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Based on kernel version 4.1. Page generated on 2015-06-28 12:08 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 min_rate,
77							unsigned long max_rate,
78							unsigned long *best_parent_rate,
79							struct clk_hw **best_parent_clk);
80			int		(*set_parent)(struct clk_hw *hw, u8 index);
81			u8		(*get_parent)(struct clk_hw *hw);
82			int		(*set_rate)(struct clk_hw *hw,
83						    unsigned long rate,
84						    unsigned long parent_rate);
85			int		(*set_rate_and_parent)(struct clk_hw *hw,
86						    unsigned long rate,
87						    unsigned long parent_rate,
88						    u8 index);
89			unsigned long	(*recalc_accuracy)(struct clk_hw *hw,
90							unsigned long parent_accuracy);
91			void		(*init)(struct clk_hw *hw);
92			int		(*debug_init)(struct clk_hw *hw,
93						      struct dentry *dentry);
94		};
95	
96		Part 3 - hardware clk implementations
97	
98	The strength of the common struct clk comes from its .ops and .hw pointers
99	which abstract the details of struct clk from the hardware-specific bits, and
100	vice versa.  To illustrate consider the simple gateable clk implementation in
101	drivers/clk/clk-gate.c:
102	
103	struct clk_gate {
104		struct clk_hw	hw;
105		void __iomem    *reg;
106		u8              bit_idx;
107		...
108	};
109	
110	struct clk_gate contains struct clk_hw hw as well as hardware-specific
111	knowledge about which register and bit controls this clk's gating.
112	Nothing about clock topology or accounting, such as enable_count or
113	notifier_count, is needed here.  That is all handled by the common
114	framework code and struct clk.
115	
116	Let's walk through enabling this clk from driver code:
117	
118		struct clk *clk;
119		clk = clk_get(NULL, "my_gateable_clk");
120	
121		clk_prepare(clk);
122		clk_enable(clk);
123	
124	The call graph for clk_enable is very simple:
125	
126	clk_enable(clk);
127		clk->ops->enable(clk->hw);
128		[resolves to...]
129			clk_gate_enable(hw);
130			[resolves struct clk gate with to_clk_gate(hw)]
131				clk_gate_set_bit(gate);
132	
133	And the definition of clk_gate_set_bit:
134	
135	static void clk_gate_set_bit(struct clk_gate *gate)
136	{
137		u32 reg;
138	
139		reg = __raw_readl(gate->reg);
140		reg |= BIT(gate->bit_idx);
141		writel(reg, gate->reg);
142	}
143	
144	Note that to_clk_gate is defined as:
145	
146	#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
147	
148	This pattern of abstraction is used for every clock hardware
149	representation.
150	
151		Part 4 - supporting your own clk hardware
152	
153	When implementing support for a new type of clock it only necessary to
154	include the following header:
155	
156	#include <linux/clk-provider.h>
157	
158	include/linux/clk.h is included within that header and clk-private.h
159	must never be included from the code which implements the operations for
160	a clock.  More on that below in Part 5.
161	
162	To construct a clk hardware structure for your platform you must define
163	the following:
164	
165	struct clk_foo {
166		struct clk_hw hw;
167		... hardware specific data goes here ...
168	};
169	
170	To take advantage of your data you'll need to support valid operations
171	for your clk:
172	
173	struct clk_ops clk_foo_ops {
174		.enable		= &clk_foo_enable;
175		.disable	= &clk_foo_disable;
176	};
177	
178	Implement the above functions using container_of:
179	
180	#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
181	
182	int clk_foo_enable(struct clk_hw *hw)
183	{
184		struct clk_foo *foo;
185	
186		foo = to_clk_foo(hw);
187	
188		... perform magic on foo ...
189	
190		return 0;
191	};
192	
193	Below is a matrix detailing which clk_ops are mandatory based upon the
194	hardware capabilities of that clock.  A cell marked as "y" means
195	mandatory, a cell marked as "n" implies that either including that
196	callback is invalid or otherwise unnecessary.  Empty cells are either
197	optional or must be evaluated on a case-by-case basis.
198	
199	                              clock hardware characteristics
200	                -----------------------------------------------------------
201	                | gate | change rate | single parent | multiplexer | root |
202	                |------|-------------|---------------|-------------|------|
203	.prepare        |      |             |               |             |      |
204	.unprepare      |      |             |               |             |      |
205	                |      |             |               |             |      |
206	.enable         | y    |             |               |             |      |
207	.disable        | y    |             |               |             |      |
208	.is_enabled     | y    |             |               |             |      |
209	                |      |             |               |             |      |
210	.recalc_rate    |      | y           |               |             |      |
211	.round_rate     |      | y [1]       |               |             |      |
212	.determine_rate |      | y [1]       |               |             |      |
213	.set_rate       |      | y           |               |             |      |
214	                |      |             |               |             |      |
215	.set_parent     |      |             | n             | y           | n    |
216	.get_parent     |      |             | n             | y           | n    |
217	                |      |             |               |             |      |
218	.recalc_accuracy|      |             |               |             |      |
219	                |      |             |               |             |      |
220	.init           |      |             |               |             |      |
221	                -----------------------------------------------------------
222	[1] either one of round_rate or determine_rate is required.
223	
224	Finally, register your clock at run-time with a hardware-specific
225	registration function.  This function simply populates struct clk_foo's
226	data and then passes the common struct clk parameters to the framework
227	with a call to:
228	
229	clk_register(...)
230	
231	See the basic clock types in drivers/clk/clk-*.c for examples.
232	
233		Part 5 - static initialization of clock data
234	
235	For platforms with many clocks (often numbering into the hundreds) it
236	may be desirable to statically initialize some clock data.  This
237	presents a problem since the definition of struct clk should be hidden
238	from everyone except for the clock core in drivers/clk/clk.c.
239	
240	To get around this problem struct clk's definition is exposed in
241	include/linux/clk-private.h along with some macros for more easily
242	initializing instances of the basic clock types.  These clocks must
243	still be initialized with the common clock framework via a call to
244	__clk_init.
245	
246	clk-private.h must NEVER be included by code which implements struct
247	clk_ops callbacks, nor must it be included by any logic which pokes
248	around inside of struct clk at run-time.  To do so is a layering
249	violation.
250	
251	To better enforce this policy, always follow this simple rule: any
252	statically initialized clock data MUST be defined in a separate file
253	from the logic that implements its ops.  Basically separate the logic
254	from the data and all is well.
255	
256		Part 6 - Disabling clock gating of unused clocks
257	
258	Sometimes during development it can be useful to be able to bypass the
259	default disabling of unused clocks. For example, if drivers aren't enabling
260	clocks properly but rely on them being on from the bootloader, bypassing
261	the disabling means that the driver will remain functional while the issues
262	are sorted out.
263	
264	To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
265	kernel.
266	
267		Part 7 - Locking
268	
269	The common clock framework uses two global locks, the prepare lock and the
270	enable lock.
271	
272	The enable lock is a spinlock and is held across calls to the .enable,
273	.disable and .is_enabled operations. Those operations are thus not allowed to
274	sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API
275	functions are allowed in atomic context.
276	
277	The prepare lock is a mutex and is held across calls to all other operations.
278	All those operations are allowed to sleep, and calls to the corresponding API
279	functions are not allowed in atomic context.
280	
281	This effectively divides operations in two groups from a locking perspective.
282	
283	Drivers don't need to manually protect resources shared between the operations
284	of one group, regardless of whether those resources are shared by multiple
285	clocks or not. However, access to resources that are shared between operations
286	of the two groups needs to be protected by the drivers. An example of such a
287	resource would be a register that controls both the clock rate and the clock
288	enable/disable state.
289	
290	The clock framework is reentrant, in that a driver is allowed to call clock
291	framework functions from within its implementation of clock operations. This
292	can for instance cause a .set_rate operation of one clock being called from
293	within the .set_rate operation of another clock. This case must be considered
294	in the driver implementations, but the code flow is usually controlled by the
295	driver in that case.
296	
297	Note that locking must also be considered when code outside of the common
298	clock framework needs to access resources used by the clock operations. This
299	is considered out of scope of this document.
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