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authorLaurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>2014-02-28 13:40:56 +0100
committerMike Turquette <mturquette@linaro.org>2014-03-19 14:56:06 -0700
commit843bad8361bb0d7e5ed6254c84abd8d2ec61b6a4 (patch)
tree19deac13af981d1c2a5d969351518a22f7920a9f /Documentation/clk.txt
parent78761147987b2750b6a848c9719967df0c5eff73 (diff)
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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diff --git a/Documentation/clk.txt b/Documentation/clk.txt
index 699ef2a323b1..c9c399af7c08 100644
--- a/Documentation/clk.txt
+++ b/Documentation/clk.txt
@@ -255,3 +255,37 @@ are sorted out.
To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
kernel.
+
+ 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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