Documentation: power/cpuidle: Document the CPU system wakeup latency QoS

Let's document how the new CPU system wakeup latency QoS limit can be used
from user space, along with how the constraint is taken into account for
s2idle and cpuidle.

Reviewed-by: Dhruva Gole <d-gole@ti.com>
Reviewed-by: Kevin Hilman (TI) <khilman@baylibre.com>
Tested-by: Kevin Hilman (TI) <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/20251125112650.329269-7-ulf.hansson@linaro.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: storulf

Documentation/admin-guide/pm/cpuidle.rst

@@ -580,6 +580,15 @@ the given CPU as the upper limit for the exit latency of the idle states that
 they are allowed to select for that CPU.  They should never select any idle
 states with exit latency beyond that limit.
 
+While the above CPU QoS constraints apply to CPU idle time management, user
+space may also request a CPU system wakeup latency QoS limit, via the
+`cpu_wakeup_latency` file.  This QoS constraint is respected when selecting a
+suitable idle state for the CPUs, while entering the system-wide suspend-to-idle
+sleep state, but also to the regular CPU idle time management.
+
+Note that, the management of the `cpu_wakeup_latency` file works according to
+the 'cpu_dma_latency' file from user space point of view.  Moreover, the unit
+is also microseconds.
 
 Idle States Control Via Kernel Command Line
 ===========================================

Documentation/power/pm_qos_interface.rst

@@ -55,23 +55,24 @@ int cpu_latency_qos_request_active(handle):
 
 From user space:
 
-The infrastructure exposes one device node, /dev/cpu_dma_latency, for the CPU
+The infrastructure exposes two separate device nodes, /dev/cpu_dma_latency for
+the CPU latency QoS and /dev/cpu_wakeup_latency for the CPU system wakeup
 latency QoS.
 
 Only processes can register a PM QoS request.  To provide for automatic
 cleanup of a process, the interface requires the process to register its
 parameter requests as follows.
 
 To register the default PM QoS target for the CPU latency QoS, the process must
-open /dev/cpu_dma_latency.
+open /dev/cpu_dma_latency.  To register a CPU system wakeup QoS limit, the
+process must open /dev/cpu_wakeup_latency.
 
 As long as the device node is held open that process has a registered
 request on the parameter.
 
 To change the requested target value, the process needs to write an s32 value to
 the open device node.  Alternatively, it can write a hex string for the value
-using the 10 char long format e.g. "0x12345678".  This translates to a
-cpu_latency_qos_update_request() call.
+using the 10 char long format e.g. "0x12345678".
 
 To remove the user mode request for a target value simply close the device
 node.