linux/Documentation/driver-api/clk.rst
Linus Torvalds 6419945e33 This time we have a good set of changes to the core framework that do some
general cleanups, but nothing too major. The majority of the diff goes to
 two SoCs, Actions Semi and Qualcomm. A brand new driver is introduced for
 Actions Semi so it takes up some lines to add all the different types, and
 the Qualcomm diff is there because we add support for two SoCs and it's quite
 a bit of data.
 
 Otherwise the big driver updates are on TI Davinci and Amlogic platforms. And
 then the long tail of driver updates for various fixes and stuff follows
 after that.
 
 Core:
  - debugfs cleanups removing error checking and an unused provider API
  - Removal of a clk init typedef that isn't used
  - Usage of match_string() to simplify parent string name matching
  - OF clk helpers moved to their own file (linux/of_clk.h)
  - Make clk warnings more readable across kernel versions
 
 New Drivers:
  - Qualcomm SDM845 GCC and Video clk controllers
  - Qualcomm MSM8998 GCC
  - Actions Semi S900 SoC support
  - Nuvoton npcm750 microcontroller clks
  - Amlogic axg AO clock controller
 
 Removed Drivers:
  - Deprecated Rockchip clk-gate driver
 
 Updates:
  - debugfs functions stopped checking return values
  - Support for the MSIOF module clocks on Rensas R-Car M3-N
  - Support for the new Rensas RZ/G1C and R-Car E3 SoCs
  - Qualcomm GDSC, RCG, and PLL updates for clk changes in new SoCs
  - Berlin and Amlogic SPDX tagging
  - Usage of of_clk_get_parent_count() in more places
  - Proper implementation of the CDEV1/2 clocks on Tegra20
  - Allwinner H6 PRCM clock support and R40 EMAC support
  - Add critical flag to meson8b's fdiv2 as temporary fixup for ethernet
  - Round closest support for meson's mpll driver
  - Support for meson8b nand clocks and gxbb video decoder clocks
  - Mediatek mali clks
  - STM32MP1 fixes
  - Uniphier LD11/LD20 stream demux system clock
 -----BEGIN PGP SIGNATURE-----
 
 iQIzBAABCgAdFiEE9L57QeeUxqYDyoaDrQKIl8bklSUFAlsWxugACgkQrQKIl8bk
 lSVs2A/9HOMsWeiYx1MESrXw6N2UknWeqeT/b1v8L/VOiptJg+OTExPbzmSylngv
 AXJAfIkCpguSMh9b310pA3DAzk5docmbQ4zL977yY+KXmOcDooCd34aG5a+tB3ie
 ugC8T2bQLrJdMp3hsqaKZsYzqe7LoW2NJgoliXDMA/QUBLpvHq+fcu2zOawingTA
 GNc3LGqP5Op7p09aPK30gtQNqLK5qGpHASa/AY7Y0PXlUeTZ8rmF06fcEAg5shkC
 CT57Zy2rSFB2RorEJarYXDPLRHMw/jxXtpMVXEy7zuz/3ajvvRiZDHv75+NaBru9
 hDt1rzslzexEN4fYzj4AtGYRKyBrHbDaxG1qdIWPWVyoE0CEb+dZ1gH7/Ski5r+s
 z5D28NogC0T0sey6yWssyG3RLvkPJ5nxUhL++siHm1lbyo16LmhB1+nFvxrlzmBB
 0V1xqEa7feYpD+JD66lJFb5ornHLwGtVYBpeiY+hrDR3ddWEe1IxaYGR2p9nHwSS
 Us/ZQdHIYBVEqoo3+BWnTn+HSQzmd/sqHqWnLlVWUHoomm5nXx18PeS87vFbcPv9
 dMr+FFJ3Elubzcy5UZJPfNw+pb+teE7tYGQkQ3nbLRxT1YZOoIJZJDqNKxM1cgne
 6c/VXJMEyBBn/w7Iru/3eWCZVQJGlmYS47DFDzduFvd3LMfmKIM=
 =KK/v
 -----END PGP SIGNATURE-----

Merge tag 'clk-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/clk/linux

Pull clk updates from Stephen Boyd:
 "This time we have a good set of changes to the core framework that do
  some general cleanups, but nothing too major. The majority of the diff
  goes to two SoCs, Actions Semi and Qualcomm. A brand new driver is
  introduced for Actions Semi so it takes up some lines to add all the
  different types, and the Qualcomm diff is there because we add support
  for two SoCs and it's quite a bit of data.

  Otherwise the big driver updates are on TI Davinci and Amlogic
  platforms. And then the long tail of driver updates for various fixes
  and stuff follows after that.

  Core:
   - debugfs cleanups removing error checking and an unused provider API
   - Removal of a clk init typedef that isn't used
   - Usage of match_string() to simplify parent string name matching
   - OF clk helpers moved to their own file (linux/of_clk.h)
   - Make clk warnings more readable across kernel versions

  New Drivers:
   - Qualcomm SDM845 GCC and Video clk controllers
   - Qualcomm MSM8998 GCC
   - Actions Semi S900 SoC support
   - Nuvoton npcm750 microcontroller clks
   - Amlogic axg AO clock controller

  Removed Drivers:
   - Deprecated Rockchip clk-gate driver

  Updates:
   - debugfs functions stopped checking return values
   - Support for the MSIOF module clocks on Rensas R-Car M3-N
   - Support for the new Rensas RZ/G1C and R-Car E3 SoCs
   - Qualcomm GDSC, RCG, and PLL updates for clk changes in new SoCs
   - Berlin and Amlogic SPDX tagging
   - Usage of of_clk_get_parent_count() in more places
   - Proper implementation of the CDEV1/2 clocks on Tegra20
   - Allwinner H6 PRCM clock support and R40 EMAC support
   - Add critical flag to meson8b's fdiv2 as temporary fixup for ethernet
   - Round closest support for meson's mpll driver
   - Support for meson8b nand clocks and gxbb video decoder clocks
   - Mediatek mali clks
   - STM32MP1 fixes
   - Uniphier LD11/LD20 stream demux system clock"

* tag 'clk-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/clk/linux: (134 commits)
  clk: qcom: Export clk_fabia_pll_configure()
  clk: bcm: Update and add Stingray clock entries
  dt-bindings: clk: Update Stingray binding doc
  clk-si544: Properly round requested frequency to nearest match
  clk: ingenic: jz4770: Add 150us delay after enabling VPU clock
  clk: ingenic: jz4770: Enable power of AHB1 bus after ungating VPU clock
  clk: ingenic: jz4770: Modify C1CLK clock to disable CPU clock stop on idle
  clk: ingenic: jz4770: Change OTG from custom to standard gated clock
  clk: ingenic: Support specifying "wait for clock stable" delay
  clk: ingenic: Add support for clocks whose gate bit is inverted
  clk: use match_string() helper
  clk: bcm2835: use match_string() helper
  clk: Return void from debug_init op
  clk: remove clk_debugfs_add_file()
  clk: tegra: no need to check return value of debugfs_create functions
  clk: davinci: no need to check return value of debugfs_create functions
  clk: bcm2835: no need to check return value of debugfs_create functions
  clk: no need to check return value of debugfs_create functions
  clk: imx6: add EPIT clock support
  clk: mvebu: use correct bit for 98DX3236 NAND
  ...
2018-06-09 12:06:24 -07:00

308 lines
12 KiB
ReStructuredText

========================
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.
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_core. This
allows for easy navigation between the two discrete halves of the common
clock interface.
Common data structures and api
==============================
Below is the common struct clk_core definition from
drivers/clk/clk.c, modified for brevity::
struct clk_core {
const char *name;
const struct clk_ops *ops;
struct clk_hw *hw;
struct module *owner;
struct clk_core *parent;
const char **parent_names;
struct clk_core **parents;
u8 num_parents;
u8 new_parent_index;
...
};
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_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::
struct clk_ops {
int (*prepare)(struct clk_hw *hw);
void (*unprepare)(struct clk_hw *hw);
int (*is_prepared)(struct clk_hw *hw);
void (*unprepare_unused)(struct clk_hw *hw);
int (*enable)(struct clk_hw *hw);
void (*disable)(struct clk_hw *hw);
int (*is_enabled)(struct clk_hw *hw);
void (*disable_unused)(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 rate,
unsigned long *parent_rate);
int (*determine_rate)(struct clk_hw *hw,
struct clk_rate_request *req);
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 rate,
unsigned long parent_rate);
int (*set_rate_and_parent)(struct clk_hw *hw,
unsigned long rate,
unsigned long parent_rate,
u8 index);
unsigned long (*recalc_accuracy)(struct clk_hw *hw,
unsigned long parent_accuracy);
int (*get_phase)(struct clk_hw *hw);
int (*set_phase)(struct clk_hw *hw, int degrees);
void (*init)(struct clk_hw *hw);
void (*debug_init)(struct clk_hw *hw,
struct dentry *dentry);
};
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::
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
framework code and struct clk_core.
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::
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::
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);
}
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.
Supporting your own clk hardware
================================
When implementing support for a new type of clock it is only necessary to
include the following header::
#include <linux/clk-provider.h>
To construct a clk hardware structure for your platform you must define
the following::
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::
struct clk_ops clk_foo_ops {
.enable = &clk_foo_enable;
.disable = &clk_foo_disable;
};
Implement the above functions using container_of::
#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw)
int clk_foo_enable(struct clk_hw *hw)
{
struct clk_foo *foo;
foo = to_clk_foo(hw);
... perform magic on foo ...
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
mandatory, a cell marked as "n" implies that either including that
callback is invalid or otherwise unnecessary. Empty cells are either
optional or must be evaluated on a case-by-case basis.
.. 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::
clk_register(...)
See the basic clock types in ``drivers/clk/clk-*.c`` for examples.
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
clocks properly but rely on them being on from the bootloader, bypassing
the disabling means that the driver will remain functional while the issues
are sorted out.
To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
kernel.
Locking
=======
The common clock framework uses two global locks, the prepare lock and the
enable lock.
The enable lock is a spinlock and is held across calls to the .enable,
.disable operations. Those operations are thus not allowed to sleep,
and calls to the clk_enable(), clk_disable() API functions are allowed in
atomic context.
For clk_is_enabled() API, it is also designed to be allowed to be used in
atomic context. However, it doesn't really make any sense to hold the enable
lock in core, unless you want to do something else with the information of
the enable state with that lock held. Otherwise, seeing if a clk is enabled is
a one-shot read of the enabled state, which could just as easily change after
the function returns because the lock is released. Thus the user of this API
needs to handle synchronizing the read of the state with whatever they're
using it for to make sure that the enable state doesn't change during that
time.
The prepare lock is a mutex and is held across calls to all other operations.
All those operations are allowed to sleep, and calls to the corresponding API
functions are not allowed in atomic context.
This effectively divides operations in two groups from a locking perspective.
Drivers don't need to manually protect resources shared between the operations
of one group, regardless of whether those resources are shared by multiple
clocks or not. However, access to resources that are shared between operations
of the two groups needs to be protected by the drivers. An example of such a
resource would be a register that controls both the clock rate and the clock
enable/disable state.
The clock framework is reentrant, in that a driver is allowed to call clock
framework functions from within its implementation of clock operations. This
can for instance cause a .set_rate operation of one clock being called from
within the .set_rate operation of another clock. This case must be considered
in the driver implementations, but the code flow is usually controlled by the
driver in that case.
Note that locking must also be considered when code outside of the common
clock framework needs to access resources used by the clock operations. This
is considered out of scope of this document.