Re: [PATCH v2 1/7] clk: Add a generic clock infrastructure

From: Saravana Kannan
Date: Wed Oct 05 2011 - 21:17:52 EST

On 09/22/2011 03:26 PM, Mike Turquette wrote:
diff --git a/include/linux/clk.h b/include/linux/clk.h
index 1d37f42..d6ae10b 100644
--- a/include/linux/clk.h
+++ b/include/linux/clk.h
+struct clk_hw {
+ struct clk *clk;
+ * struct clk_hw_ops - Callback operations for hardware clocks; these are to
+ * be provided by the clock implementation, and will be called by drivers
+ * through the clk_* API.
+ *
+ * @prepare: Prepare the clock for enabling. This must not return until
+ * the clock is fully prepared, and it's safe to call clk_enable.
+ * This callback is intended to allow clock implementations to
+ * do any initialisation that may sleep. Called with
+ * prepare_lock held.
+ *
+ * @unprepare: Release the clock from its prepared state. This will typically
+ * undo any work done in the @prepare callback. Called with
+ * prepare_lock held.
+ *
+ * @enable: Enable the clock atomically. This must not return until the
+ * clock is generating a valid clock signal, usable by consumer
+ * devices. Called with enable_lock held. This function must not
+ * sleep.
+ *
+ * @disable: Disable the clock atomically. Called with enable_lock held.
+ * This function must not sleep.
+ *
+ * @recalc_rate Recalculate the rate of this clock, by quering hardware
+ * and/or the clock's parent. Called with the global clock mutex
+ * held. Optional, but recommended - if this op is not set,
+ * clk_get_rate will return 0.
+ *
+ * @get_parent Query the parent of this clock; for clocks with multiple
+ * possible parents, query the hardware for the current
+ * parent. Currently only called when the clock is first
+ * registered.
+ *
+ * The clk_enable/clk_disable and clk_prepare/clk_unprepare pairs allow
+ * implementations to split any work between atomic (enable) and sleepable
+ * (prepare) contexts. If a clock requires sleeping code to be turned on, this
+ * should be done in clk_prepare. Switching that will not sleep should be done
+ * in clk_enable.
+ *
+ * Typically, drivers will call clk_prepare when a clock may be needed later
+ * (eg. when a device is opened), and clk_enable when the clock is actually
+ * required (eg. from an interrupt). Note that clk_prepare *must* have been
+ * called before clk_enable.
+struct clk_hw_ops {
+ int (*prepare)(struct clk_hw *);
+ void (*unprepare)(struct clk_hw *);
+ int (*enable)(struct clk_hw *);
+ void (*disable)(struct clk_hw *);
+ unsigned long (*recalc_rate)(struct clk_hw *);
+ long (*round_rate)(struct clk_hw *, unsigned long);
+ struct clk * (*get_parent)(struct clk_hw *);

I would like to understand the need for recalc rate if that's something that we want to go into the common framework (even if it's optional). I have mostly heard only second hand explanations of the need for recalc_rate(), so I might not have the full picture. But for all the cases that I can think of, recalc_rate seems like a paradox.

If recalc_rate() is used to make sure the "current rate" of a "clock A" is always known even if it's parent "clock B"'s rate is changed, then it also means that the rate of "clock A" can change without clk_set_rate(clock A, new rate). That in turn means that the clk_get_rate() just gives the instantaneous snapshot of the rate. So, any use of clk_get_rate(clock A) for anything other than printing/logging the return value is broken code. In which case, do we really care for recalc_rate()? We could just return the rate that it was set to when clk_set_rate() was called and call it a day or return 0 for such clocks to indicate that the clock rate is "unknown".

The whole concept of trying to recalculate the rate for a clock makes me feel uneasy since it promotes misunderstanding the behavior of the clock and writing bad code based on that misunderstanding.

I would like to hear to real usecases before I propose some alternatives that I have in mind.


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