Meaning of clk_round_rate()?
Stephen Boyd
(include/linux/clk.h)? The function documentation says "adjust a rate to
the exact rate a clock can provide". That seems pretty vague. I'm lead
to believe that it rounds the rate to the closest rate supported. Is
that correct? Is there some sort of error margin where beyond that it's
no longer possible to be rounded? 0.5%? 1%?
Assuming it's doing closest matching, I don't see how it's very useful
in practice. Some users of clk_round_rate() are blindly searching up and
down in the frequency space until they find a suitable rate (see
sound/atmel/abdac.c and sound/spi/at73c213.c). These drivers might be
better served by something like a clk_round_rate_up() and a
clk_round_rate_down() which would round the rate to the nearest higher
and lower frequency respectively without requiring complex loops around
clk_round_rate().
In addition, an up/down rounding approach would make it simpler for
drivers to find a min/max rate (for example display panels have a max
frequency they can support).
A similar approach was suggested by David Brownell [1] but nothing came
of it.
[1] http://article.gmane.org/gmane.linux.ports.arm.kernel/38076
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Russell King - ARM Linux
> What is the meaning of clk_round_rate() in the clk API
> (include/linux/clk.h)? The function documentation says "adjust a rate to
> the exact rate a clock can provide". That seems pretty vague. I'm lead
> to believe that it rounds the rate to the closest rate supported. Is
> that correct? Is there some sort of error margin where beyond that it's
> no longer possible to be rounded? 0.5%? 1%?
clk_round_rate() returns the clock rate which will be set if you ask
clk_set_rate() to set that rate. It provides a way to query from
the implementation exactly what rate you'll get if you use clk_set_rate()
with that same argument.
So essentially, clk_set_rate() should be:
static int clk_set_rate(struct clk *clk, unsigned long rate)
{
rate = clk_round_rate(clk, rate);
return set_actual_rate(clk, rate);
Stephen Boyd
> clk_round_rate() returns the clock rate which will be set if you ask
> clk_set_rate() to set that rate. It provides a way to query from
> the implementation exactly what rate you'll get if you use clk_set_rate()
> with that same argument.
>
> So essentially, clk_set_rate() should be:
>
> static int clk_set_rate(struct clk *clk, unsigned long rate)
> {
> rate = clk_round_rate(clk, rate);
> return set_actual_rate(clk, rate);
> }
From what I understand, you're saying clk_round_rate() is defined as
what clk_set_rate() would do, which is call clk_round_rate() and then
set the rate with whatever is returned by clk_round_rate()? Isn't that a
recursive definition?
I'll play along though. The use of the function is to determine what the
rate will be if I call clk_set_rate(), but what is the implementation of
it suppose to be. I guess now I'm asking what should clk_set_rate() do?
Round up, down, to the closest value, or just fail if it's not exact.
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Saravana Kannan
> clk_round_rate() returns the clock rate which will be set if you ask
> clk_set_rate() to set that rate. It provides a way to query from
> the implementation exactly what rate you'll get if you use clk_set_rate()
> with that same argument.
Fair enough explanation for clk_round_rate(). I guess I should take it
as "it's up to the specific clock implementation on what it wants to do".
But what about the problem of a clock consumer trying to find a suitable
frequency amongst the ones provided by a particular clock?
What are your thoughts on adding the following two APIs to linux/clk.h?
clk_round_rate_down/floor()
clk_round_rate_up/ceil()
Thanks,
Saravana
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