[PATCH] clk: divider: fix rate calculation for fractional rates
tomi.valkeinen at ti.com
Wed Nov 6 06:48:44 EST 2013
On 2013-11-06 13:15, Russell King - ARM Linux wrote:
> On Wed, Nov 06, 2013 at 01:06:48PM +0200, Tomi Valkeinen wrote:
>> This means that the following code works a bit oddly:
>> rate = clk_round_rate(clk, 123428572);
>> clk_set_rate(clk, rate);
> You're right, but the above sequence is quite a crass thing to do. Why?
Do you mean that you think the fix is right, but the above example
sequence is silly, or that the fix is not needed either?
> clk_round_rate() returns the clock rate that clk_set_rate() would give
> you if you were to use this sequence:
> clk_rate_rate(clk, 123428572);
> rate = clk_get_rate(clk);
> The difference is that it doesn't change the actual clock rate itself;
> clk_round_rate() is meant to answer the question:
> "If I were to set _this_ rate, what clock rate would
> the clock give me?"
> thereby providing a method for drivers to inquire what the effect would
> be when changing such a clock without actually affecting it.
> So please, don't use clk_round_rate() followed by clk_set_rate().
Ok, if defined like that, then the current behavior is logical.
The comment in clk.h says "adjust a rate to the exact rate a clock can
provide", which does not contradict with what you said, but doesn't
really confirm it either. If I get "the exact rate a clock can provide"
I don't see why I can't use that exact clock rate for clk_set_rate.
Maybe the comment should be improved to state explicitly what it does.
However, how about the following sequence:
rate = clk_get_rate(clk);
I didn't test that but it should result in the clock first set to
123428571, and then to 108000000. Obviously pointless if done exactly
like that, but I don't see why the above code sequence is wrong, yet it
gives a bit surprising result.
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 901 bytes
Desc: OpenPGP digital signature
More information about the linux-arm-kernel