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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 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 for easy 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			long		(*determine_rate)(struct clk_hw *hw,
74							unsigned long rate,
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			void		(*init)(struct clk_hw *hw);
81		};
82	
83		Part 3 - hardware clk implementations
84	
85	The strength of the common struct clk comes from its .ops and .hw pointers
86	which abstract the details of struct clk from the hardware-specific bits, and
87	vice versa.  To illustrate consider the simple gateable clk implementation in
88	drivers/clk/clk-gate.c:
89	
90	struct clk_gate {
91		struct clk_hw	hw;
92		void __iomem    *reg;
93		u8              bit_idx;
94		...
95	};
96	
97	struct clk_gate contains struct clk_hw hw as well as hardware-specific
98	knowledge about which register and bit controls this clk's gating.
99	Nothing about clock topology or accounting, such as enable_count or
100	notifier_count, is needed here.  That is all handled by the common
101	framework code and struct clk.
102	
103	Let's walk through enabling this clk from driver code:
104	
105		struct clk *clk;
106		clk = clk_get(NULL, "my_gateable_clk");
107	
108		clk_prepare(clk);
109		clk_enable(clk);
110	
111	The call graph for clk_enable is very simple:
112	
113	clk_enable(clk);
114		clk->ops->enable(clk->hw);
115		[resolves to...]
116			clk_gate_enable(hw);
117			[resolves struct clk gate with to_clk_gate(hw)]
118				clk_gate_set_bit(gate);
119	
120	And the definition of clk_gate_set_bit:
121	
122	static void clk_gate_set_bit(struct clk_gate *gate)
123	{
124		u32 reg;
125	
126		reg = __raw_readl(gate->reg);
127		reg |= BIT(gate->bit_idx);
128		writel(reg, gate->reg);
129	}
130	
131	Note that to_clk_gate is defined as:
132	
133	#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
134	
135	This pattern of abstraction is used for every clock hardware
136	representation.
137	
138		Part 4 - supporting your own clk hardware
139	
140	When implementing support for a new type of clock it only necessary to
141	include the following header:
142	
143	#include <linux/clk-provider.h>
144	
145	include/linux/clk.h is included within that header and clk-private.h
146	must never be included from the code which implements the operations for
147	a clock.  More on that below in Part 5.
148	
149	To construct a clk hardware structure for your platform you must define
150	the following:
151	
152	struct clk_foo {
153		struct clk_hw hw;
154		... hardware specific data goes here ...
155	};
156	
157	To take advantage of your data you'll need to support valid operations
158	for your clk:
159	
160	struct clk_ops clk_foo_ops {
161		.enable		= &clk_foo_enable;
162		.disable	= &clk_foo_disable;
163	};
164	
165	Implement the above functions using container_of:
166	
167	#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
168	
169	int clk_foo_enable(struct clk_hw *hw)
170	{
171		struct clk_foo *foo;
172	
173		foo = to_clk_foo(hw);
174	
175		... perform magic on foo ...
176	
177		return 0;
178	};
179	
180	Below is a matrix detailing which clk_ops are mandatory based upon the
181	hardware capabilities of that clock.  A cell marked as "y" means
182	mandatory, a cell marked as "n" implies that either including that
183	callback is invalid or otherwise unnecessary.  Empty cells are either
184	optional or must be evaluated on a case-by-case basis.
185	
186	                              clock hardware characteristics
187	                -----------------------------------------------------------
188	                | gate | change rate | single parent | multiplexer | root |
189	                |------|-------------|---------------|-------------|------|
190	.prepare        |      |             |               |             |      |
191	.unprepare      |      |             |               |             |      |
192	                |      |             |               |             |      |
193	.enable         | y    |             |               |             |      |
194	.disable        | y    |             |               |             |      |
195	.is_enabled     | y    |             |               |             |      |
196	                |      |             |               |             |      |
197	.recalc_rate    |      | y           |               |             |      |
198	.round_rate     |      | y [1]       |               |             |      |
199	.determine_rate |      | y [1]       |               |             |      |
200	.set_rate       |      | y           |               |             |      |
201	                |      |             |               |             |      |
202	.set_parent     |      |             | n             | y           | n    |
203	.get_parent     |      |             | n             | y           | n    |
204	                |      |             |               |             |      |
205	.init           |      |             |               |             |      |
206	                -----------------------------------------------------------
207	[1] either one of round_rate or determine_rate is required.
208	
209	Finally, register your clock at run-time with a hardware-specific
210	registration function.  This function simply populates struct clk_foo's
211	data and then passes the common struct clk parameters to the framework
212	with a call to:
213	
214	clk_register(...)
215	
216	See the basic clock types in drivers/clk/clk-*.c for examples.
217	
218		Part 5 - static initialization of clock data
219	
220	For platforms with many clocks (often numbering into the hundreds) it
221	may be desirable to statically initialize some clock data.  This
222	presents a problem since the definition of struct clk should be hidden
223	from everyone except for the clock core in drivers/clk/clk.c.
224	
225	To get around this problem struct clk's definition is exposed in
226	include/linux/clk-private.h along with some macros for more easily
227	initializing instances of the basic clock types.  These clocks must
228	still be initialized with the common clock framework via a call to
229	__clk_init.
230	
231	clk-private.h must NEVER be included by code which implements struct
232	clk_ops callbacks, nor must it be included by any logic which pokes
233	around inside of struct clk at run-time.  To do so is a layering
234	violation.
235	
236	To better enforce this policy, always follow this simple rule: any
237	statically initialized clock data MUST be defined in a separate file
238	from the logic that implements its ops.  Basically separate the logic
239	from the data and all is well.
240	
241		Part 6 - Disabling clock gating of unused clocks
242	
243	Sometimes during development it can be useful to be able to bypass the
244	default disabling of unused clocks. For example, if drivers aren't enabling
245	clocks properly but rely on them being on from the bootloader, bypassing
246	the disabling means that the driver will remain functional while the issues
247	are sorted out.
248	
249	To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
250	kernel.
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