Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with
or
.
Download ZIP
Newer
Older
100644 234 lines (182 sloc) 8.663 kB
69fe8a8 @mturquette Documentation: common clk API
mturquette authored
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 capbilities 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 uneccesary. 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.
Something went wrong with that request. Please try again.