On Wed, 27 Mar 2013, Mike Turquette wrote:
Reentrancy into the clock framework is necessary for clock operations that result in nested calls to the clk api. A common example is a clock that is prepared via an i2c transaction, such as a clock inside of a discrete audio chip or a power management IC. The i2c subsystem itself will use the clk api resulting in a deadlock:
clk_prepare(audio_clk) i2c_transfer(..) clk_prepare(i2c_controller_clk)
The ability to reenter the clock framework prevents this deadlock.
Other use cases exist such as allowing .set_rate callbacks to call clk_set_parent to achieve the best rate, or to save power in certain configurations. Yet another example is performing pinctrl operations from a clk_ops callback. Calls into the pinctrl subsystem may call clk_{un}prepare on an unrelated clock. Allowing for nested calls to reenter the clock framework enables both of these use cases.
Reentrancy is implemented by two global pointers that track the owner currently holding a global lock. One pointer tracks the owner during sleepable, mutex-protected operations and the other one tracks the owner during non-interruptible, spinlock-protected operations.
When the clk framework is entered we try to hold the global lock. If it is held we compare the current task id against the current owner; a
s/task id/task/ We store a the task pointer in the owner variable.
Reviewed-by: Thomas Gleixner tglx@linutronix.de