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 for easy 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                long            (*determine_rate)(struct clk_hw *hw,
  74                                                unsigned long rate,
  75                                                unsigned long *best_parent_rate,
  76                                                struct clk **best_parent_clk);
  77                int             (*set_parent)(struct clk_hw *hw, u8 index);
  78                u8              (*get_parent)(struct clk_hw *hw);
  79                int             (*set_rate)(struct clk_hw *hw, unsigned long);
  80                void            (*init)(struct clk_hw *hw);
  81        };
  82
  83        Part 3 - hardware clk implementations
  84
  85The strength of the common struct clk comes from its .ops and .hw pointers
  86which abstract the details of struct clk from the hardware-specific bits, and
  87vice versa.  To illustrate consider the simple gateable clk implementation in
  88drivers/clk/clk-gate.c:
  89
  90struct clk_gate {
  91        struct clk_hw   hw;
  92        void __iomem    *reg;
  93        u8              bit_idx;
  94        ...
  95};
  96
  97struct clk_gate contains struct clk_hw hw as well as hardware-specific
  98knowledge about which register and bit controls this clk's gating.
  99Nothing about clock topology or accounting, such as enable_count or
 100notifier_count, is needed here.  That is all handled by the common
 101framework code and struct clk.
 102
 103Let's walk through enabling this clk from driver code:
 104
 105        struct clk *clk;
 106        clk = clk_get(NULL, "my_gateable_clk");
 107
 108        clk_prepare(clk);
 109        clk_enable(clk);
 110
 111The call graph for clk_enable is very simple:
 112
 113clk_enable(clk);
 114        clk->ops->enable(clk->hw);
 115        [resolves to...]
 116                clk_gate_enable(hw);
 117                [resolves struct clk gate with to_clk_gate(hw)]
 118                        clk_gate_set_bit(gate);
 119
 120And the definition of clk_gate_set_bit:
 121
 122static void clk_gate_set_bit(struct clk_gate *gate)
 123{
 124        u32 reg;
 125
 126        reg = __raw_readl(gate->reg);
 127        reg |= BIT(gate->bit_idx);
 128        writel(reg, gate->reg);
 129}
 130
 131Note that to_clk_gate is defined as:
 132
 133#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
 134
 135This pattern of abstraction is used for every clock hardware
 136representation.
 137
 138        Part 4 - supporting your own clk hardware
 139
 140When implementing support for a new type of clock it only necessary to
 141include the following header:
 142
 143#include <linux/clk-provider.h>
 144
 145include/linux/clk.h is included within that header and clk-private.h
 146must never be included from the code which implements the operations for
 147a clock.  More on that below in Part 5.
 148
 149To construct a clk hardware structure for your platform you must define
 150the following:
 151
 152struct clk_foo {
 153        struct clk_hw hw;
 154        ... hardware specific data goes here ...
 155};
 156
 157To take advantage of your data you'll need to support valid operations
 158for your clk:
 159
 160struct clk_ops clk_foo_ops {
 161        .enable         = &clk_foo_enable;
 162        .disable        = &clk_foo_disable;
 163};
 164
 165Implement the above functions using container_of:
 166
 167#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
 168
 169int clk_foo_enable(struct clk_hw *hw)
 170{
 171        struct clk_foo *foo;
 172
 173        foo = to_clk_foo(hw);
 174
 175        ... perform magic on foo ...
 176
 177        return 0;
 178};
 179
 180Below is a matrix detailing which clk_ops are mandatory based upon the
 181hardware capabilities of that clock.  A cell marked as "y" means
 182mandatory, a cell marked as "n" implies that either including that
 183callback is invalid or otherwise unnecessary.  Empty cells are either
 184optional or must be evaluated on a case-by-case basis.
 185
 186                              clock hardware characteristics
 187                -----------------------------------------------------------
 188                | gate | change rate | single parent | multiplexer | root |
 189                |------|-------------|---------------|-------------|------|
 190.prepare        |      |             |               |             |      |
 191.unprepare      |      |             |               |             |      |
 192                |      |             |               |             |      |
 193.enable         | y    |             |               |             |      |
 194.disable        | y    |             |               |             |      |
 195.is_enabled     | y    |             |               |             |      |
 196                |      |             |               |             |      |
 197.recalc_rate    |      | y           |               |             |      |
 198.round_rate     |      | y [1]       |               |             |      |
 199.determine_rate |      | y [1]       |               |             |      |
 200.set_rate       |      | y           |               |             |      |
 201                |      |             |               |             |      |
 202.set_parent     |      |             | n             | y           | n    |
 203.get_parent     |      |             | n             | y           | n    |
 204                |      |             |               |             |      |
 205.init           |      |             |               |             |      |
 206                -----------------------------------------------------------
 207[1] either one of round_rate or determine_rate is required.
 208
 209Finally, register your clock at run-time with a hardware-specific
 210registration function.  This function simply populates struct clk_foo's
 211data and then passes the common struct clk parameters to the framework
 212with a call to:
 213
 214clk_register(...)
 215
 216See the basic clock types in drivers/clk/clk-*.c for examples.
 217
 218        Part 5 - static initialization of clock data
 219
 220For platforms with many clocks (often numbering into the hundreds) it
 221may be desirable to statically initialize some clock data.  This
 222presents a problem since the definition of struct clk should be hidden
 223from everyone except for the clock core in drivers/clk/clk.c.
 224
 225To get around this problem struct clk's definition is exposed in
 226include/linux/clk-private.h along with some macros for more easily
 227initializing instances of the basic clock types.  These clocks must
 228still be initialized with the common clock framework via a call to
 229__clk_init.
 230
 231clk-private.h must NEVER be included by code which implements struct
 232clk_ops callbacks, nor must it be included by any logic which pokes
 233around inside of struct clk at run-time.  To do so is a layering
 234violation.
 235
 236To better enforce this policy, always follow this simple rule: any
 237statically initialized clock data MUST be defined in a separate file
 238from the logic that implements its ops.  Basically separate the logic
 239from the data and all is well.
 240
 241        Part 6 - Disabling clock gating of unused clocks
 242
 243Sometimes during development it can be useful to be able to bypass the
 244default disabling of unused clocks. For example, if drivers aren't enabling
 245clocks properly but rely on them being on from the bootloader, bypassing
 246the disabling means that the driver will remain functional while the issues
 247are sorted out.
 248
 249To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
 250kernel.
 251
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