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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 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			int		(*set_rate_and_parent)(struct clk_hw *hw,
81						    unsigned long rate,
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);
86		};
87	
88		Part 3 - hardware clk implementations
89	
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:
94	
95	struct clk_gate {
96		struct clk_hw	hw;
97		void __iomem    *reg;
98		u8              bit_idx;
99		...
100	};
101	
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.
107	
108	Let's walk through enabling this clk from driver code:
109	
110		struct clk *clk;
111		clk = clk_get(NULL, "my_gateable_clk");
112	
113		clk_prepare(clk);
114		clk_enable(clk);
115	
116	The call graph for clk_enable is very simple:
117	
118	clk_enable(clk);
119		clk->ops->enable(clk->hw);
120		[resolves to...]
121			clk_gate_enable(hw);
122			[resolves struct clk gate with to_clk_gate(hw)]
123				clk_gate_set_bit(gate);
124	
125	And the definition of clk_gate_set_bit:
126	
127	static void clk_gate_set_bit(struct clk_gate *gate)
128	{
129		u32 reg;
130	
131		reg = __raw_readl(gate->reg);
132		reg |= BIT(gate->bit_idx);
133		writel(reg, gate->reg);
134	}
135	
136	Note that to_clk_gate is defined as:
137	
138	#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
139	
140	This pattern of abstraction is used for every clock hardware
141	representation.
142	
143		Part 4 - supporting your own clk hardware
144	
145	When implementing support for a new type of clock it only necessary to
146	include the following header:
147	
148	#include <linux/clk-provider.h>
149	
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.
153	
154	To construct a clk hardware structure for your platform you must define
155	the following:
156	
157	struct clk_foo {
158		struct clk_hw hw;
159		... hardware specific data goes here ...
160	};
161	
162	To take advantage of your data you'll need to support valid operations
163	for your clk:
164	
165	struct clk_ops clk_foo_ops {
166		.enable		= &clk_foo_enable;
167		.disable	= &clk_foo_disable;
168	};
169	
170	Implement the above functions using container_of:
171	
172	#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
173	
174	int clk_foo_enable(struct clk_hw *hw)
175	{
176		struct clk_foo *foo;
177	
178		foo = to_clk_foo(hw);
179	
180		... perform magic on foo ...
181	
182		return 0;
183	};
184	
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.
190	
191	                              clock hardware characteristics
192	                -----------------------------------------------------------
193	                | gate | change rate | single parent | multiplexer | root |
194	                |------|-------------|---------------|-------------|------|
195	.prepare        |      |             |               |             |      |
196	.unprepare      |      |             |               |             |      |
197	                |      |             |               |             |      |
198	.enable         | y    |             |               |             |      |
199	.disable        | y    |             |               |             |      |
200	.is_enabled     | y    |             |               |             |      |
201	                |      |             |               |             |      |
202	.recalc_rate    |      | y           |               |             |      |
203	.round_rate     |      | y [1]       |               |             |      |
204	.determine_rate |      | y [1]       |               |             |      |
205	.set_rate       |      | y           |               |             |      |
206	                |      |             |               |             |      |
207	.set_parent     |      |             | n             | y           | n    |
208	.get_parent     |      |             | n             | y           | n    |
209	                |      |             |               |             |      |
210	.recalc_accuracy|      |             |               |             |      |
211	                |      |             |               |             |      |
212	.init           |      |             |               |             |      |
213	                -----------------------------------------------------------
214	[1] either one of round_rate or determine_rate is required.
215	
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
219	with a call to:
220	
221	clk_register(...)
222	
223	See the basic clock types in drivers/clk/clk-*.c for examples.
224	
225		Part 5 - static initialization of clock data
226	
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.
231	
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
236	__clk_init.
237	
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
241	violation.
242	
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.
247	
248		Part 6 - Disabling clock gating of unused clocks
249	
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
254	are sorted out.
255	
256	To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
257	kernel.
258	
259		Part 7 - Locking
260	
261	The common clock framework uses two global locks, the prepare lock and the
262	enable lock.
263	
264	The enable lock is a spinlock and is held across calls to the .enable,
265	.disable and .is_enabled operations. Those operations are thus not allowed to
266	sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API
267	functions are allowed in atomic context.
268	
269	The prepare lock is a mutex and is held across calls to all other operations.
270	All those operations are allowed to sleep, and calls to the corresponding API
271	functions are not allowed in atomic context.
272	
273	This effectively divides operations in two groups from a locking perspective.
274	
275	Drivers don't need to manually protect resources shared between the operations
276	of one group, regardless of whether those resources are shared by multiple
277	clocks or not. However, access to resources that are shared between operations
278	of the two groups needs to be protected by the drivers. An example of such a
279	resource would be a register that controls both the clock rate and the clock
280	enable/disable state.
281	
282	The clock framework is reentrant, in that a driver is allowed to call clock
283	framework functions from within its implementation of clock operations. This
284	can for instance cause a .set_rate operation of one clock being called from
285	within the .set_rate operation of another clock. This case must be considered
286	in the driver implementations, but the code flow is usually controlled by the
287	driver in that case.
288	
289	Note that locking must also be considered when code outside of the common
290	clock framework needs to access resources used by the clock operations. This
291	is considered out of scope of this document.
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