1e435256d6
This is primarily useful when there's a driver that doesn't claim clocks properly, but the bootloader leaves them on. It's not expected to be used in normal cases, but for bringup and debug it's very useful to have the option to not gate unclaimed clocks that are still on. Signed-off-by: Olof Johansson <olof@lixom.net> Signed-off-by: Mike Turquette <mturquette@linaro.org> [mturquette@linaro.org: fixed up trivial merge issue]
245 lines
8.9 KiB
Plaintext
245 lines
8.9 KiB
Plaintext
The Common Clk Framework
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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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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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gating, rate adjustment, muxing or other operations. This framework is
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enabled with the CONFIG_COMMON_CLK option.
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The interface itself is divided into two halves, each shielded from the
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details of its counterpart. First is the common definition of struct
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clk which unifies the framework-level accounting and infrastructure that
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has traditionally been duplicated across a variety of platforms. Second
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is a common implementation of the clk.h api, defined in
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drivers/clk/clk.c. Finally there is struct clk_ops, whose operations
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are invoked by the clk api implementation.
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The second half of the interface is comprised of the hardware-specific
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callbacks registered with struct clk_ops and the corresponding
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hardware-specific structures needed to model a particular clock. For
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the remainder of this document any reference to a callback in struct
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clk_ops, such as .enable or .set_rate, implies the hardware-specific
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implementation of that code. Likewise, references to struct clk_foo
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serve as a convenient shorthand for the implementation of the
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hardware-specific bits for the hypothetical "foo" hardware.
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Tying the two halves of this interface together is struct clk_hw, which
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is defined in struct clk_foo and pointed to within struct clk. This
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allows easy for 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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Below is the common struct clk definition from
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include/linux/clk-private.h, modified for brevity:
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struct clk {
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const char *name;
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const struct clk_ops *ops;
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struct clk_hw *hw;
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char **parent_names;
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struct clk **parents;
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struct clk *parent;
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struct hlist_head children;
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struct hlist_node child_node;
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...
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};
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The members above make up the core of the clk tree topology. The clk
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api itself defines several driver-facing functions which operate on
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struct clk. That api is documented in include/linux/clk.h.
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Platforms and devices utilizing the common struct clk use the struct
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clk_ops pointer in struct clk to perform the hardware-specific parts of
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the operations defined in clk.h:
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struct clk_ops {
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int (*prepare)(struct clk_hw *hw);
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void (*unprepare)(struct clk_hw *hw);
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int (*enable)(struct clk_hw *hw);
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void (*disable)(struct clk_hw *hw);
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int (*is_enabled)(struct clk_hw *hw);
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unsigned long (*recalc_rate)(struct clk_hw *hw,
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unsigned long parent_rate);
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long (*round_rate)(struct clk_hw *hw, unsigned long,
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unsigned long *);
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int (*set_parent)(struct clk_hw *hw, u8 index);
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u8 (*get_parent)(struct clk_hw *hw);
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int (*set_rate)(struct clk_hw *hw, unsigned long);
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void (*init)(struct clk_hw *hw);
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};
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Part 3 - hardware clk implementations
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The strength of the common struct clk 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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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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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.
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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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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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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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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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Note that to_clk_gate is defined as:
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#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk)
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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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When implementing support for a new type of clock it only necessary to
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include the following header:
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#include <linux/clk-provider.h>
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include/linux/clk.h is included within that header and clk-private.h
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must never be included from the code which implements the operations for
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a clock. More on that below in Part 5.
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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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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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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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#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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foo = to_clk_foo(hw);
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... perform magic on foo ...
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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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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 | | | |
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.set_rate | | y | | | |
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.set_parent | | | n | y | n |
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.get_parent | | | n | y | n |
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.init | | | | | |
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-----------------------------------------------------------
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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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clk_register(...)
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See the basic clock types in drivers/clk/clk-*.c for examples.
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Part 5 - static initialization of clock data
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For platforms with many clocks (often numbering into the hundreds) it
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may be desirable to statically initialize some clock data. This
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presents a problem since the definition of struct clk should be hidden
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from everyone except for the clock core in drivers/clk/clk.c.
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To get around this problem struct clk's definition is exposed in
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include/linux/clk-private.h along with some macros for more easily
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initializing instances of the basic clock types. These clocks must
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still be initialized with the common clock framework via a call to
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__clk_init.
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clk-private.h must NEVER be included by code which implements struct
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clk_ops callbacks, nor must it be included by any logic which pokes
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around inside of struct clk at run-time. To do so is a layering
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violation.
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To better enforce this policy, always follow this simple rule: any
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statically initialized clock data MUST be defined in a separate file
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from the logic that implements its ops. Basically separate the logic
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from the data and all is well.
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Part 6 - Disabling clock gating of unused clocks
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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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clocks properly but rely on them being on from the bootloader, bypassing
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the disabling means that the driver will remain functional while the issues
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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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