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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:40 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			int		(*determine_rate)(struct clk_hw *hw,
75							  struct clk_rate_request *req);
76			int		(*set_parent)(struct clk_hw *hw, u8 index);
77			u8		(*get_parent)(struct clk_hw *hw);
78			int		(*set_rate)(struct clk_hw *hw,
79						    unsigned long rate,
80						    unsigned long parent_rate);
81			int		(*set_rate_and_parent)(struct clk_hw *hw,
82						    unsigned long rate,
83						    unsigned long parent_rate,
84						    u8 index);
85			unsigned long	(*recalc_accuracy)(struct clk_hw *hw,
86							unsigned long parent_accuracy);
87			void		(*init)(struct clk_hw *hw);
88			int		(*debug_init)(struct clk_hw *hw,
89						      struct dentry *dentry);
90		};
91	
92		Part 3 - hardware clk implementations
93	
94	The strength of the common struct clk comes from its .ops and .hw pointers
95	which abstract the details of struct clk from the hardware-specific bits, and
96	vice versa.  To illustrate consider the simple gateable clk implementation in
97	drivers/clk/clk-gate.c:
98	
99	struct clk_gate {
100		struct clk_hw	hw;
101		void __iomem    *reg;
102		u8              bit_idx;
103		...
104	};
105	
106	struct clk_gate contains struct clk_hw hw as well as hardware-specific
107	knowledge about which register and bit controls this clk's gating.
108	Nothing about clock topology or accounting, such as enable_count or
109	notifier_count, is needed here.  That is all handled by the common
110	framework code and struct clk.
111	
112	Let's walk through enabling this clk from driver code:
113	
114		struct clk *clk;
115		clk = clk_get(NULL, "my_gateable_clk");
116	
117		clk_prepare(clk);
118		clk_enable(clk);
119	
120	The call graph for clk_enable is very simple:
121	
122	clk_enable(clk);
123		clk->ops->enable(clk->hw);
124		[resolves to...]
125			clk_gate_enable(hw);
126			[resolves struct clk gate with to_clk_gate(hw)]
127				clk_gate_set_bit(gate);
128	
129	And the definition of clk_gate_set_bit:
130	
131	static void clk_gate_set_bit(struct clk_gate *gate)
132	{
133		u32 reg;
134	
135		reg = __raw_readl(gate->reg);
136		reg |= BIT(gate->bit_idx);
137		writel(reg, gate->reg);
138	}
139	
140	Note that to_clk_gate is defined as:
141	
142	#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
143	
144	This pattern of abstraction is used for every clock hardware
145	representation.
146	
147		Part 4 - supporting your own clk hardware
148	
149	When implementing support for a new type of clock it only necessary to
150	include the following header:
151	
152	#include <linux/clk-provider.h>
153	
154	include/linux/clk.h is included within that header and clk-private.h
155	must never be included from the code which implements the operations for
156	a clock.  More on that below in Part 5.
157	
158	To construct a clk hardware structure for your platform you must define
159	the following:
160	
161	struct clk_foo {
162		struct clk_hw hw;
163		... hardware specific data goes here ...
164	};
165	
166	To take advantage of your data you'll need to support valid operations
167	for your clk:
168	
169	struct clk_ops clk_foo_ops {
170		.enable		= &clk_foo_enable;
171		.disable	= &clk_foo_disable;
172	};
173	
174	Implement the above functions using container_of:
175	
176	#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
177	
178	int clk_foo_enable(struct clk_hw *hw)
179	{
180		struct clk_foo *foo;
181	
182		foo = to_clk_foo(hw);
183	
184		... perform magic on foo ...
185	
186		return 0;
187	};
188	
189	Below is a matrix detailing which clk_ops are mandatory based upon the
190	hardware capabilities of that clock.  A cell marked as "y" means
191	mandatory, a cell marked as "n" implies that either including that
192	callback is invalid or otherwise unnecessary.  Empty cells are either
193	optional or must be evaluated on a case-by-case basis.
194	
195	                              clock hardware characteristics
196	                -----------------------------------------------------------
197	                | gate | change rate | single parent | multiplexer | root |
198	                |------|-------------|---------------|-------------|------|
199	.prepare        |      |             |               |             |      |
200	.unprepare      |      |             |               |             |      |
201	                |      |             |               |             |      |
202	.enable         | y    |             |               |             |      |
203	.disable        | y    |             |               |             |      |
204	.is_enabled     | y    |             |               |             |      |
205	                |      |             |               |             |      |
206	.recalc_rate    |      | y           |               |             |      |
207	.round_rate     |      | y [1]       |               |             |      |
208	.determine_rate |      | y [1]       |               |             |      |
209	.set_rate       |      | y           |               |             |      |
210	                |      |             |               |             |      |
211	.set_parent     |      |             | n             | y           | n    |
212	.get_parent     |      |             | n             | y           | n    |
213	                |      |             |               |             |      |
214	.recalc_accuracy|      |             |               |             |      |
215	                |      |             |               |             |      |
216	.init           |      |             |               |             |      |
217	                -----------------------------------------------------------
218	[1] either one of round_rate or determine_rate is required.
219	
220	Finally, register your clock at run-time with a hardware-specific
221	registration function.  This function simply populates struct clk_foo's
222	data and then passes the common struct clk parameters to the framework
223	with a call to:
224	
225	clk_register(...)
226	
227	See the basic clock types in drivers/clk/clk-*.c for examples.
228	
229		Part 5 - Disabling clock gating of unused clocks
230	
231	Sometimes during development it can be useful to be able to bypass the
232	default disabling of unused clocks. For example, if drivers aren't enabling
233	clocks properly but rely on them being on from the bootloader, bypassing
234	the disabling means that the driver will remain functional while the issues
235	are sorted out.
236	
237	To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
238	kernel.
239	
240		Part 6 - Locking
241	
242	The common clock framework uses two global locks, the prepare lock and the
243	enable lock.
244	
245	The enable lock is a spinlock and is held across calls to the .enable,
246	.disable and .is_enabled operations. Those operations are thus not allowed to
247	sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API
248	functions are allowed in atomic context.
249	
250	The prepare lock is a mutex and is held across calls to all other operations.
251	All those operations are allowed to sleep, and calls to the corresponding API
252	functions are not allowed in atomic context.
253	
254	This effectively divides operations in two groups from a locking perspective.
255	
256	Drivers don't need to manually protect resources shared between the operations
257	of one group, regardless of whether those resources are shared by multiple
258	clocks or not. However, access to resources that are shared between operations
259	of the two groups needs to be protected by the drivers. An example of such a
260	resource would be a register that controls both the clock rate and the clock
261	enable/disable state.
262	
263	The clock framework is reentrant, in that a driver is allowed to call clock
264	framework functions from within its implementation of clock operations. This
265	can for instance cause a .set_rate operation of one clock being called from
266	within the .set_rate operation of another clock. This case must be considered
267	in the driver implementations, but the code flow is usually controlled by the
268	driver in that case.
269	
270	Note that locking must also be considered when code outside of the common
271	clock framework needs to access resources used by the clock operations. This
272	is considered out of scope of this document.
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