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Documentation: common clk API

Provide documentation for the common clk structures and APIs.  This code
can be found in drivers/clk/ and include/linux/clk*.h.

Signed-off-by: Mike Turquette <>
Signed-off-by: Mike Turquette <>
Reviewed-by: Andrew Lunn <>
Cc: Russell King <>
Cc: Jeremy Kerr <>
Cc: Thomas Gleixner <>
Cc: Arnd Bergman <>
Cc: Paul Walmsley <>
Cc: Shawn Guo <>
Cc: Sascha Hauer <>
Cc: Richard Zhao <>
Cc: Saravana Kannan <>
Cc: Magnus Damm <>
Cc: Rob Herring <>
Cc: Mark Brown <>
Cc: Linus Walleij <>
Cc: Stephen Boyd <>
Cc: Amit Kucheria <>
Cc: Deepak Saxena <>
Cc: Grant Likely <>
Signed-off-by: Arnd Bergmann <>
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+ The Common Clk Framework
+ Mike Turquette <>
+This document endeavours to explain the common clk framework details,
+and how to port a platform over to this framework. It is not yet a
+detailed explanation of the clock api in include/linux/clk.h, but
+perhaps someday it will include that information.
+ Part 1 - introduction and interface split
+The common clk framework is an interface to control the clock nodes
+available on various devices today. This may come in the form of clock
+gating, rate adjustment, muxing or other operations. This framework is
+enabled with the CONFIG_COMMON_CLK option.
+The interface itself is divided into two halves, each shielded from the
+details of its counterpart. First is the common definition of struct
+clk which unifies the framework-level accounting and infrastructure that
+has traditionally been duplicated across a variety of platforms. Second
+is a common implementation of the clk.h api, defined in
+drivers/clk/clk.c. Finally there is struct clk_ops, whose operations
+are invoked by the clk api implementation.
+The second half of the interface is comprised of the hardware-specific
+callbacks registered with struct clk_ops and the corresponding
+hardware-specific structures needed to model a particular clock. For
+the remainder of this document any reference to a callback in struct
+clk_ops, such as .enable or .set_rate, implies the hardware-specific
+implementation of that code. Likewise, references to struct clk_foo
+serve as a convenient shorthand for the implementation of the
+hardware-specific bits for the hypothetical "foo" hardware.
+Tying the two halves of this interface together is struct clk_hw, which
+is defined in struct clk_foo and pointed to within struct clk. This
+allows easy for navigation between the two discrete halves of the common
+clock interface.
+ Part 2 - common data structures and api
+Below is the common struct clk definition from
+include/linux/clk-private.h, modified for brevity:
+ struct clk {
+ const char *name;
+ const struct clk_ops *ops;
+ struct clk_hw *hw;
+ char **parent_names;
+ struct clk **parents;
+ struct clk *parent;
+ struct hlist_head children;
+ struct hlist_node child_node;
+ ...
+ };
+The members above make up the core of the clk tree topology. The clk
+api itself defines several driver-facing functions which operate on
+struct clk. That api is documented in include/linux/clk.h.
+Platforms and devices utilizing the common struct clk use the struct
+clk_ops pointer in struct clk to perform the hardware-specific parts of
+the operations defined in clk.h:
+ struct clk_ops {
+ int (*prepare)(struct clk_hw *hw);
+ void (*unprepare)(struct clk_hw *hw);
+ int (*enable)(struct clk_hw *hw);
+ void (*disable)(struct clk_hw *hw);
+ int (*is_enabled)(struct clk_hw *hw);
+ unsigned long (*recalc_rate)(struct clk_hw *hw,
+ unsigned long parent_rate);
+ long (*round_rate)(struct clk_hw *hw, unsigned long,
+ unsigned long *);
+ int (*set_parent)(struct clk_hw *hw, u8 index);
+ u8 (*get_parent)(struct clk_hw *hw);
+ int (*set_rate)(struct clk_hw *hw, unsigned long);
+ void (*init)(struct clk_hw *hw);
+ };
+ Part 3 - hardware clk implementations
+The strength of the common struct clk comes from its .ops and .hw pointers
+which abstract the details of struct clk from the hardware-specific bits, and
+vice versa. To illustrate consider the simple gateable clk implementation in
+struct clk_gate {
+ struct clk_hw hw;
+ void __iomem *reg;
+ u8 bit_idx;
+ ...
+struct clk_gate contains struct clk_hw hw as well as hardware-specific
+knowledge about which register and bit controls this clk's gating.
+Nothing about clock topology or accounting, such as enable_count or
+notifier_count, is needed here. That is all handled by the common
+framework code and struct clk.
+Let's walk through enabling this clk from driver code:
+ struct clk *clk;
+ clk = clk_get(NULL, "my_gateable_clk");
+ clk_prepare(clk);
+ clk_enable(clk);
+The call graph for clk_enable is very simple:
+ clk->ops->enable(clk->hw);
+ [resolves to...]
+ clk_gate_enable(hw);
+ [resolves struct clk gate with to_clk_gate(hw)]
+ clk_gate_set_bit(gate);
+And the definition of clk_gate_set_bit:
+static void clk_gate_set_bit(struct clk_gate *gate)
+ u32 reg;
+ reg = __raw_readl(gate->reg);
+ reg |= BIT(gate->bit_idx);
+ writel(reg, gate->reg);
+Note that to_clk_gate is defined as:
+#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
+This pattern of abstraction is used for every clock hardware
+ Part 4 - supporting your own clk hardware
+When implementing support for a new type of clock it only necessary to
+include the following header:
+#include <linux/clk-provider.h>
+include/linux/clk.h is included within that header and clk-private.h
+must never be included from the code which implements the operations for
+a clock. More on that below in Part 5.
+To construct a clk hardware structure for your platform you must define
+the following:
+struct clk_foo {
+ struct clk_hw hw;
+ ... hardware specific data goes here ...
+To take advantage of your data you'll need to support valid operations
+for your clk:
+struct clk_ops clk_foo_ops {
+ .enable = &clk_foo_enable;
+ .disable = &clk_foo_disable;
+Implement the above functions using container_of:
+#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
+int clk_foo_enable(struct clk_hw *hw)
+ struct clk_foo *foo;
+ foo = to_clk_foo(hw);
+ ... perform magic on foo ...
+ return 0;
+Below is a matrix detailing which clk_ops are mandatory based upon the
+hardware capbilities of that clock. A cell marked as "y" means
+mandatory, a cell marked as "n" implies that either including that
+callback is invalid or otherwise uneccesary. Empty cells are either
+optional or must be evaluated on a case-by-case basis.
+ clock hardware characteristics
+ -----------------------------------------------------------
+ | gate | change rate | single parent | multiplexer | root |
+ |------|-------------|---------------|-------------|------|
+.prepare | | | | | |
+.unprepare | | | | | |
+ | | | | | |
+.enable | y | | | | |
+.disable | y | | | | |
+.is_enabled | y | | | | |
+ | | | | | |
+.recalc_rate | | y | | | |
+.round_rate | | y | | | |
+.set_rate | | y | | | |
+ | | | | | |
+.set_parent | | | n | y | n |
+.get_parent | | | n | y | n |
+ | | | | | |
+.init | | | | | |
+ -----------------------------------------------------------
+Finally, register your clock at run-time with a hardware-specific
+registration function. This function simply populates struct clk_foo's
+data and then passes the common struct clk parameters to the framework
+with a call to:
+See the basic clock types in drivers/clk/clk-*.c for examples.
+ Part 5 - static initialization of clock data
+For platforms with many clocks (often numbering into the hundreds) it
+may be desirable to statically initialize some clock data. This
+presents a problem since the definition of struct clk should be hidden
+from everyone except for the clock core in drivers/clk/clk.c.
+To get around this problem struct clk's definition is exposed in
+include/linux/clk-private.h along with some macros for more easily
+initializing instances of the basic clock types. These clocks must
+still be initialized with the common clock framework via a call to
+clk-private.h must NEVER be included by code which implements struct
+clk_ops callbacks, nor must it be included by any logic which pokes
+around inside of struct clk at run-time. To do so is a layering
+To better enforce this policy, always follow this simple rule: any
+statically initialized clock data MUST be defined in a separate file
+from the logic that implements its ops. Basically separate the logic
+from the data and all is well.
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