Documentation/clk.txt

   1                 The Common Clk Framework
   2                 Mike Turquette <mturquette@ti.com>
   3
   4 This document endeavours to explain the common clk framework details,
   5 and how to port a platform over to this framework.  It is not yet a
   6 detailed explanation of the clock api in include/linux/clk.h, but
   7 perhaps someday it will include that information.
   8
   9         Part 1 - introduction and interface split
  10
  11 The common clk framework is an interface to control the clock nodes
  12 available on various devices today.  This may come in the form of clock
  13 gating, rate adjustment, muxing or other operations.  This framework is
  14 enabled with the CONFIG_COMMON_CLK option.
  15
  16 The interface itself is divided into two halves, each shielded from the
  17 details of its counterpart.  First is the common definition of struct
  18 clk which unifies the framework-level accounting and infrastructure that
  19 has traditionally been duplicated across a variety of platforms.  Second
  20 is a common implementation of the clk.h api, defined in
  21 drivers/clk/clk.c.  Finally there is struct clk_ops, whose operations
  22 are invoked by the clk api implementation.
  23
  24 The second half of the interface is comprised of the hardware-specific
  25 callbacks registered with struct clk_ops and the corresponding
  26 hardware-specific structures needed to model a particular clock.  For
  27 the remainder of this document any reference to a callback in struct
  28 clk_ops, such as .enable or .set_rate, implies the hardware-specific
  29 implementation of that code.  Likewise, references to struct clk_foo
  30 serve as a convenient shorthand for the implementation of the
  31 hardware-specific bits for the hypothetical "foo" hardware.
  32
  33 Tying the two halves of this interface together is struct clk_hw, which
  34 is defined in struct clk_foo and pointed to within struct clk.  This
  35 allows easy for navigation between the two discrete halves of the common
  36 clock interface.
  37
  38         Part 2 - common data structures and api
  39
  40 Below is the common struct clk definition from
  41 include/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
  55 The members above make up the core of the clk tree topology.  The clk
  56 api itself defines several driver-facing functions which operate on
  57 struct clk.  That api is documented in include/linux/clk.h.
  58
  59 Platforms and devices utilizing the common struct clk use the struct
  60 clk_ops pointer in struct clk to perform the hardware-specific parts of
  61 the 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
  81 The strength of the common struct clk comes from its .ops and .hw pointers
  82 which abstract the details of struct clk from the hardware-specific bits, and
  83 vice versa.  To illustrate consider the simple gateable clk implementation in
  84 drivers/clk/clk-gate.c:
  85
  86 struct clk_gate {
  87         struct clk_hw   hw;
  88         void __iomem    *reg;
  89         u8              bit_idx;
  90         ...
  91 };
  92
  93 struct clk_gate contains struct clk_hw hw as well as hardware-specific
  94 knowledge about which register and bit controls this clk's gating.
  95 Nothing about clock topology or accounting, such as enable_count or
  96 notifier_count, is needed here.  That is all handled by the common
  97 framework code and struct clk.
  98
  99 Let'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
 107 The call graph for clk_enable is very simple:
 108
 109 clk_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
 116 And the definition of clk_gate_set_bit:
 117
 118 static 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
 127 Note that to_clk_gate is defined as:
 128
 129 #define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
 130
 131 This pattern of abstraction is used for every clock hardware
 132 representation.
 133
 134         Part 4 - supporting your own clk hardware
 135
 136 When implementing support for a new type of clock it only necessary to
 137 include the following header:
 138
 139 #include <linux/clk-provider.h>
 140
 141 include/linux/clk.h is included within that header and clk-private.h
 142 must never be included from the code which implements the operations for
 143 a clock.  More on that below in Part 5.
 144
 145 To construct a clk hardware structure for your platform you must define
 146 the following:
 147
 148 struct clk_foo {
 149         struct clk_hw hw;
 150         ... hardware specific data goes here ...
 151 };
 152
 153 To take advantage of your data you'll need to support valid operations
 154 for your clk:
 155
 156 struct clk_ops clk_foo_ops {
 157         .enable         = &clk_foo_enable;
 158         .disable        = &clk_foo_disable;
 159 };
 160
 161 Implement the above functions using container_of:
 162
 163 #define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
 164
 165 int 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
 176 Below is a matrix detailing which clk_ops are mandatory based upon the
 177 hardware capabilities of that clock.  A cell marked as "y" means
 178 mandatory, a cell marked as "n" implies that either including that
 179 callback is invalid or otherwise unnecessary.  Empty cells are either
 180 optional 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
 203 Finally, register your clock at run-time with a hardware-specific
 204 registration function.  This function simply populates struct clk_foo's
 205 data and then passes the common struct clk parameters to the framework
 206 with a call to:
 207
 208 clk_register(...)
 209
 210 See the basic clock types in drivers/clk/clk-*.c for examples.
 211
 212         Part 5 - static initialization of clock data
 213
 214 For platforms with many clocks (often numbering into the hundreds) it
 215 may be desirable to statically initialize some clock data.  This
 216 presents a problem since the definition of struct clk should be hidden
 217 from everyone except for the clock core in drivers/clk/clk.c.
 218
 219 To get around this problem struct clk's definition is exposed in
 220 include/linux/clk-private.h along with some macros for more easily
 221 initializing instances of the basic clock types.  These clocks must
 222 still be initialized with the common clock framework via a call to
 223 __clk_init.
 224
 225 clk-private.h must NEVER be included by code which implements struct
 226 clk_ops callbacks, nor must it be included by any logic which pokes
 227 around inside of struct clk at run-time.  To do so is a layering
 228 violation.
 229
 230 To better enforce this policy, always follow this simple rule: any
 231 statically initialized clock data MUST be defined in a separate file
 232 from the logic that implements its ops.  Basically separate the logic
 233 from the data and all is well.
 234
 235         Part 6 - Disabling clock gating of unused clocks
 236
 237 Sometimes during development it can be useful to be able to bypass the
 238 default disabling of unused clocks. For example, if drivers aren't enabling
 239 clocks properly but rely on them being on from the bootloader, bypassing
 240 the disabling means that the driver will remain functional while the issues
 241 are sorted out.
 242
 243 To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
 244 kernel.