Hi Uwe,

On Thu, 2014-05-15 at 09:38AM +0200, Uwe Kleine-K?nig wrote:

Hello,

it's great you pick that up.

On Wed, May 14, 2014 at 03:30:52PM -0700, Soren Brinkmann wrote:

quoted

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 <redacted>
---

 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,
	S?ren