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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 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,
72							unsigned long rate,
73							unsigned long *parent_rate);
74			long		(*determine_rate)(struct clk_hw *hw,
75							unsigned long rate,
76							unsigned long *best_parent_rate,
77							struct clk **best_parent_clk);
78			int		(*set_parent)(struct clk_hw *hw, u8 index);
79			u8		(*get_parent)(struct clk_hw *hw);
80			int		(*set_rate)(struct clk_hw *hw,
81						    unsigned long rate,
82						    unsigned long parent_rate);
83			int		(*set_rate_and_parent)(struct clk_hw *hw,
84						    unsigned long rate,
85						    unsigned long parent_rate,
86						    u8 index);
87			unsigned long	(*recalc_accuracy)(struct clk_hw *hw,
88							unsigned long parent_accuracy);
89			void		(*init)(struct clk_hw *hw);
90			int		(*debug_init)(struct clk_hw *hw,
91						      struct dentry *dentry);
92		};
93	
94		Part 3 - hardware clk implementations
95	
96	The strength of the common struct clk comes from its .ops and .hw pointers
97	which abstract the details of struct clk from the hardware-specific bits, and
98	vice versa.  To illustrate consider the simple gateable clk implementation in
99	drivers/clk/clk-gate.c:
100	
101	struct clk_gate {
102		struct clk_hw	hw;
103		void __iomem    *reg;
104		u8              bit_idx;
105		...
106	};
107	
108	struct clk_gate contains struct clk_hw hw as well as hardware-specific
109	knowledge about which register and bit controls this clk's gating.
110	Nothing about clock topology or accounting, such as enable_count or
111	notifier_count, is needed here.  That is all handled by the common
112	framework code and struct clk.
113	
114	Let'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	
122	The call graph for clk_enable is very simple:
123	
124	clk_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	
131	And the definition of clk_gate_set_bit:
132	
133	static 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	
142	Note that to_clk_gate is defined as:
143	
144	#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
145	
146	This pattern of abstraction is used for every clock hardware
147	representation.
148	
149		Part 4 - supporting your own clk hardware
150	
151	When implementing support for a new type of clock it only necessary to
152	include the following header:
153	
154	#include <linux/clk-provider.h>
155	
156	include/linux/clk.h is included within that header and clk-private.h
157	must never be included from the code which implements the operations for
158	a clock.  More on that below in Part 5.
159	
160	To construct a clk hardware structure for your platform you must define
161	the following:
162	
163	struct clk_foo {
164		struct clk_hw hw;
165		... hardware specific data goes here ...
166	};
167	
168	To take advantage of your data you'll need to support valid operations
169	for your clk:
170	
171	struct clk_ops clk_foo_ops {
172		.enable		= &clk_foo_enable;
173		.disable	= &clk_foo_disable;
174	};
175	
176	Implement the above functions using container_of:
177	
178	#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
179	
180	int 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	
191	Below is a matrix detailing which clk_ops are mandatory based upon the
192	hardware capabilities of that clock.  A cell marked as "y" means
193	mandatory, a cell marked as "n" implies that either including that
194	callback is invalid or otherwise unnecessary.  Empty cells are either
195	optional or must be evaluated on a case-by-case basis.
196	
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	                |      |             |               |             |      |
216	.recalc_accuracy|      |             |               |             |      |
217	                |      |             |               |             |      |
218	.init           |      |             |               |             |      |
219	                -----------------------------------------------------------
220	[1] either one of round_rate or determine_rate is required.
221	
222	Finally, register your clock at run-time with a hardware-specific
223	registration function.  This function simply populates struct clk_foo's
224	data and then passes the common struct clk parameters to the framework
225	with a call to:
226	
227	clk_register(...)
228	
229	See the basic clock types in drivers/clk/clk-*.c for examples.
230	
231		Part 5 - static initialization of clock data
232	
233	For platforms with many clocks (often numbering into the hundreds) it
234	may be desirable to statically initialize some clock data.  This
235	presents a problem since the definition of struct clk should be hidden
236	from everyone except for the clock core in drivers/clk/clk.c.
237	
238	To get around this problem struct clk's definition is exposed in
239	include/linux/clk-private.h along with some macros for more easily
240	initializing instances of the basic clock types.  These clocks must
241	still be initialized with the common clock framework via a call to
242	__clk_init.
243	
244	clk-private.h must NEVER be included by code which implements struct
245	clk_ops callbacks, nor must it be included by any logic which pokes
246	around inside of struct clk at run-time.  To do so is a layering
247	violation.
248	
249	To better enforce this policy, always follow this simple rule: any
250	statically initialized clock data MUST be defined in a separate file
251	from the logic that implements its ops.  Basically separate the logic
252	from the data and all is well.
253	
254		Part 6 - Disabling clock gating of unused clocks
255	
256	Sometimes during development it can be useful to be able to bypass the
257	default disabling of unused clocks. For example, if drivers aren't enabling
258	clocks properly but rely on them being on from the bootloader, bypassing
259	the disabling means that the driver will remain functional while the issues
260	are sorted out.
261	
262	To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
263	kernel.
264	
265		Part 7 - Locking
266	
267	The common clock framework uses two global locks, the prepare lock and the
268	enable lock.
269	
270	The 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
272	sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API
273	functions are allowed in atomic context.
274	
275	The prepare lock is a mutex and is held across calls to all other operations.
276	All those operations are allowed to sleep, and calls to the corresponding API
277	functions are not allowed in atomic context.
278	
279	This effectively divides operations in two groups from a locking perspective.
280	
281	Drivers don't need to manually protect resources shared between the operations
282	of one group, regardless of whether those resources are shared by multiple
283	clocks or not. However, access to resources that are shared between operations
284	of the two groups needs to be protected by the drivers. An example of such a
285	resource would be a register that controls both the clock rate and the clock
286	enable/disable state.
287	
288	The clock framework is reentrant, in that a driver is allowed to call clock
289	framework functions from within its implementation of clock operations. This
290	can for instance cause a .set_rate operation of one clock being called from
291	within the .set_rate operation of another clock. This case must be considered
292	in the driver implementations, but the code flow is usually controlled by the
293	driver in that case.
294	
295	Note that locking must also be considered when code outside of the common
296	clock framework needs to access resources used by the clock operations. This
297	is considered out of scope of this document.
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