Commit 843bad83 authored by Laurent Pinchart's avatar Laurent Pinchart Committed by Mike Turquette

Documentation: clk: Add locking documentation

Briefly document the common clock framework locking scheme from a clock
driver point of view.
Signed-off-by: default avatarLaurent Pinchart <>
Signed-off-by: default avatarMike Turquette <>
parent 78761147
......@@ -255,3 +255,37 @@ are sorted out.
To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
Part 7 - Locking
The common clock framework uses two global locks, the prepare lock and the
enable lock.
The enable lock is a spinlock and is held across calls to the .enable,
.disable and .is_enabled operations. Those operations are thus not allowed to
sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API
functions are allowed in atomic context.
The prepare lock is a mutex and is held across calls to all other operations.
All those operations are allowed to sleep, and calls to the corresponding API
functions are not allowed in atomic context.
This effectively divides operations in two groups from a locking perspective.
Drivers don't need to manually protect resources shared between the operations
of one group, regardless of whether those resources are shared by multiple
clocks or not. However, access to resources that are shared between operations
of the two groups needs to be protected by the drivers. An example of such a
resource would be a register that controls both the clock rate and the clock
enable/disable state.
The clock framework is reentrant, in that a driver is allowed to call clock
framework functions from within its implementation of clock operations. This
can for instance cause a .set_rate operation of one clock being called from
within the .set_rate operation of another clock. This case must be considered
in the driver implementations, but the code flow is usually controlled by the
driver in that case.
Note that locking must also be considered when code outside of the common
clock framework needs to access resources used by the clock operations. This
is considered out of scope of this document.
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