Clocks, resets, and power

Device-tree references connect S31 peripherals to the Linux clock, reset and power-domain providers. Clock tree describes the registered clock relationships; Power domains describes the domain policies.

Acquire and release driver resources

Use the provider APIs for the resources declared by a device’s binding. Preserve acquisition errors, including -EPROBE_DEFER, so probe can be retried when a required provider becomes available.

API	Role
`devm_clk_get()`	Acquire a clock reference; does not enable it
`clk_prepare_enable()`	Prepare and enable the acquired clock
`clk_get_rate()`	Read the clock rate used to calculate peripheral timing
`clk_disable_unprepare()`	Balance a successful prepare/enable
`devm_reset_control_get_optional_exclusive()`	Acquire an optional reset control; an absent optional control may be `NULL`
`reset_control_reset()`	Request a reset pulse through that control

The S31 I2C probe is a concrete example: map registers, acquire and enable the clock, register clk_disable_unprepare() with devm_add_action_or_reset(), acquire and pulse the optional reset, then configure the controller and request its IRQ. Its error path preserves provider errors. This ordering belongs to that controller; follow the binding and hardware requirements for another device. See I2C probe and cleanup.

Before releasing a clock or buffer, stop the device’s transfers and interrupt activity. DMA cleanup has additional synchronization requirements described in DMA and cache coherency. Do not bypass a provider with direct clock or PMU register writes from a client driver.

Inspect provider state

for file in /sys/bus/platform/devices/*/clocks; do
    [ -r "$file" ] && cat "$file"
done
for file in /sys/bus/platform/devices/*/domains; do
    [ -r "$file" ] && cat "$file"
done

Each clocks line has these fields:

Field	Meaning
Leading integer	Clock ID from the S31 clock binding
Name	Registered CCF clock name
`state=on` / `state=off`	Result of `clk_hw_is_prepared()`; CCF preparation state
`critical=0` / `critical=1`	Whether CCF marks the clock critical
`rate=`	Rate reported by the provider, in Hz

state=on does not independently read back an electrical clock signal or prove that a peripheral is operational. Interpret it together with the rate, probe messages and device state. The precise output comes from clocks_show().

The domains output separates policy, software state and force-register settings; see its field definitions. For CPU frequency and system sleep controls, see Power management.