1 The Common Clk Framework
2 Mike Turquette <mturquette@ti.com>
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.
9 Part 1 - introduction and interface split
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.
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.
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.
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 for easy navigation between the two discrete halves of the common
38 Part 2 - common data structures and api
40 Below is the common struct clk definition from
41 include/linux/clk-private.h, modified for brevity:
45 const struct clk_ops *ops;
50 struct hlist_head children;
51 struct hlist_node child_node;
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.
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:
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,
73 long (*determine_rate)(struct clk_hw *hw,
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 unsigned long (*recalc_accuracy)(struct clk_hw *hw,
81 unsigned long parent_accuracy);
82 void (*init)(struct clk_hw *hw);
85 Part 3 - hardware clk implementations
87 The strength of the common struct clk comes from its .ops and .hw pointers
88 which abstract the details of struct clk from the hardware-specific bits, and
89 vice versa. To illustrate consider the simple gateable clk implementation in
90 drivers/clk/clk-gate.c:
99 struct clk_gate contains struct clk_hw hw as well as hardware-specific
100 knowledge about which register and bit controls this clk's gating.
101 Nothing about clock topology or accounting, such as enable_count or
102 notifier_count, is needed here. That is all handled by the common
103 framework code and struct clk.
105 Let's walk through enabling this clk from driver code:
108 clk = clk_get(NULL, "my_gateable_clk");
113 The call graph for clk_enable is very simple:
116 clk->ops->enable(clk->hw);
119 [resolves struct clk gate with to_clk_gate(hw)]
120 clk_gate_set_bit(gate);
122 And the definition of clk_gate_set_bit:
124 static void clk_gate_set_bit(struct clk_gate *gate)
128 reg = __raw_readl(gate->reg);
129 reg |= BIT(gate->bit_idx);
130 writel(reg, gate->reg);
133 Note that to_clk_gate is defined as:
135 #define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
137 This pattern of abstraction is used for every clock hardware
140 Part 4 - supporting your own clk hardware
142 When implementing support for a new type of clock it only necessary to
143 include the following header:
145 #include <linux/clk-provider.h>
147 include/linux/clk.h is included within that header and clk-private.h
148 must never be included from the code which implements the operations for
149 a clock. More on that below in Part 5.
151 To construct a clk hardware structure for your platform you must define
156 ... hardware specific data goes here ...
159 To take advantage of your data you'll need to support valid operations
162 struct clk_ops clk_foo_ops {
163 .enable = &clk_foo_enable;
164 .disable = &clk_foo_disable;
167 Implement the above functions using container_of:
169 #define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
171 int clk_foo_enable(struct clk_hw *hw)
175 foo = to_clk_foo(hw);
177 ... perform magic on foo ...
182 Below is a matrix detailing which clk_ops are mandatory based upon the
183 hardware capabilities of that clock. A cell marked as "y" means
184 mandatory, a cell marked as "n" implies that either including that
185 callback is invalid or otherwise unnecessary. Empty cells are either
186 optional or must be evaluated on a case-by-case basis.
188 clock hardware characteristics
189 -----------------------------------------------------------
190 | gate | change rate | single parent | multiplexer | root |
191 |------|-------------|---------------|-------------|------|
193 .unprepare | | | | | |
195 .enable | y | | | | |
196 .disable | y | | | | |
197 .is_enabled | y | | | | |
199 .recalc_rate | | y | | | |
200 .round_rate | | y [1] | | | |
201 .determine_rate | | y [1] | | | |
202 .set_rate | | y | | | |
204 .set_parent | | | n | y | n |
205 .get_parent | | | n | y | n |
207 .recalc_accuracy| | | | | |
210 -----------------------------------------------------------
211 [1] either one of round_rate or determine_rate is required.
213 Finally, register your clock at run-time with a hardware-specific
214 registration function. This function simply populates struct clk_foo's
215 data and then passes the common struct clk parameters to the framework
220 See the basic clock types in drivers/clk/clk-*.c for examples.
222 Part 5 - static initialization of clock data
224 For platforms with many clocks (often numbering into the hundreds) it
225 may be desirable to statically initialize some clock data. This
226 presents a problem since the definition of struct clk should be hidden
227 from everyone except for the clock core in drivers/clk/clk.c.
229 To get around this problem struct clk's definition is exposed in
230 include/linux/clk-private.h along with some macros for more easily
231 initializing instances of the basic clock types. These clocks must
232 still be initialized with the common clock framework via a call to
235 clk-private.h must NEVER be included by code which implements struct
236 clk_ops callbacks, nor must it be included by any logic which pokes
237 around inside of struct clk at run-time. To do so is a layering
240 To better enforce this policy, always follow this simple rule: any
241 statically initialized clock data MUST be defined in a separate file
242 from the logic that implements its ops. Basically separate the logic
243 from the data and all is well.
245 Part 6 - Disabling clock gating of unused clocks
247 Sometimes during development it can be useful to be able to bypass the
248 default disabling of unused clocks. For example, if drivers aren't enabling
249 clocks properly but rely on them being on from the bootloader, bypassing
250 the disabling means that the driver will remain functional while the issues
253 To bypass this disabling, include "clk_ignore_unused" in the bootargs to the