linux-kernel - [PATCH v2 4/4] Documentation: power/cpuidle: Document the CPU system-wakeup latency QoS

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Message-ID: <20251016151929.75863-5-ulf.hansson@linaro.org>
Date: Thu, 16 Oct 2025 17:19:24 +0200
From: Ulf Hansson <ulf.hansson@...aro.org>
To: "Rafael J . Wysocki" <rafael@...nel.org>,
	linux-pm@...r.kernel.org
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Kevin Hilman <khilman@...libre.com>,
	Pavel Machek <pavel@...nel.org>,
	Len Brown <len.brown@...el.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Maulik Shah <quic_mkshah@...cinc.com>,
	Prasad Sodagudi <psodagud@...cinc.com>,
	Dhruva Gole <d-gole@...com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	linux-kernel@...r.kernel.org,
	Jonathan Corbet <corbet@....net>
Subject: [PATCH v2 4/4] Documentation: power/cpuidle: Document the CPU system-wakeup latency QoS

Let's document how the new CPU system-wakeup latency QoS can be used from
user space, along with how the corresponding latency constraint gets
respected during s2idle.

Cc: Jonathan Corbet <corbet@....net>
Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
---

Changes in v2:
	- New patch.

---
 Documentation/admin-guide/pm/cpuidle.rst | 7 +++++++
 Documentation/power/pm_qos_interface.rst | 9 +++++----
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/pm/cpuidle.rst b/Documentation/admin-guide/pm/cpuidle.rst
index 0c090b076224..3f6f79a9bc8f 100644
--- a/Documentation/admin-guide/pm/cpuidle.rst
+++ b/Documentation/admin-guide/pm/cpuidle.rst
@@ -580,6 +580,13 @@ 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 applies to a running system, 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.
+
+Note that, in regards how to manage the QoS request from user space for the
+`cpu_wakeup_latency` file, it works according to the `cpu_dma_latency` file.
 
 Idle States Control Via Kernel Command Line
 ===========================================
diff --git a/Documentation/power/pm_qos_interface.rst b/Documentation/power/pm_qos_interface.rst
index 5019c79c7710..723f4714691a 100644
--- a/Documentation/power/pm_qos_interface.rst
+++ b/Documentation/power/pm_qos_interface.rst
@@ -55,7 +55,8 @@ 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
@@ -63,15 +64,15 @@ 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.
-- 
2.43.0