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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
13541950 35allows for easy navigation between the two discrete halves of the common
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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);
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71 long (*round_rate)(struct clk_hw *hw,
72 unsigned long rate,
73 unsigned long *parent_rate);
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74 long (*determine_rate)(struct clk_hw *hw,
75 unsigned long rate,
76 unsigned long *best_parent_rate,
77 struct clk **best_parent_clk);
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78 int (*set_parent)(struct clk_hw *hw, u8 index);
79 u8 (*get_parent)(struct clk_hw *hw);
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80 int (*set_rate)(struct clk_hw *hw,
81 unsigned long rate,
82 unsigned long parent_rate);
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83 int (*set_rate_and_parent)(struct clk_hw *hw,
84 unsigned long rate,
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85 unsigned long parent_rate,
86 u8 index);
5279fc40 87 unsigned long (*recalc_accuracy)(struct clk_hw *hw,
54e73016 88 unsigned long parent_accuracy);
69fe8a8e 89 void (*init)(struct clk_hw *hw);
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90 int (*debug_init)(struct clk_hw *hw,
91 struct dentry *dentry);
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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
a368a6a3 192hardware capabilities of that clock. A cell marked as "y" means
69fe8a8e 193mandatory, a cell marked as "n" implies that either including that
a368a6a3 194callback is invalid or otherwise unnecessary. Empty cells are either
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195optional or must be evaluated on a case-by-case basis.
196
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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 | | | | | |
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216.recalc_accuracy| | | | | |
217 | | | | | |
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218.init | | | | | |
219 -----------------------------------------------------------
220[1] either one of round_rate or determine_rate is required.
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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.
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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.
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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.