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SubjectRe: [PATCH 1/4] clk: Introduce 'clk_find_nearest_rate()'
On Mon, Jun 30, 2014 at 09:56:33AM -0700, Soren Brinkmann wrote:
> Introduce a new API function to find the rate a clock can provide which
> is closest to a given rate.
>
> clk_round_rate() leaves it to the clock driver how rounding is done.
> Commonly implementations round down due to use-cases that have a certain
> frequency maximum that must not be exceeded.
>
> The new API call enables use-cases where accuracy is preferred. E.g.
> Ethernet clocks.
>
> Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
> ---
>
> drivers/clk/clk.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/clk.h | 9 +++++++++
> 2 files changed, 66 insertions(+)
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index 8b73edef151d..fce1165cd879 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -1030,6 +1030,63 @@ long clk_round_rate(struct clk *clk, unsigned long rate)
> EXPORT_SYMBOL_GPL(clk_round_rate);
>
> /**
> + * clk_find_nearest_rate - 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 finds the closest rate that the clk
> + * can actually use which is then returned.
> + * Note: This function relies on the clock's clk_round_rate() implementation.
> + * For cases clk_round_rate() rounds up, not the closest but the rounded up
> + * rate is found.
> + */
> +long clk_find_nearest_rate(struct clk *clk, unsigned long rate)
> +{
> + long ret, lower, upper;
> + unsigned long tmp;
> +
> + clk_prepare_lock();
> +
> + lower = __clk_round_rate(clk, rate);
> + if (lower >= rate || lower < 0) {
> + ret = lower;
> + goto unlock;
> + }
> +
> + tmp = rate + (rate - lower) - 1;
> + if (tmp > LONG_MAX)
> + upper = LONG_MAX;
> + else
> + upper = tmp;
Consider rate = 0xf0000000, lower = 0x7fffffff (= LONG_MAX). Then tmp =
(unsigned long)0x160000000 = 0x60000000. In this case you pick upper =
0x60000000 while you should use upper = LONG_MAX.

I think you need

- if (tmp > LONG_MAX)
+ if (tmp > LONG_MAX || tmp < rate)

(and a comment)

> +
> + upper = __clk_round_rate(clk, upper);
> + if (upper <= lower || upper < 0) {
Is it an idea to do something like:

if (IS_ENABLED(CONFIG_CLK_SANITY_CHECKS))
WARN_ON(upper < lower && upper >= 0);

here?

> + ret = lower;
> + goto unlock;
> + }
> +
> + lower = rate + 1;
> + while (lower < upper) {
> + long rounded, mid;
> +
> + mid = lower + ((upper - lower) >> 1);
> + rounded = __clk_round_rate(clk, mid);
> + if (rounded < lower)
> + lower = mid + 1;
> + else
> + upper = rounded;
> + }
This is broken if you don't assume that __clk_round_rate rounds down.
Consider an implementation that already does round_nearest and clk can
assume the values 0x60000 and 0x85000 (and nothing in between), and rate
= 0x70000. This results in

lower = 0x60000;
tmp = 0x7ffff;
upper = __clk_round_rate(clk, 0x7ffff) = 0x85000

before the loop and the loop then doesn't even terminate.

Best regards
Uwe

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