1		The Common Clk Framework
2		Mike Turquette <mturquette@ti.com>
3
4This document endeavours to explain the common clk framework details,
5and how to port a platform over to this framework.  It is not yet a
6detailed explanation of the clock api in include/linux/clk.h, but
7perhaps someday it will include that information.
8
9	Part 1 - introduction and interface split
10
11The common clk framework is an interface to control the clock nodes
12available on various devices today.  This may come in the form of clock
13gating, rate adjustment, muxing or other operations.  This framework is
14enabled with the CONFIG_COMMON_CLK option.
15
16The interface itself is divided into two halves, each shielded from the
17details of its counterpart.  First is the common definition of struct
18clk which unifies the framework-level accounting and infrastructure that
19has traditionally been duplicated across a variety of platforms.  Second
20is a common implementation of the clk.h api, defined in
21drivers/clk/clk.c.  Finally there is struct clk_ops, whose operations
22are invoked by the clk api implementation.
23
24The second half of the interface is comprised of the hardware-specific
25callbacks registered with struct clk_ops and the corresponding
26hardware-specific structures needed to model a particular clock.  For
27the remainder of this document any reference to a callback in struct
28clk_ops, such as .enable or .set_rate, implies the hardware-specific
29implementation of that code.  Likewise, references to struct clk_foo
30serve as a convenient shorthand for the implementation of the
31hardware-specific bits for the hypothetical "foo" hardware.
32
33Tying the two halves of this interface together is struct clk_hw, which
34is defined in struct clk_foo and pointed to within struct clk.  This
35allows easy for navigation between the two discrete halves of the common
36clock interface.
37
38	Part 2 - common data structures and api
39
40Below is the common struct clk definition from
41include/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
55The members above make up the core of the clk tree topology.  The clk
56api itself defines several driver-facing functions which operate on
57struct clk.  That api is documented in include/linux/clk.h.
58
59Platforms and devices utilizing the common struct clk use the struct
60clk_ops pointer in struct clk to perform the hardware-specific parts of
61the 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		int		(*set_parent)(struct clk_hw *hw, u8 index);
74		u8		(*get_parent)(struct clk_hw *hw);
75		int		(*set_rate)(struct clk_hw *hw, unsigned long);
76		void		(*init)(struct clk_hw *hw);
77	};
78
79	Part 3 - hardware clk implementations
80
81The strength of the common struct clk comes from its .ops and .hw pointers
82which abstract the details of struct clk from the hardware-specific bits, and
83vice versa.  To illustrate consider the simple gateable clk implementation in
84drivers/clk/clk-gate.c:
85
86struct clk_gate {
87	struct clk_hw	hw;
88	void __iomem    *reg;
89	u8              bit_idx;
90	...
91};
92
93struct clk_gate contains struct clk_hw hw as well as hardware-specific
94knowledge about which register and bit controls this clk's gating.
95Nothing about clock topology or accounting, such as enable_count or
96notifier_count, is needed here.  That is all handled by the common
97framework code and struct clk.
98
99Let's walk through enabling this clk from driver code:
100
101	struct clk *clk;
102	clk = clk_get(NULL, "my_gateable_clk");
103
104	clk_prepare(clk);
105	clk_enable(clk);
106
107The call graph for clk_enable is very simple:
108
109clk_enable(clk);
110	clk->ops->enable(clk->hw);
111	[resolves to...]
112		clk_gate_enable(hw);
113		[resolves struct clk gate with to_clk_gate(hw)]
114			clk_gate_set_bit(gate);
115
116And the definition of clk_gate_set_bit:
117
118static void clk_gate_set_bit(struct clk_gate *gate)
119{
120	u32 reg;
121
122	reg = __raw_readl(gate->reg);
123	reg |= BIT(gate->bit_idx);
124	writel(reg, gate->reg);
125}
126
127Note that to_clk_gate is defined as:
128
129#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
130
131This pattern of abstraction is used for every clock hardware
132representation.
133
134	Part 4 - supporting your own clk hardware
135
136When implementing support for a new type of clock it only necessary to
137include the following header:
138
139#include <linux/clk-provider.h>
140
141include/linux/clk.h is included within that header and clk-private.h
142must never be included from the code which implements the operations for
143a clock.  More on that below in Part 5.
144
145To construct a clk hardware structure for your platform you must define
146the following:
147
148struct clk_foo {
149	struct clk_hw hw;
150	... hardware specific data goes here ...
151};
152
153To take advantage of your data you'll need to support valid operations
154for your clk:
155
156struct clk_ops clk_foo_ops {
157	.enable		= &clk_foo_enable;
158	.disable	= &clk_foo_disable;
159};
160
161Implement the above functions using container_of:
162
163#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
164
165int clk_foo_enable(struct clk_hw *hw)
166{
167	struct clk_foo *foo;
168
169	foo = to_clk_foo(hw);
170
171	... perform magic on foo ...
172
173	return 0;
174};
175
176Below is a matrix detailing which clk_ops are mandatory based upon the
177hardware capbilities of that clock.  A cell marked as "y" means
178mandatory, a cell marked as "n" implies that either including that
179callback is invalid or otherwise uneccesary.  Empty cells are either
180optional or must be evaluated on a case-by-case basis.
181
182                           clock hardware characteristics
183	     -----------------------------------------------------------
184             | gate | change rate | single parent | multiplexer | root |
185             |------|-------------|---------------|-------------|------|
186.prepare     |      |             |               |             |      |
187.unprepare   |      |             |               |             |      |
188             |      |             |               |             |      |
189.enable      | y    |             |               |             |      |
190.disable     | y    |             |               |             |      |
191.is_enabled  | y    |             |               |             |      |
192             |      |             |               |             |      |
193.recalc_rate |      | y           |               |             |      |
194.round_rate  |      | y           |               |             |      |
195.set_rate    |      | y           |               |             |      |
196             |      |             |               |             |      |
197.set_parent  |      |             | n             | y           | n    |
198.get_parent  |      |             | n             | y           | n    |
199             |      |             |               |             |      |
200.init        |      |             |               |             |      |
201	     -----------------------------------------------------------
202
203Finally, register your clock at run-time with a hardware-specific
204registration function.  This function simply populates struct clk_foo's
205data and then passes the common struct clk parameters to the framework
206with a call to:
207
208clk_register(...)
209
210See the basic clock types in drivers/clk/clk-*.c for examples.
211
212	Part 5 - static initialization of clock data
213
214For platforms with many clocks (often numbering into the hundreds) it
215may be desirable to statically initialize some clock data.  This
216presents a problem since the definition of struct clk should be hidden
217from everyone except for the clock core in drivers/clk/clk.c.
218
219To get around this problem struct clk's definition is exposed in
220include/linux/clk-private.h along with some macros for more easily
221initializing instances of the basic clock types.  These clocks must
222still be initialized with the common clock framework via a call to
223__clk_init.
224
225clk-private.h must NEVER be included by code which implements struct
226clk_ops callbacks, nor must it be included by any logic which pokes
227around inside of struct clk at run-time.  To do so is a layering
228violation.
229
230To better enforce this policy, always follow this simple rule: any
231statically initialized clock data MUST be defined in a separate file
232from the logic that implements its ops.  Basically separate the logic
233from the data and all is well.
234