest - round the given rate for a clk
> + * @clk: the clk for which we are rounding a rate
> + * @rate: the rate which is to be rounded
> + *
> + * Takes in a rate as input and rounds it to the closest rate that the clk
> + * can actually use which is then returned. If clk doesn't support
> + * round_rate operation then the parent rate is returned.
> + */
> +long clk_round_rate_nearest(struct clk *clk, unsigned long rate)
> +{
> +	long lower_limit = clk_round_rate(clk, rate);
> +	long upper_limit = clk_round_rate(clk, rate + (rate - lower_limit));
> +
> +	if (rate - lower_limit < upper_limit - rate)
> +		return lower_limit;
> +	else
> +		return upper_limit;
I wanted to suggest to add some comment to describe why the calculation
works here. While trying to proove it, I noticed that this
implementation is buggy.
Consider a clock that can provide the following frequencies: 38000,
38401, 38600.

	clk_round_rate_nearest(clk, 38400)
	  lower_limit = clk_round_rate(clk, 38400) -> 38000
	  upper_limit = clk_round_rate(clk, 38800) -> 38600

	  return 38600

but 38401 would have been the better/correct answer. I think you cannot
implement clk_round_rate_nearest without iteration if you don't want to
add specific logic to the clock providers.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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