clk.txt: standardize document format
Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: - Use section/title markups; - Use :Author: for authorship; - Mark literals and literal blocks; - Mark tables; - Use ReST notation for footnotes. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mauro Carvalho Chehab
Jonathan Corbet
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@ -1,12 +1,16 @@
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The Common Clk Framework
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Mike Turquette <mturquette@ti.com>
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========================
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The Common Clk Framework
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========================
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:Author: Mike Turquette <mturquette@ti.com>
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This document endeavours to explain the common clk framework details,
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and how to port a platform over to this framework. It is not yet a
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detailed explanation of the clock api in include/linux/clk.h, but
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perhaps someday it will include that information.
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Part 1 - introduction and interface split
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Introduction and interface split
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================================
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The common clk framework is an interface to control the clock nodes
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available on various devices today. This may come in the form of clock
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@ -35,10 +39,11 @@ is defined in struct clk_foo and pointed to within struct clk_core. This
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allows for easy navigation between the two discrete halves of the common
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clock interface.
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Part 2 - common data structures and api
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Common data structures and api
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==============================
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Below is the common struct clk_core definition from
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drivers/clk/clk.c, modified for brevity:
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drivers/clk/clk.c, modified for brevity::
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struct clk_core {
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const char *name;
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@ -59,7 +64,7 @@ struct clk. That api is documented in include/linux/clk.h.
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Platforms and devices utilizing the common struct clk_core use the struct
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clk_ops pointer in struct clk_core to perform the hardware-specific parts of
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the operations defined in clk-provider.h:
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the operations defined in clk-provider.h::
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struct clk_ops {
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int (*prepare)(struct clk_hw *hw);
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@ -95,19 +100,20 @@ the operations defined in clk-provider.h:
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struct dentry *dentry);
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};
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Part 3 - hardware clk implementations
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Hardware clk implementations
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============================
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The strength of the common struct clk_core comes from its .ops and .hw pointers
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which abstract the details of struct clk from the hardware-specific bits, and
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vice versa. To illustrate consider the simple gateable clk implementation in
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drivers/clk/clk-gate.c:
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drivers/clk/clk-gate.c::
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struct clk_gate {
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struct clk_hw hw;
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void __iomem *reg;
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u8 bit_idx;
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...
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};
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struct clk_gate {
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struct clk_hw hw;
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void __iomem *reg;
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u8 bit_idx;
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...
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};
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struct clk_gate contains struct clk_hw hw as well as hardware-specific
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knowledge about which register and bit controls this clk's gating.
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@ -115,7 +121,7 @@ Nothing about clock topology or accounting, such as enable_count or
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notifier_count, is needed here. That is all handled by the common
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framework code and struct clk_core.
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Let's walk through enabling this clk from driver code:
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Let's walk through enabling this clk from driver code::
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struct clk *clk;
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clk = clk_get(NULL, "my_gateable_clk");
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@ -123,70 +129,71 @@ Let's walk through enabling this clk from driver code:
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clk_prepare(clk);
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clk_enable(clk);
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The call graph for clk_enable is very simple:
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The call graph for clk_enable is very simple::
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clk_enable(clk);
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clk->ops->enable(clk->hw);
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[resolves to...]
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clk_gate_enable(hw);
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[resolves struct clk gate with to_clk_gate(hw)]
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clk_gate_set_bit(gate);
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clk_enable(clk);
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clk->ops->enable(clk->hw);
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[resolves to...]
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clk_gate_enable(hw);
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[resolves struct clk gate with to_clk_gate(hw)]
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clk_gate_set_bit(gate);
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And the definition of clk_gate_set_bit:
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And the definition of clk_gate_set_bit::
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static void clk_gate_set_bit(struct clk_gate *gate)
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{
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u32 reg;
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static void clk_gate_set_bit(struct clk_gate *gate)
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{
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u32 reg;
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reg = __raw_readl(gate->reg);
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reg |= BIT(gate->bit_idx);
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writel(reg, gate->reg);
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}
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reg = __raw_readl(gate->reg);
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reg |= BIT(gate->bit_idx);
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writel(reg, gate->reg);
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}
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Note that to_clk_gate is defined as:
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Note that to_clk_gate is defined as::
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#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, hw)
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#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, hw)
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This pattern of abstraction is used for every clock hardware
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representation.
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Part 4 - supporting your own clk hardware
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Supporting your own clk hardware
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================================
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When implementing support for a new type of clock it is only necessary to
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include the following header:
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include the following header::
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#include <linux/clk-provider.h>
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#include <linux/clk-provider.h>
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To construct a clk hardware structure for your platform you must define
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the following:
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the following::
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struct clk_foo {
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struct clk_hw hw;
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... hardware specific data goes here ...
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};
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struct clk_foo {
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struct clk_hw hw;
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... hardware specific data goes here ...
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};
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To take advantage of your data you'll need to support valid operations
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for your clk:
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for your clk::
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struct clk_ops clk_foo_ops {
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.enable = &clk_foo_enable;
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.disable = &clk_foo_disable;
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};
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struct clk_ops clk_foo_ops {
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.enable = &clk_foo_enable;
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.disable = &clk_foo_disable;
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};
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Implement the above functions using container_of:
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Implement the above functions using container_of::
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#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
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#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
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int clk_foo_enable(struct clk_hw *hw)
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{
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struct clk_foo *foo;
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int clk_foo_enable(struct clk_hw *hw)
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{
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struct clk_foo *foo;
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foo = to_clk_foo(hw);
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foo = to_clk_foo(hw);
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... perform magic on foo ...
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... perform magic on foo ...
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return 0;
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};
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return 0;
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};
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Below is a matrix detailing which clk_ops are mandatory based upon the
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hardware capabilities of that clock. A cell marked as "y" means
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@ -194,41 +201,56 @@ mandatory, a cell marked as "n" implies that either including that
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callback is invalid or otherwise unnecessary. Empty cells are either
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optional or must be evaluated on a case-by-case basis.
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clock hardware characteristics
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-----------------------------------------------------------
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| gate | change rate | single parent | multiplexer | root |
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|------|-------------|---------------|-------------|------|
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.prepare | | | | | |
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.unprepare | | | | | |
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| | | | | |
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.enable | y | | | | |
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.disable | y | | | | |
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.is_enabled | y | | | | |
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| | | | | |
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.recalc_rate | | y | | | |
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.round_rate | | y [1] | | | |
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.determine_rate | | y [1] | | | |
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.set_rate | | y | | | |
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| | | | | |
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.set_parent | | | n | y | n |
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.get_parent | | | n | y | n |
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| | | | | |
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.recalc_accuracy| | | | | |
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| | | | | |
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.init | | | | | |
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-----------------------------------------------------------
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[1] either one of round_rate or determine_rate is required.
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.. table:: clock hardware characteristics
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+----------------+------+-------------+---------------+-------------+------+
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| | gate | change rate | single parent | multiplexer | root |
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+================+======+=============+===============+=============+======+
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|.prepare | | | | | |
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+----------------+------+-------------+---------------+-------------+------+
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|.unprepare | | | | | |
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+----------------+------+-------------+---------------+-------------+------+
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+----------------+------+-------------+---------------+-------------+------+
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|.enable | y | | | | |
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+----------------+------+-------------+---------------+-------------+------+
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|.disable | y | | | | |
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+----------------+------+-------------+---------------+-------------+------+
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|.is_enabled | y | | | | |
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+----------------+------+-------------+---------------+-------------+------+
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+----------------+------+-------------+---------------+-------------+------+
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|.recalc_rate | | y | | | |
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+----------------+------+-------------+---------------+-------------+------+
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|.round_rate | | y [1]_ | | | |
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+----------------+------+-------------+---------------+-------------+------+
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|.determine_rate | | y [1]_ | | | |
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+----------------+------+-------------+---------------+-------------+------+
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|.set_rate | | y | | | |
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+----------------+------+-------------+---------------+-------------+------+
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+----------------+------+-------------+---------------+-------------+------+
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|.set_parent | | | n | y | n |
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+----------------+------+-------------+---------------+-------------+------+
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|.get_parent | | | n | y | n |
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+----------------+------+-------------+---------------+-------------+------+
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+----------------+------+-------------+---------------+-------------+------+
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|.recalc_accuracy| | | | | |
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+----------------+------+-------------+---------------+-------------+------+
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+----------------+------+-------------+---------------+-------------+------+
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|.init | | | | | |
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+----------------+------+-------------+---------------+-------------+------+
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.. [1] either one of round_rate or determine_rate is required.
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Finally, register your clock at run-time with a hardware-specific
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registration function. This function simply populates struct clk_foo's
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data and then passes the common struct clk parameters to the framework
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with a call to:
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with a call to::
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clk_register(...)
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clk_register(...)
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See the basic clock types in drivers/clk/clk-*.c for examples.
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See the basic clock types in ``drivers/clk/clk-*.c`` for examples.
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Part 5 - Disabling clock gating of unused clocks
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Disabling clock gating of unused clocks
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=======================================
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Sometimes during development it can be useful to be able to bypass the
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default disabling of unused clocks. For example, if drivers aren't enabling
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@ -239,7 +261,8 @@ are sorted out.
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To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
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kernel.
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Part 6 - Locking
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Locking
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=======
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The common clock framework uses two global locks, the prepare lock and the
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enable lock.
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