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,
  72                                                unsigned long rate,
  73                                                unsigned long *parent_rate);
  74                long            (*determine_rate)(struct clk_hw *hw,
  75                                                unsigned long rate,
  76                                                unsigned long *best_parent_rate,
  77                                                struct clk **best_parent_clk);
  78                int             (*set_parent)(struct clk_hw *hw, u8 index);
  79                u8              (*get_parent)(struct clk_hw *hw);
  80                int             (*set_rate)(struct clk_hw *hw,
  81                                            unsigned long rate,
  82                                            unsigned long parent_rate);
  83                int             (*set_rate_and_parent)(struct clk_hw *hw,
  84                                            unsigned long rate,
  85                                            unsigned long parent_rate,
  86                                            u8 index);
  87                unsigned long   (*recalc_accuracy)(struct clk_hw *hw,
  88                                                unsigned long parent_accuracy);
  89                void            (*init)(struct clk_hw *hw);
  90                int             (*debug_init)(struct clk_hw *hw,
  91                                              struct dentry *dentry);
  92        };
  93
  94        Part 3 - hardware clk implementations
  95
  96The strength of the common struct clk comes from its .ops and .hw pointers
  97which abstract the details of struct clk from the hardware-specific bits, and
  98vice versa.  To illustrate consider the simple gateable clk implementation in
  99drivers/clk/clk-gate.c:
 100
 101struct clk_gate {
 102        struct clk_hw   hw;
 103        void __iomem    *reg;
 104        u8              bit_idx;
 105        ...
 106};
 107
 108struct clk_gate contains struct clk_hw hw as well as hardware-specific
 109knowledge about which register and bit controls this clk's gating.
 110Nothing about clock topology or accounting, such as enable_count or
 111notifier_count, is needed here.  That is all handled by the common
 112framework code and struct clk.
 113
 114Let's walk through enabling this clk from driver code:
 115
 116        struct clk *clk;
 117        clk = clk_get(NULL, "my_gateable_clk");
 118
 119        clk_prepare(clk);
 120        clk_enable(clk);
 121
 122The call graph for clk_enable is very simple:
 123
 124clk_enable(clk);
 125        clk->ops->enable(clk->hw);
 126        [resolves to...]
 127                clk_gate_enable(hw);
 128                [resolves struct clk gate with to_clk_gate(hw)]
 129                        clk_gate_set_bit(gate);
 130
 131And the definition of clk_gate_set_bit:
 132
 133static void clk_gate_set_bit(struct clk_gate *gate)
 134{
 135        u32 reg;
 136
 137        reg = __raw_readl(gate->reg);
 138        reg |= BIT(gate->bit_idx);
 139        writel(reg, gate->reg);
 140}
 141
 142Note that to_clk_gate is defined as:
 143
 144#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
 145
 146This pattern of abstraction is used for every clock hardware
 147representation.
 148
 149        Part 4 - supporting your own clk hardware
 150
 151When implementing support for a new type of clock it only necessary to
 152include the following header:
 153
 154#include <linux/clk-provider.h>
 155
 156include/linux/clk.h is included within that header and clk-private.h
 157must never be included from the code which implements the operations for
 158a clock.  More on that below in Part 5.
 159
 160To construct a clk hardware structure for your platform you must define
 161the following:
 162
 163struct clk_foo {
 164        struct clk_hw hw;
 165        ... hardware specific data goes here ...
 166};
 167
 168To take advantage of your data you'll need to support valid operations
 169for your clk:
 170
 171struct clk_ops clk_foo_ops {
 172        .enable         = &clk_foo_enable;
 173        .disable        = &clk_foo_disable;
 174};
 175
 176Implement the above functions using container_of:
 177
 178#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
 179
 180int clk_foo_enable(struct clk_hw *hw)
 181{
 182        struct clk_foo *foo;
 183
 184        foo = to_clk_foo(hw);
 185
 186        ... perform magic on foo ...
 187
 188        return 0;
 189};
 190
 191Below is a matrix detailing which clk_ops are mandatory based upon the
 192hardware capabilities of that clock.  A cell marked as "y" means
 193mandatory, a cell marked as "n" implies that either including that
 194callback is invalid or otherwise unnecessary.  Empty cells are either
 195optional or must be evaluated on a case-by-case basis.
 196
 197                              clock hardware characteristics
 198                -----------------------------------------------------------
 199                | gate | change rate | single parent | multiplexer | root |
 200                |------|-------------|---------------|-------------|------|
 201.prepare        |      |             |               |             |      |
 202.unprepare      |      |             |               |             |      |
 203                |      |             |               |             |      |
 204.enable         | y    |             |               |             |      |
 205.disable        | y    |             |               |             |      |
 206.is_enabled     | y    |             |               |             |      |
 207                |      |             |               |             |      |
 208.recalc_rate    |      | y           |               |             |      |
 209.round_rate     |      | y [1]       |               |             |      |
 210.determine_rate |      | y [1]       |               |             |      |
 211.set_rate       |      | y           |               |             |      |
 212                |      |             |               |             |      |
 213.set_parent     |      |             | n             | y           | n    |
 214.get_parent     |      |             | n             | y           | n    |
 215                |      |             |               |             |      |
 216.recalc_accuracy|      |             |               |             |      |
 217                |      |             |               |             |      |
 218.init           |      |             |               |             |      |
 219                -----------------------------------------------------------
 220[1] either one of round_rate or determine_rate is required.
 221
 222Finally, register your clock at run-time with a hardware-specific
 223registration function.  This function simply populates struct clk_foo's
 224data and then passes the common struct clk parameters to the framework
 225with a call to:
 226
 227clk_register(...)
 228
 229See the basic clock types in drivers/clk/clk-*.c for examples.
 230
 231        Part 5 - static initialization of clock data
 232
 233For platforms with many clocks (often numbering into the hundreds) it
 234may be desirable to statically initialize some clock data.  This
 235presents a problem since the definition of struct clk should be hidden
 236from everyone except for the clock core in drivers/clk/clk.c.
 237
 238To get around this problem struct clk's definition is exposed in
 239include/linux/clk-private.h along with some macros for more easily
 240initializing instances of the basic clock types.  These clocks must
 241still be initialized with the common clock framework via a call to
 242__clk_init.
 243
 244clk-private.h must NEVER be included by code which implements struct
 245clk_ops callbacks, nor must it be included by any logic which pokes
 246around inside of struct clk at run-time.  To do so is a layering
 247violation.
 248
 249To better enforce this policy, always follow this simple rule: any
 250statically initialized clock data MUST be defined in a separate file
 251from the logic that implements its ops.  Basically separate the logic
 252from the data and all is well.
 253
 254        Part 6 - Disabling clock gating of unused clocks
 255
 256Sometimes during development it can be useful to be able to bypass the
 257default disabling of unused clocks. For example, if drivers aren't enabling
 258clocks properly but rely on them being on from the bootloader, bypassing
 259the disabling means that the driver will remain functional while the issues
 260are sorted out.
 261
 262To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
 263kernel.
 264
 265        Part 7 - Locking
 266
 267The common clock framework uses two global locks, the prepare lock and the
 268enable lock.
 269
 270The enable lock is a spinlock and is held across calls to the .enable,
 271.disable and .is_enabled operations. Those operations are thus not allowed to
 272sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API
 273functions are allowed in atomic context.
 274
 275The prepare lock is a mutex and is held across calls to all other operations.
 276All those operations are allowed to sleep, and calls to the corresponding API
 277functions are not allowed in atomic context.
 278
 279This effectively divides operations in two groups from a locking perspective.
 280
 281Drivers don't need to manually protect resources shared between the operations
 282of one group, regardless of whether those resources are shared by multiple
 283clocks or not. However, access to resources that are shared between operations
 284of the two groups needs to be protected by the drivers. An example of such a
 285resource would be a register that controls both the clock rate and the clock
 286enable/disable state.
 287
 288The clock framework is reentrant, in that a driver is allowed to call clock
 289framework functions from within its implementation of clock operations. This
 290can for instance cause a .set_rate operation of one clock being called from
 291within the .set_rate operation of another clock. This case must be considered
 292in the driver implementations, but the code flow is usually controlled by the
 293driver in that case.
 294
 295Note that locking must also be considered when code outside of the common
 296clock framework needs to access resources used by the clock operations. This
 297is considered out of scope of this document.
 298
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