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|
The Common Clk Framework
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
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
is defined in struct clk_foo and pointed to within struct clk. This
allows easy for navigation between the two discrete halves of the common
clock interface.
Part 2 - common data structures and api
Below is the common struct clk definition from
include/linux/clk-private.h, modified for brevity:
struct clk {
const char *name;
const struct clk_ops *ops;
struct clk_hw *hw;
char **parent_names;
struct clk **parents;
struct clk *parent;
struct hlist_head children;
struct hlist_node child_node;
...
};
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.
Platforms and devices utilizing the common struct clk use the struct
clk_ops pointer in struct clk to perform the hardware-specific parts of
the operations defined in clk.h:
struct clk_ops {
int (*prepare)(struct clk_hw *hw);
void (*unprepare)(struct clk_hw *hw);
int (*enable)(struct clk_hw *hw);
void (*disable)(struct clk_hw *hw);
int (*is_enabled)(struct clk_hw *hw);
unsigned long (*recalc_rate)(struct clk_hw *hw,
unsigned long parent_rate);
long (*round_rate)(struct clk_hw *hw, unsigned long,
unsigned long *);
int (*set_parent)(struct clk_hw *hw, u8 index);
u8 (*get_parent)(struct clk_hw *hw);
int (*set_rate)(struct clk_hw *hw, unsigned long);
void (*init)(struct clk_hw *hw);
};
Part 3 - hardware clk implementations
The strength of the common struct clk 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:
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.
Nothing about clock topology or accounting, such as enable_count or
notifier_count, is needed here. That is all handled by the common
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