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Based on kernel version 3.9. Page generated on 2013-05-02 22:55 EST.

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.
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