linux/Documentation/clk.txt
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   1                The Common Clk Framework
   2                Mike Turquette <mturquette@ti.com>
   3
   4This document endeavours to explain the common clk framework details,
   5and how to port a platform over to this framework.  It is not yet a
   6detailed explanation of the clock api in include/linux/clk.h, but
   7perhaps someday it will include that information.
   8
   9        Part 1 - introduction and interface split
  10
  11The common clk framework is an interface to control the clock nodes
  12available on various devices today.  This may come in the form of clock
  13gating, rate adjustment, muxing or other operations.  This framework is
  14enabled with the CONFIG_COMMON_CLK option.
  15
  16The interface itself is divided into two halves, each shielded from the
  17details of its counterpart.  First is the common definition of struct
  18clk which unifies the framework-level accounting and infrastructure that
  19has traditionally been duplicated across a variety of platforms.  Second
  20is a common implementation of the clk.h api, defined in
  21drivers/clk/clk.c.  Finally there is struct clk_ops, whose operations
  22are invoked by the clk api implementation.
  23
  24The second half of the interface is comprised of the hardware-specific
  25callbacks registered with struct clk_ops and the corresponding
  26hardware-specific structures needed to model a particular clock.  For
  27the remainder of this document any reference to a callback in struct
  28clk_ops, such as .enable or .set_rate, implies the hardware-specific
  29implementation of that code.  Likewise, references to struct clk_foo
  30serve as a convenient shorthand for the implementation of the
  31hardware-specific bits for the hypothetical "foo" hardware.
  32
  33Tying the two halves of this interface together is struct clk_hw, which
  34is defined in struct clk_foo and pointed to within struct clk.  This
  35allows easy for navigation between the two discrete halves of the common
  36clock interface.
  37
  38        Part 2 - common data structures and api
  39
  40Below is the common struct clk definition from
  41include/linux/clk-private.h, modified for brevity:
  42
  43        struct clk {
  44                const char              *name;
  45                const struct clk_ops    *ops;
  46                struct clk_hw           *hw;
  47                char                    **parent_names;
  48                struct clk              **parents;
  49                struct clk              *parent;
  50                struct hlist_head       children;
  51                struct hlist_node       child_node;
  52                ...
  53        };
  54
  55The members above make up the core of the clk tree topology.  The clk
  56api itself defines several driver-facing functions which operate on
  57struct clk.  That api is documented in include/linux/clk.h.
  58
  59Platforms and devices utilizing the common struct clk use the struct
  60clk_ops pointer in struct clk to perform the hardware-specific parts of
  61the operations defined in clk.h:
  62
  63        struct clk_ops {
  64                int             (*prepare)(struct clk_hw *hw);
  65                void            (*unprepare)(struct clk_hw *hw);
  66                int             (*enable)(struct clk_hw *hw);
  67                void            (*disable)(struct clk_hw *hw);
  68                int             (*is_enabled)(struct clk_hw *hw);
  69                unsigned long   (*recalc_rate)(struct clk_hw *hw,
  70                                                unsigned long parent_rate);
  71                long            (*round_rate)(struct clk_hw *hw, unsigned long,
  72                                                unsigned long *);
  73                int             (*set_parent)(struct clk_hw *hw, u8 index);
  74                u8              (*get_parent)(struct clk_hw *hw);
  75                int             (*set_rate)(struct clk_hw *hw, unsigned long);
  76                void            (*init)(struct clk_hw *hw);
  77        };
  78
  79        Part 3 - hardware clk implementations
  80
  81The strength of the common struct clk comes from its .ops and .hw pointers
  82which abstract the details of struct clk from the hardware-specific bits, and
  83vice versa.  To illustrate consider the simple gateable clk implementation in
  84drivers/clk/clk-gate.c:
  85
  86struct clk_gate {
  87        struct clk_hw   hw;
  88        void __iomem    *reg;
  89        u8              bit_idx;
  90        ...
  91};
  92
  93struct clk_gate contains struct clk_hw hw as well as hardware-specific
  94knowledge about which register and bit controls this clk's gating.
  95Nothing about clock topology or accounting, such as enable_count or
  96notifier_count, is needed here.  That is all handled by the common
  97framework code and struct clk.
  98
  99Let's walk through enabling this clk from driver code:
 100
 101        struct clk *clk;
 102        clk = clk_get(NULL, "my_gateable_clk");
 103
 104        clk_prepare(clk);
 105        clk_enable(clk);
 106
 107The call graph for clk_enable is very simple:
 108
 109clk_enable(clk);
 110        clk->ops->enable(clk->hw);
 111        [resolves to...]
 112                clk_gate_enable(hw);
 113                [resolves struct clk gate with to_clk_gate(hw)]
 114                        clk_gate_set_bit(gate);
 115
 116And the definition of clk_gate_set_bit:
 117
 118static void clk_gate_set_bit(struct clk_gate *gate)
 119{
 120        u32 reg;
 121
 122        reg = __raw_readl(gate->reg);
 123        reg |= BIT(gate->bit_idx);
 124        writel(reg, gate->reg);
 125}
 126
 127Note that to_clk_gate is defined as:
 128
 129#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
 130
 131This pattern of abstraction is used for every clock hardware
 132representation.
 133
 134        Part 4 - supporting your own clk hardware
 135
 136When implementing support for a new type of clock it only necessary to
 137include the following header:
 138
 139#include <linux/clk-provider.h>
 140
 141include/linux/clk.h is included within that header and clk-private.h
 142must never be included from the code which implements the operations for
 143a clock.  More on that below in Part 5.
 144
 145To construct a clk hardware structure for your platform you must define
 146the following:
 147
 148struct clk_foo {
 149        struct clk_hw hw;
 150        ... hardware specific data goes here ...
 151};
 152
 153To take advantage of your data you'll need to support valid operations
 154for your clk:
 155
 156struct clk_ops clk_foo_ops {
 157        .enable         = &clk_foo_enable;
 158        .disable        = &clk_foo_disable;
 159};
 160
 161Implement the above functions using container_of:
 162
 163#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
 164
 165int clk_foo_enable(struct clk_hw *hw)
 166{
 167        struct clk_foo *foo;
 168
 169        foo = to_clk_foo(hw);
 170
 171        ... perform magic on foo ...
 172
 173        return 0;
 174};
 175
 176Below is a matrix detailing which clk_ops are mandatory based upon the
 177hardware capbilities of that clock.  A cell marked as "y" means
 178mandatory, a cell marked as "n" implies that either including that
 179callback is invalid or otherwise uneccesary.  Empty cells are either
 180optional or must be evaluated on a case-by-case basis.
 181
 182                           clock hardware characteristics
 183             -----------------------------------------------------------
 184             | gate | change rate | single parent | multiplexer | root |
 185             |------|-------------|---------------|-------------|------|
 186.prepare     |      |             |               |             |      |
 187.unprepare   |      |             |               |             |      |
 188             |      |             |               |             |      |
 189.enable      | y    |             |               |             |      |
 190.disable     | y    |             |               |             |      |
 191.is_enabled  | y    |             |               |             |      |
 192             |      |             |               |             |      |
 193.recalc_rate |      | y           |               |             |      |
 194.round_rate  |      | y           |               |             |      |
 195.set_rate    |      | y           |               |             |      |
 196             |      |             |               |             |      |
 197.set_parent  |      |             | n             | y           | n    |
 198.get_parent  |      |             | n             | y           | n    |
 199             |      |             |               |             |      |
 200.init        |      |             |               |             |      |
 201             -----------------------------------------------------------
 202
 203Finally, register your clock at run-time with a hardware-specific
 204registration function.  This function simply populates struct clk_foo's
 205data and then passes the common struct clk parameters to the framework
 206with a call to:
 207
 208clk_register(...)
 209
 210See the basic clock types in drivers/clk/clk-*.c for examples.
 211
 212        Part 5 - static initialization of clock data
 213
 214For platforms with many clocks (often numbering into the hundreds) it
 215may be desirable to statically initialize some clock data.  This
 216presents a problem since the definition of struct clk should be hidden
 217from everyone except for the clock core in drivers/clk/clk.c.
 218
 219To get around this problem struct clk's definition is exposed in
 220include/linux/clk-private.h along with some macros for more easily
 221initializing instances of the basic clock types.  These clocks must
 222still be initialized with the common clock framework via a call to
 223__clk_init.
 224
 225clk-private.h must NEVER be included by code which implements struct
 226clk_ops callbacks, nor must it be included by any logic which pokes
 227around inside of struct clk at run-time.  To do so is a layering
 228violation.
 229
 230To better enforce this policy, always follow this simple rule: any
 231statically initialized clock data MUST be defined in a separate file
 232from the logic that implements its ops.  Basically separate the logic
 233from the data and all is well.
 234
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