clk.txt: standardize document format

========================

The Common Clk Framework

		Mike Turquette <mturquette@ti.com>

========================

:Author: Mike Turquette <mturquette@ti.com>

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.

	Part 1 - introduction and interface split

Introduction and interface split

================================

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

allows for easy navigation between the two discrete halves of the common

clock interface.

	Part 2 - common data structures and api

Common data structures and api

==============================

Below is the common struct clk_core definition from

drivers/clk/clk.c, modified for brevity:

drivers/clk/clk.c, modified for brevity::

	struct clk_core {

		const char		*name;

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

the operations defined in clk-provider.h:

the operations defined in clk-provider.h::

	struct clk_ops {

		int		(*prepare)(struct clk_hw *hw);

					      struct dentry *dentry);

	};

	Part 3 - hardware clk implementations

Hardware clk implementations

============================

The strength of the common struct clk_core comes from its .ops and .hw pointers

which abstract the details of struct clk from the hardware-specific bits, and

vice versa.  To illustrate consider the simple gateable clk implementation in

drivers/clk/clk-gate.c:

drivers/clk/clk-gate.c::

	struct clk_gate {

		struct clk_hw	hw;

notifier_count, is needed here.  That is all handled by the common

framework code and struct clk_core.

Let's walk through enabling this clk from driver code:

Let's walk through enabling this clk from driver code::

	struct clk *clk;

	clk = clk_get(NULL, "my_gateable_clk");

	clk_prepare(clk);

	clk_enable(clk);

The call graph for clk_enable is very simple:

The call graph for clk_enable is very simple::

	clk_enable(clk);

		clk->ops->enable(clk->hw);

			[resolves struct clk gate with to_clk_gate(hw)]

				clk_gate_set_bit(gate);

And the definition of clk_gate_set_bit:

And the definition of clk_gate_set_bit::

	static void clk_gate_set_bit(struct clk_gate *gate)

	{

		writel(reg, gate->reg);

	}

Note that to_clk_gate is defined as:

Note that to_clk_gate is defined as::

	#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, hw)

This pattern of abstraction is used for every clock hardware

representation.

	Part 4 - supporting your own clk hardware

Supporting your own clk hardware

================================

When implementing support for a new type of clock it is only necessary to

include the following header:

include the following header::

	#include <linux/clk-provider.h>

To construct a clk hardware structure for your platform you must define

the following:

the following::

	struct clk_foo {

		struct clk_hw hw;

	};

To take advantage of your data you'll need to support valid operations

for your clk:

for your clk::

	struct clk_ops clk_foo_ops {

		.enable		= &clk_foo_enable;

		.disable	= &clk_foo_disable;

	};

Implement the above functions using container_of:

Implement the above functions using container_of::

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

callback is invalid or otherwise unnecessary.  Empty cells are either

optional or must be evaluated on a case-by-case basis.

                              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.

.. 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.

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

with a call to:

with a call to::

	clk_register(...)

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

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

	Part 5 - Disabling clock gating of unused clocks

Disabling clock gating of unused clocks

=======================================

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

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

kernel.

	Part 6 - Locking

Locking

=======

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

enable lock.