Messages in this thread | | | Date | Wed, 05 Oct 2011 18:17:50 -0700 | From | Saravana Kannan <> | Subject | Re: [PATCH v2 1/7] clk: Add a generic clock infrastructure |
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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 > +#ifdef CONFIG_GENERIC_CLK > + > +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.
Thanks, Saravana
-- Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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