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 int (*set_rate_and_parent)(struct clk_hw *hw,
82 unsigned long parent_rate, u8 index);
83 unsigned long (*recalc_accuracy)(struct clk_hw *hw,
84 unsigned long parent_accuracy);
85 void (*init)(struct clk_hw *hw);
88 Part 3 - hardware clk implementations
90 The strength of the common struct clk comes from its .ops and .hw pointers
91 which abstract the details of struct clk from the hardware-specific bits, and
92 vice versa. To illustrate consider the simple gateable clk implementation in
93 drivers/clk/clk-gate.c:
102 struct clk_gate contains struct clk_hw hw as well as hardware-specific
103 knowledge about which register and bit controls this clk's gating.
104 Nothing about clock topology or accounting, such as enable_count or
105 notifier_count, is needed here. That is all handled by the common
106 framework code and struct clk.
108 Let's walk through enabling this clk from driver code:
111 clk = clk_get(NULL, "my_gateable_clk");
116 The call graph for clk_enable is very simple:
119 clk->ops->enable(clk->hw);
122 [resolves struct clk gate with to_clk_gate(hw)]
123 clk_gate_set_bit(gate);
125 And the definition of clk_gate_set_bit:
127 static void clk_gate_set_bit(struct clk_gate *gate)
131 reg = __raw_readl(gate->reg);
132 reg |= BIT(gate->bit_idx);
133 writel(reg, gate->reg);
136 Note that to_clk_gate is defined as:
138 #define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
140 This pattern of abstraction is used for every clock hardware
143 Part 4 - supporting your own clk hardware
145 When implementing support for a new type of clock it only necessary to
146 include the following header:
148 #include <linux/clk-provider.h>
150 include/linux/clk.h is included within that header and clk-private.h
151 must never be included from the code which implements the operations for
152 a clock. More on that below in Part 5.
154 To construct a clk hardware structure for your platform you must define
159 ... hardware specific data goes here ...
162 To take advantage of your data you'll need to support valid operations
165 struct clk_ops clk_foo_ops {
166 .enable = &clk_foo_enable;
167 .disable = &clk_foo_disable;
170 Implement the above functions using container_of:
172 #define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
174 int clk_foo_enable(struct clk_hw *hw)
178 foo = to_clk_foo(hw);
180 ... perform magic on foo ...
185 Below is a matrix detailing which clk_ops are mandatory based upon the
186 hardware capabilities of that clock. A cell marked as "y" means
187 mandatory, a cell marked as "n" implies that either including that
188 callback is invalid or otherwise unnecessary. Empty cells are either
189 optional or must be evaluated on a case-by-case basis.
191 clock hardware characteristics
192 -----------------------------------------------------------
193 | gate | change rate | single parent | multiplexer | root |
194 |------|-------------|---------------|-------------|------|
196 .unprepare | | | | | |
198 .enable | y | | | | |
199 .disable | y | | | | |
200 .is_enabled | y | | | | |
202 .recalc_rate | | y | | | |
203 .round_rate | | y [1] | | | |
204 .determine_rate | | y [1] | | | |
205 .set_rate | | y | | | |
207 .set_parent | | | n | y | n |
208 .get_parent | | | n | y | n |
210 .recalc_accuracy| | | | | |
213 -----------------------------------------------------------
214 [1] either one of round_rate or determine_rate is required.
216 Finally, register your clock at run-time with a hardware-specific
217 registration function. This function simply populates struct clk_foo's
218 data and then passes the common struct clk parameters to the framework
223 See the basic clock types in drivers/clk/clk-*.c for examples.
225 Part 5 - static initialization of clock data
227 For platforms with many clocks (often numbering into the hundreds) it
228 may be desirable to statically initialize some clock data. This
229 presents a problem since the definition of struct clk should be hidden
230 from everyone except for the clock core in drivers/clk/clk.c.
232 To get around this problem struct clk's definition is exposed in
233 include/linux/clk-private.h along with some macros for more easily
234 initializing instances of the basic clock types. These clocks must
235 still be initialized with the common clock framework via a call to
238 clk-private.h must NEVER be included by code which implements struct
239 clk_ops callbacks, nor must it be included by any logic which pokes
240 around inside of struct clk at run-time. To do so is a layering
243 To better enforce this policy, always follow this simple rule: any
244 statically initialized clock data MUST be defined in a separate file
245 from the logic that implements its ops. Basically separate the logic
246 from the data and all is well.
248 Part 6 - Disabling clock gating of unused clocks
250 Sometimes during development it can be useful to be able to bypass the
251 default disabling of unused clocks. For example, if drivers aren't enabling
252 clocks properly but rely on them being on from the bootloader, bypassing
253 the disabling means that the driver will remain functional while the issues
256 To bypass this disabling, include "clk_ignore_unused" in the bootargs to the