clk.txt 11.6 KB
Newer Older
1 2 3 4 5
========================
The Common Clk Framework
========================

:Author: Mike Turquette <mturquette@ti.com>
6 7 8 9 10 11

This document endeavours to explain the common clk framework details,
and how to port a platform over to this framework.  It is not yet a
detailed explanation of the clock api in include/linux/clk.h, but
perhaps someday it will include that information.

12 13
Introduction and interface split
================================
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

The common clk framework is an interface to control the clock nodes
available on various devices today.  This may come in the form of clock
gating, rate adjustment, muxing or other operations.  This framework is
enabled with the CONFIG_COMMON_CLK option.

The interface itself is divided into two halves, each shielded from the
details of its counterpart.  First is the common definition of struct
clk which unifies the framework-level accounting and infrastructure that
has traditionally been duplicated across a variety of platforms.  Second
is a common implementation of the clk.h api, defined in
drivers/clk/clk.c.  Finally there is struct clk_ops, whose operations
are invoked by the clk api implementation.

The second half of the interface is comprised of the hardware-specific
callbacks registered with struct clk_ops and the corresponding
hardware-specific structures needed to model a particular clock.  For
the remainder of this document any reference to a callback in struct
clk_ops, such as .enable or .set_rate, implies the hardware-specific
implementation of that code.  Likewise, references to struct clk_foo
serve as a convenient shorthand for the implementation of the
hardware-specific bits for the hypothetical "foo" hardware.

Tying the two halves of this interface together is struct clk_hw, which
38
is defined in struct clk_foo and pointed to within struct clk_core.  This
39
allows for easy navigation between the two discrete halves of the common
40 41
clock interface.

42 43
Common data structures and api
==============================
44

45
Below is the common struct clk_core definition from
46
drivers/clk/clk.c, modified for brevity::
47

48
	struct clk_core {
49 50 51
		const char		*name;
		const struct clk_ops	*ops;
		struct clk_hw		*hw;
52 53 54 55 56 57
		struct module		*owner;
		struct clk_core		*parent;
		const char		**parent_names;
		struct clk_core		**parents;
		u8			num_parents;
		u8			new_parent_index;
58 59 60 61 62 63 64
		...
	};

The members above make up the core of the clk tree topology.  The clk
api itself defines several driver-facing functions which operate on
struct clk.  That api is documented in include/linux/clk.h.

65 66
Platforms and devices utilizing the common struct clk_core use the struct
clk_ops pointer in struct clk_core to perform the hardware-specific parts of
67
the operations defined in clk-provider.h::
68 69 70 71

	struct clk_ops {
		int		(*prepare)(struct clk_hw *hw);
		void		(*unprepare)(struct clk_hw *hw);
72 73
		int		(*is_prepared)(struct clk_hw *hw);
		void		(*unprepare_unused)(struct clk_hw *hw);
74 75 76
		int		(*enable)(struct clk_hw *hw);
		void		(*disable)(struct clk_hw *hw);
		int		(*is_enabled)(struct clk_hw *hw);
77
		void		(*disable_unused)(struct clk_hw *hw);
78 79
		unsigned long	(*recalc_rate)(struct clk_hw *hw,
						unsigned long parent_rate);
80 81 82
		long		(*round_rate)(struct clk_hw *hw,
						unsigned long rate,
						unsigned long *parent_rate);
83 84
		int		(*determine_rate)(struct clk_hw *hw,
						  struct clk_rate_request *req);
85 86
		int		(*set_parent)(struct clk_hw *hw, u8 index);
		u8		(*get_parent)(struct clk_hw *hw);
87 88 89
		int		(*set_rate)(struct clk_hw *hw,
					    unsigned long rate,
					    unsigned long parent_rate);
90 91
		int		(*set_rate_and_parent)(struct clk_hw *hw,
					    unsigned long rate,
92 93
					    unsigned long parent_rate,
					    u8 index);
94
		unsigned long	(*recalc_accuracy)(struct clk_hw *hw,
95
						unsigned long parent_accuracy);
96 97
		int		(*get_phase)(struct clk_hw *hw);
		int		(*set_phase)(struct clk_hw *hw, int degrees);
98
		void		(*init)(struct clk_hw *hw);
99 100
		int		(*debug_init)(struct clk_hw *hw,
					      struct dentry *dentry);
101 102
	};

103 104
Hardware clk implementations
============================
105

106
The strength of the common struct clk_core comes from its .ops and .hw pointers
107 108
which abstract the details of struct clk from the hardware-specific bits, and
vice versa.  To illustrate consider the simple gateable clk implementation in
109
drivers/clk/clk-gate.c::
110

111 112 113 114 115 116
	struct clk_gate {
		struct clk_hw	hw;
		void __iomem    *reg;
		u8              bit_idx;
		...
	};
117 118 119 120 121

struct clk_gate contains struct clk_hw hw as well as hardware-specific
knowledge about which register and bit controls this clk's gating.
Nothing about clock topology or accounting, such as enable_count or
notifier_count, is needed here.  That is all handled by the common
122
framework code and struct clk_core.
123

124
Let's walk through enabling this clk from driver code::
125 126 127 128 129 130 131

	struct clk *clk;
	clk = clk_get(NULL, "my_gateable_clk");

	clk_prepare(clk);
	clk_enable(clk);

132
The call graph for clk_enable is very simple::
133

134 135 136 137 138 139
	clk_enable(clk);
		clk->ops->enable(clk->hw);
		[resolves to...]
			clk_gate_enable(hw);
			[resolves struct clk gate with to_clk_gate(hw)]
				clk_gate_set_bit(gate);
140

141
And the definition of clk_gate_set_bit::
142

143 144 145
	static void clk_gate_set_bit(struct clk_gate *gate)
	{
		u32 reg;
146

147 148 149 150
		reg = __raw_readl(gate->reg);
		reg |= BIT(gate->bit_idx);
		writel(reg, gate->reg);
	}
151

152
Note that to_clk_gate is defined as::
153

154
	#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, hw)
155 156 157 158

This pattern of abstraction is used for every clock hardware
representation.

159 160
Supporting your own clk hardware
================================
161

162
When implementing support for a new type of clock it is only necessary to
163
include the following header::
164

165
	#include <linux/clk-provider.h>
166 167

To construct a clk hardware structure for your platform you must define
168
the following::
169

170 171 172 173
	struct clk_foo {
		struct clk_hw hw;
		... hardware specific data goes here ...
	};
174 175

To take advantage of your data you'll need to support valid operations
176
for your clk::
177

178 179 180 181
	struct clk_ops clk_foo_ops {
		.enable		= &clk_foo_enable;
		.disable	= &clk_foo_disable;
	};
182

183
Implement the above functions using container_of::
184

185
	#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
186

187 188 189
	int clk_foo_enable(struct clk_hw *hw)
	{
		struct clk_foo *foo;
190

191
		foo = to_clk_foo(hw);
192

193
		... perform magic on foo ...
194

195 196
		return 0;
	};
197 198

Below is a matrix detailing which clk_ops are mandatory based upon the
199
hardware capabilities of that clock.  A cell marked as "y" means
200
mandatory, a cell marked as "n" implies that either including that
201
callback is invalid or otherwise unnecessary.  Empty cells are either
202 203
optional or must be evaluated on a case-by-case basis.

204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241
.. table:: clock hardware characteristics

   +----------------+------+-------------+---------------+-------------+------+
   |                | gate | change rate | single parent | multiplexer | root |
   +================+======+=============+===============+=============+======+
   |.prepare        |      |             |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   |.unprepare      |      |             |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   +----------------+------+-------------+---------------+-------------+------+
   |.enable         | y    |             |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   |.disable        | y    |             |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   |.is_enabled     | y    |             |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   +----------------+------+-------------+---------------+-------------+------+
   |.recalc_rate    |      | y           |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   |.round_rate     |      | y [1]_      |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   |.determine_rate |      | y [1]_      |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   |.set_rate       |      | y           |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   +----------------+------+-------------+---------------+-------------+------+
   |.set_parent     |      |             | n             | y           | n    |
   +----------------+------+-------------+---------------+-------------+------+
   |.get_parent     |      |             | n             | y           | n    |
   +----------------+------+-------------+---------------+-------------+------+
   +----------------+------+-------------+---------------+-------------+------+
   |.recalc_accuracy|      |             |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+
   +----------------+------+-------------+---------------+-------------+------+
   |.init           |      |             |               |             |      |
   +----------------+------+-------------+---------------+-------------+------+

.. [1] either one of round_rate or determine_rate is required.
242 243 244 245

Finally, register your clock at run-time with a hardware-specific
registration function.  This function simply populates struct clk_foo's
data and then passes the common struct clk parameters to the framework
246
with a call to::
247

248
	clk_register(...)
249

250
See the basic clock types in ``drivers/clk/clk-*.c`` for examples.
251

252 253
Disabling clock gating of unused clocks
=======================================
254 255 256 257 258 259 260 261 262

Sometimes during development it can be useful to be able to bypass the
default disabling of unused clocks. For example, if drivers aren't enabling
clocks properly but rely on them being on from the bootloader, bypassing
the disabling means that the driver will remain functional while the issues
are sorted out.

To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
kernel.
263

264 265
Locking
=======
266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297

The common clock framework uses two global locks, the prepare lock and the
enable lock.

The enable lock is a spinlock and is held across calls to the .enable,
.disable and .is_enabled operations. Those operations are thus not allowed to
sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API
functions are allowed in atomic context.

The prepare lock is a mutex and is held across calls to all other operations.
All those operations are allowed to sleep, and calls to the corresponding API
functions are not allowed in atomic context.

This effectively divides operations in two groups from a locking perspective.

Drivers don't need to manually protect resources shared between the operations
of one group, regardless of whether those resources are shared by multiple
clocks or not. However, access to resources that are shared between operations
of the two groups needs to be protected by the drivers. An example of such a
resource would be a register that controls both the clock rate and the clock
enable/disable state.

The clock framework is reentrant, in that a driver is allowed to call clock
framework functions from within its implementation of clock operations. This
can for instance cause a .set_rate operation of one clock being called from
within the .set_rate operation of another clock. This case must be considered
in the driver implementations, but the code flow is usually controlled by the
driver in that case.

Note that locking must also be considered when code outside of the common
clock framework needs to access resources used by the clock operations. This
is considered out of scope of this document.