clk: Clarify clk_get_rate() expectations

As shown by a number of clock users already, clk_get_rate() can be
called whether or not the clock is enabled.

Similarly, a number of clock drivers will return a rate of 0 whenever
the rate cannot be figured out.

Since it was a bit ambiguous before, let's make it clear in the
clk_get_rate() documentation.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220816112530.1837489-6-maxime@cerno.tech
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Author: mripard

drivers/clk/clk.c

@@ -1672,8 +1672,9 @@ static unsigned long clk_core_get_rate_recalc(struct clk_core *core)
  * @clk: the clk whose rate is being returned
  *
  * Simply returns the cached rate of the clk, unless CLK_GET_RATE_NOCACHE flag
- * is set, which means a recalc_rate will be issued.
- * If clk is NULL then returns 0.
+ * is set, which means a recalc_rate will be issued. Can be called regardless of
+ * the clock enabledness. If clk is NULL, or if an error occurred, then returns
+ * 0.
  */
 unsigned long clk_get_rate(struct clk *clk)
 {