Hi Uwe, On Thu, 2014-05-15 at 09:38AM +0200, Uwe Kleine-König wrote: > 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. You're right. Apparently we haven't had such a case. Anyway, this needs to be implemented differently. Thanks, Sören -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
