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+		The Common Clk Framework
+		Mike Turquette <mturquette@ti.com>
+
+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 for easy 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 rate,
+						unsigned long *parent_rate);
+		long		(*determine_rate)(struct clk_hw *hw,
+						unsigned long rate,
+						unsigned long min_rate,
+						unsigned long max_rate,
+						unsigned long *best_parent_rate,
+						struct clk_hw **best_parent_clk);
+		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 rate,
+					    unsigned long parent_rate);
+		int		(*set_rate_and_parent)(struct clk_hw *hw,
+					    unsigned long rate,
+					    unsigned long parent_rate,
+					    u8 index);
+		unsigned long	(*recalc_accuracy)(struct clk_hw *hw,
+						unsigned long parent_accuracy);
+		void		(*init)(struct clk_hw *hw);
+		int		(*debug_init)(struct clk_hw *hw,
+					      struct dentry *dentry);
+	};
+
+	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
+drivers/clk/clk-gate.c:
+
+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_enable(clk);
+	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
+representation.
+
+	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 capabilities 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 unnecessary.  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 [1]       |               |             |      |
+.determine_rate |      | y [1]       |               |             |      |
+.set_rate       |      | y           |               |             |      |
+                |      |             |               |             |      |
+.set_parent     |      |             | n             | y           | n    |
+.get_parent     |      |             | n             | y           | n    |
+                |      |             |               |             |      |
+.recalc_accuracy|      |             |               |             |      |
+                |      |             |               |             |      |
+.init           |      |             |               |             |      |
+                -----------------------------------------------------------
+[1] either one of round_rate or determine_rate is required.
+
+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:
+
+clk_register(...)
+
+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_init.
+
+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
+violation.
+
+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.
+
+	Part 6 - Disabling clock gating of unused clocks
+
+Sometimes during development it can be useful to be able to bypass the
+default disabling of unused clocks. For example, if drivers aren't enabling
+clocks properly but rely on them being on from the bootloader, bypassing
+the disabling means that the driver will remain functional while the issues
+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.