applications: nrf_desktop: Require zero latency in PM while USB active

Require zero latency in Power Management while USB is active to ensure
high performance. While USB is active, application power down is blocked
anyway.

Jira: NCSDK-30503

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>
Signed-off-by: Pekka Niskanen <pekka.niskanen@nordicsemi.no>

Author: MarekPieta

applications/nrf_desktop/doc/usb_state_pm.rst

@@ -7,8 +7,11 @@ USB state power manager module
    :local:
    :depth: 2
 
-The |usb_state_pm| is minor, stateless module that imposes an application power level restriction related to the USB state.
-The application power level is managed by the :ref:`power manager module <caf_power_manager>`.
+The |usb_state_pm| is a minor, stateless module that imposes the following power state restrictions related to the USB state:
+
+* Application power level restrictions.
+  The application power level is managed by the :ref:`caf_power_manager`.
+* Zephyr's :ref:`zephyr:pm-system` latency restrictions.
 
 Module events
 *************
@@ -24,20 +27,41 @@ Configuration
 *************
 
 To enable the module, use the :ref:`CONFIG_DESKTOP_USB_PM_ENABLE <config_desktop_app_options>` Kconfig option.
-It depends on the options :ref:`CONFIG_DESKTOP_USB_ENABLE <config_desktop_app_options>` and :kconfig:option:`CONFIG_CAF_POWER_MANAGER`.
+It depends on the options :ref:`CONFIG_DESKTOP_USB_ENABLE <config_desktop_app_options>` and :kconfig:option:`CONFIG_CAF_PM_EVENTS`.
 
 The log level is inherited from the :ref:`nrf_desktop_usb_state`.
 
+System Power Management integration
+===================================
+
+Zephyr's System Power Management (:kconfig:option:`CONFIG_PM`) does not automatically take into account (expect) wakeups related to user input and finalized HID report transfers over USB.
+This results in entering low power states if no work is scheduled to be done in the nearest future.
+If you use Zephyr's System Power Management, the module automatically requires zero latency in the Power Management while USB is active.
+This is done to prevent entering power states that introduce wakeup latency and ensure high performance.
+You can control this feature using the :ref:`CONFIG_DESKTOP_USB_PM_REQ_NO_PM_LATENCY <config_desktop_app_options>` Kconfig option.
+
 Implementation details
 **********************
 
-For the change of the restricted power level, the module reacts to :c:struct:`usb_state_event`.
-Upon reception of the event and depending on the current USB state, the module requests different power restrictions:
+The module reacts to the :c:struct:`usb_state_event`.
+Upon reception of the event and depending on the current USB state, the module requests different power restrictions.
+For more information about the USB states in nRF Desktop, see the :ref:`nrf_desktop_usb_state`.
+
+Application power level
+=======================
+
+The application power level is imposed using the :c:struct:`power_manager_restrict_event`.
 
 * If the USB state is set to :c:enum:`USB_STATE_POWERED`, the module restricts the power down level to the :c:enum:`POWER_MANAGER_LEVEL_SUSPENDED`.
 * If the USB state is set to :c:enum:`USB_STATE_ACTIVE`, the :c:enum:`POWER_MANAGER_LEVEL_ALIVE` is required.
 * If the USB state is set to :c:enum:`USB_STATE_DISCONNECTED`, any power level is allowed.
 * If the USB state is set to :c:enum:`USB_STATE_SUSPENDED`, the :c:enum:`POWER_MANAGER_LEVEL_SUSPENDED` is imposed.
-  The module restricts the power down level to the :c:enum:`POWER_MANAGER_LEVEL_SUSPENDED` and generates :c:struct:`force_power_down_event`.
+  The module restricts the power down level to the :c:enum:`POWER_MANAGER_LEVEL_SUSPENDED`.
+  The module also submits a :c:struct:`force_power_down_event` to force a quick power down.
 
-For more information about the USB states in nRF Desktop, see the :ref:`nrf_desktop_usb_state`.
+System Power Management latency
+===============================
+
+The latency requirements of the System Power Management are updated using the :c:func:`pm_policy_latency_request_add` and :c:func:`pm_policy_latency_request_remove` functions.
+The zero latency requirement is added when USB state is set to :c:enum:`USB_STATE_ACTIVE`.
+The requirement is removed if USB enters another state.

applications/nrf_desktop/src/modules/Kconfig.usb_state

@@ -47,6 +47,16 @@ config DESKTOP_USB_PM_ENABLE
 	help
 	  This enables small module that blocks power down if the USB is active.
 
+config DESKTOP_USB_PM_REQ_NO_PM_LATENCY
+	bool "Require zero latency in Power Management while USB is active"
+	default y
+	depends on PM
+	depends on DESKTOP_USB_PM_ENABLE
+	help
+	  The zero latency prevents entering power states that introduce wakeup
+	  latency and ensures high performance. While USB is active, application
+	  power down is blocked anyway.
+
 config DESKTOP_USB_HID_REPORT_SENT_ON_SOF
 	bool "Submit HID report sent event on USB Start of Frame (SOF) [EXPERIMENTAL]"
 	select EXPERIMENTAL

applications/nrf_desktop/src/modules/usb_state_pm.c

@@ -5,6 +5,7 @@
  */
 #include <zephyr/kernel.h>
 #include <zephyr/types.h>
+#include <zephyr/pm/policy.h>
 
 #include "usb_event.h"
 
@@ -18,13 +19,37 @@ LOG_MODULE_REGISTER(MODULE, CONFIG_DESKTOP_USB_STATE_LOG_LEVEL);
 #include <caf/events/force_power_down_event.h>
 
 
+static void update_pm_policy_latency_req(bool enable)
+{
+	static bool enabled;
+	static struct pm_policy_latency_request req;
+
+	if (enabled == enable) {
+		/* Nothing to do. */
+		return;
+	}
+
+	if (enable) {
+		/* Ensure no latency. */
+		pm_policy_latency_request_add(&req, 0U);
+	} else {
+		pm_policy_latency_request_remove(&req);
+	}
+
+	enabled = enable;
+}
+
 static bool app_event_handler(const struct app_event_header *aeh)
 {
 	if (is_usb_state_event(aeh)) {
 		const struct usb_state_event *event = cast_usb_state_event(aeh);
 
 		LOG_DBG("USB state change detected");
 
+		if (IS_ENABLED(CONFIG_DESKTOP_USB_PM_REQ_NO_PM_LATENCY)) {
+			update_pm_policy_latency_req(event->state == USB_STATE_ACTIVE);
+		}
+
 		switch (event->state) {
 		case USB_STATE_POWERED:
 			power_manager_restrict(MODULE_IDX(MODULE), POWER_MANAGER_LEVEL_SUSPENDED);

doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst

@@ -287,6 +287,8 @@ nRF Desktop
     For details, see the :ref:`nrf_desktop_board_configuration`.
   * The ``dongle_small`` configuration for the nRF52833 DK.
     The configuration enables logs and mimics the dongle configuration used for small SoCs.
+  * Requirement for zero latency in Zephyr's :ref:`zephyr:pm-system` while USB is active (:ref:`CONFIG_DESKTOP_USB_PM_REQ_NO_PM_LATENCY <config_desktop_app_options>` Kconfig option of the :ref:`nrf_desktop_usb_state_pm`).
+    The feature is enabled by default if Zephyr Power Management (:kconfig:option:`CONFIG_PM`) is enabled to prevent entering power states that introduce wakeup latency and ensure high performance.
 
 * Removed an imply from the nRF Desktop Bluetooth connectivity Kconfig option (:ref:`CONFIG_DESKTOP_BT <config_desktop_app_options>`).
   The imply enabled a separate workqueue for connection TX notify processing (:kconfig:option:`CONFIG_BT_CONN_TX_NOTIFY_WQ`) if MPSL was used for synchronization between the flash memory driver and radio (:kconfig:option:`CONFIG_SOC_FLASH_NRF_RADIO_SYNC_MPSL`).