- The Common Clk Framework
- Mike Turquette <mturquette@ti.com>
+========================
+The Common Clk Framework
+========================
+
+: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;
- void __iomem *reg;
- u8 bit_idx;
- ...
-};
+ struct clk_gate {
+ struct clk_hw hw;
+ void __iomem *reg;
+ u8 bit_idx;
+ ...
+ };
struct clk_gate contains struct clk_hw hw as well as hardware-specific
knowledge about which register and bit controls this clk's gating.
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 to...]
- clk_gate_enable(hw);
- [resolves struct clk gate with to_clk_gate(hw)]
- clk_gate_set_bit(gate);
+ 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);
-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)
-{
- u32 reg;
+ static void clk_gate_set_bit(struct clk_gate *gate)
+ {
+ u32 reg;
- reg = __raw_readl(gate->reg);
- reg |= BIT(gate->bit_idx);
- writel(reg, gate->reg);
-}
+ reg = __raw_readl(gate->reg);
+ reg |= BIT(gate->bit_idx);
+ 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)
+ #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>
+ #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;
- ... hardware specific data goes here ...
-};
+ struct clk_foo {
+ struct clk_hw hw;
+ ... hardware specific data goes here ...
+ };
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;
-};
+ 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)
+ #define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
-int clk_foo_enable(struct clk_hw *hw)
-{
- struct clk_foo *foo;
+ int clk_foo_enable(struct clk_hw *hw)
+ {
+ struct clk_foo *foo;
- foo = to_clk_foo(hw);
+ foo = to_clk_foo(hw);
- ... perform magic on foo ...
+ ... perform magic on foo ...
- return 0;
-};
+ return 0;
+ };
Below is a matrix detailing which clk_ops are mandatory based upon the
hardware capabilities of that clock. A cell marked as "y" means
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(...)
+ 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.
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