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This binding is a work-in-progress, and are based on some experimental
work by benh[1].
Sources of clock signal can be represented by any node in the device
tree. Those nodes are designated as clock providers. Clock consumer
nodes use a phandle and clock specifier pair to connect clock provider
outputs to clock inputs. Similar to the gpio specifiers, a clock
specifier is an array of zero, one or more cells identifying the clock
output on a device. The length of a clock specifier is defined by the
value of a #clock-cells property in the clock provider node.
[1] https://patchwork.ozlabs.org/patch/31551/
==Clock providers==
Required properties:
#clock-cells: Number of cells in a clock specifier; Typically 0 for nodes
with a single clock output and 1 for nodes with multiple
clock outputs.
Optional properties:
clock-output-names: Recommended to be a list of strings of clock output signal
names indexed by the first cell in the clock specifier.
However, the meaning of clock-output-names is domain
specific to the clock provider, and is only provided to
encourage using the same meaning for the majority of clock
providers. This format may not work for clock providers
using a complex clock specifier format. In those cases it
is recommended to omit this property and create a binding
specific names property.
Clock consumer nodes must never directly reference
the provider's clock-output-names property.
For example:
oscillator {
#clock-cells = <1>;
clock-output-names = "ckil", "ckih";
};
- this node defines a device with two clock outputs, the first named
"ckil" and the second named "ckih". Consumer nodes always reference
clocks by index. The names should reflect the clock output signal
names for the device.
clock-indices: If the identifying number for the clocks in the node
is not linear from zero, then this allows the mapping of
identifiers into the clock-output-names array.
For example, if we have two clocks <&oscillator 1> and <&oscillator 3>:
oscillator {
compatible = "myclocktype";
#clock-cells = <1>;
clock-indices = <1>, <3>;
clock-output-names = "clka", "clkb";
}
This ensures we do not have any empty strings in clock-output-names
==Clock consumers==
Required properties:
clocks: List of phandle and clock specifier pairs, one pair
for each clock input to the device. Note: if the
clock provider specifies '0' for #clock-cells, then
only the phandle portion of the pair will appear.
Optional properties:
clock-names: List of clock input name strings sorted in the same
order as the clocks property. Consumers drivers
will use clock-names to match clock input names
with clocks specifiers.
clock-ranges: Empty property indicating that child nodes can inherit named
clocks from this node. Useful for bus nodes to provide a
clock to their children.
For example:
device {
clocks = <&osc 1>, <&ref 0>;
clock-names = "baud", "register";
};
This represents a device with two clock inputs, named "baud" and "register".
The baud clock is connected to output 1 of the &osc device, and the register
clock is connected to output 0 of the &ref.
==Example==
/* external oscillator */
osc: oscillator {
compatible = "fixed-clock";
#clock-cells = <0>;
clock-frequency = <32678>;
clock-output-names = "osc";
};
/* phase-locked-loop device, generates a higher frequency clock
* from the external oscillator reference */
pll: pll@4c000 {
compatible = "vendor,some-pll-interface"
#clock-cells = <1>;
clocks = <&osc 0>;
clock-names = "ref";
reg = <0x4c000 0x1000>;
clock-output-names = "pll", "pll-switched";
};
/* UART, using the low frequency oscillator for the baud clock,
* and the high frequency switched PLL output for register
* clocking */
uart@a000 {
compatible = "fsl,imx-uart";
reg = <0xa000 0x1000>;
interrupts = <33>;
clocks = <&osc 0>, <&pll 1>;
clock-names = "baud", "register";
};
This DT fragment defines three devices: an external oscillator to provide a
low-frequency reference clock, a PLL device to generate a higher frequency
clock signal, and a UART.
* The oscillator is fixed-frequency, and provides one clock output, named "osc".
* The PLL is both a clock provider and a clock consumer. It uses the clock
signal generated by the external oscillator, and provides two output signals
("pll" and "pll-switched").
* The UART has its baud clock connected the external oscillator and its
register clock connected to the PLL clock (the "pll-switched" signal)
==Assigned clock parents and rates==
Some platforms may require initial configuration of default parent clocks
and clock frequencies. Such a configuration can be specified in a device tree
node through assigned-clocks, assigned-clock-parents and assigned-clock-rates
properties. The assigned-clock-parents property should contain a list of parent
clocks in the form of a phandle and clock specifier pair and the
assigned-clock-rates property should contain a list of frequencies in Hz. Both
these properties should correspond to the clocks listed in the assigned-clocks
property.
To skip setting parent or rate of a clock its corresponding entry should be
set to 0, or can be omitted if it is not followed by any non-zero entry.
uart@a000 {
compatible = "fsl,imx-uart";
reg = <0xa000 0x1000>;
...
clocks = <&osc 0>, <&pll 1>;
clock-names = "baud", "register";
assigned-clocks = <&clkcon 0>, <&pll 2>;
assigned-clock-parents = <&pll 2>;
assigned-clock-rates = <0>, <460800>;
};
In this example the <&pll 2> clock is set as parent of clock <&clkcon 0> and
the <&pll 2> clock is assigned a frequency value of 460800 Hz.
Configuring a clock's parent and rate through the device node that consumes
the clock can be done only for clocks that have a single user. Specifying
conflicting parent or rate configuration in multiple consumer nodes for
a shared clock is forbidden.
Configuration of common clocks, which affect multiple consumer devices can
be similarly specified in the clock provider node.
==Protected clocks==
Some platforms or firmwares may not fully expose all the clocks to the OS, such
as in situations where those clks are used by drivers running in ARM secure
execution levels. Such a configuration can be specified in device tree with the
protected-clocks property in the form of a clock specifier list. This property should
only be specified in the node that is providing the clocks being protected:
clock-controller@a000f000 {
compatible = "vendor,clk95;
reg = <0xa000f000 0x1000>
#clocks-cells = <1>;
...
protected-clocks = <UART3_CLK>, <SPI5_CLK>;
};
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