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Documentation: clk: Add locking documentation
Briefly document the common clock framework locking scheme from a clock driver point of view. Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com> Signed-off-by: Mike Turquette <mturquette@linaro.org>
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@ -255,3 +255,37 @@ are sorted out.
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To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
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kernel.
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Part 7 - Locking
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The common clock framework uses two global locks, the prepare lock and the
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enable lock.
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The enable lock is a spinlock and is held across calls to the .enable,
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.disable and .is_enabled operations. Those operations are thus not allowed to
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sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API
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functions are allowed in atomic context.
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The prepare lock is a mutex and is held across calls to all other operations.
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All those operations are allowed to sleep, and calls to the corresponding API
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functions are not allowed in atomic context.
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This effectively divides operations in two groups from a locking perspective.
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Drivers don't need to manually protect resources shared between the operations
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of one group, regardless of whether those resources are shared by multiple
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clocks or not. However, access to resources that are shared between operations
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of the two groups needs to be protected by the drivers. An example of such a
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resource would be a register that controls both the clock rate and the clock
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enable/disable state.
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The clock framework is reentrant, in that a driver is allowed to call clock
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framework functions from within its implementation of clock operations. This
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can for instance cause a .set_rate operation of one clock being called from
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within the .set_rate operation of another clock. This case must be considered
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in the driver implementations, but the code flow is usually controlled by the
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driver in that case.
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Note that locking must also be considered when code outside of the common
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clock framework needs to access resources used by the clock operations. This
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is considered out of scope of this document.
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