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 <mturquette@linaro.org>
Signed-off-by: Mike Turquette <mturquette@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Jeremy Kerr <jeremy.kerr@canonical.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergman <arnd.bergmann@linaro.org>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Shawn Guo <shawn.guo@freescale.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Richard Zhao <richard.zhao@linaro.org>
Cc: Saravana Kannan <skannan@codeaurora.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Amit Kucheria <amit.kucheria@linaro.org>
Cc: Deepak Saxena <dsaxena@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

1 files changed, 233 insertions, 0 deletions

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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 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
+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 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:
+
+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.