s390/idle: Introduce cpuidle for s390

Introduce generic cpuidle driver on s390. Use a two stage approach to
handle idle scenarios and use idle governor for idle stage selection.
Two stages are, from shallow to deep, idle polling and enabled wait.

Suggested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Mete Durlu <meted@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Acked-by: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

MAINTAINERS

@@ -6851,6 +6851,13 @@ L:	linux-riscv@lists.infradead.org
 S:	Maintained
 F:	drivers/cpuidle/cpuidle-riscv-sbi.c
 
+CPUIDLE DRIVER - S390
+M:	Mete Durlu <meted@linux.ibm.com>
+L:	linux-pm@vger.kernel.org
+L:	linux-s390@vger.kernel.org
+S:	Maintained
+F:	drivers/cpuidle/cpuidle-s390.c
+
 CPUMASK API [RUST]
 M:	Viresh Kumar <viresh.kumar@linaro.org>
 R:	Yury Norov <yury.norov@gmail.com>
@@ -23790,6 +23797,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux.git
 F:	Documentation/driver-api/s390-drivers.rst
 F:	Documentation/arch/s390/
 F:	arch/s390/
+F:	drivers/cpuidle/cpuidle-s390.c
 F:	drivers/s390/
 F:	drivers/watchdog/diag288_wdt.c
 

arch/s390/Kconfig

@@ -20,6 +20,9 @@ config ARCH_HAS_ILOG2_U64
 config ARCH_PROC_KCORE_TEXT
 	def_bool y
 
+config ARCH_HAS_CPU_RELAX
+	def_bool y
+
 config GENERIC_HWEIGHT
 	def_bool !HAVE_MARCH_Z196_FEATURES
 
@@ -705,6 +708,8 @@ config KERNEL_IMAGE_BASE
 
 endmenu
 
+source "drivers/cpuidle/Kconfig"
+
 menu "Memory setup"
 
 config ARCH_SPARSEMEM_ENABLE

drivers/cpuidle/Kconfig

@@ -71,6 +71,11 @@ depends on RISCV
 source "drivers/cpuidle/Kconfig.riscv"
 endmenu
 
+menu "S390 CPU Idle Drivers"
+depends on S390
+source "drivers/cpuidle/Kconfig.s390"
+endmenu
+
 config HALTPOLL_CPUIDLE
 	tristate "Halt poll cpuidle driver"
 	depends on X86 && KVM_GUEST

drivers/cpuidle/Kconfig.s390

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# S390 CPU Idle drivers
+#
+
+config S390_CPUIDLE
+	bool "S390 CPU idle driver"
+	default y
+	help
+	  Select this option to enable processor idle state management
+	  through cpuidle subsystem.

drivers/cpuidle/Makefile

@@ -42,3 +42,7 @@ obj-$(CONFIG_POWERNV_CPUIDLE)		+= cpuidle-powernv.o
 ###############################################################################
 # RISC-V drivers
 obj-$(CONFIG_RISCV_SBI_CPUIDLE)		+= cpuidle-riscv-sbi.o
+
+###############################################################################
+# S390 drivers
+obj-$(CONFIG_S390_CPUIDLE)		+= cpuidle-s390.o

drivers/cpuidle/cpuidle-s390.c

@@ -0,0 +1,115 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * s390 generic CPU idle driver.
+ *
+ * Copyright IBM Corp. 2026
+ */
+
+#define pr_fmt(fmt) "CPUidle s390: " fmt
+
+#include <linux/init.h>
+#include <linux/cpuidle.h>
+#include <linux/cpu.h>
+#include <linux/sched/clock.h>
+
+static __cpuidle int s390_enter_idle(struct cpuidle_device *dev,
+				     struct cpuidle_driver *drv,
+				     int index)
+{
+	arch_cpu_idle();
+	return index;
+}
+
+static struct cpuidle_driver s390_cpuidle_driver = {
+	.cpumask = (struct cpumask *)cpu_present_mask,
+	.name = "s390-idle",
+	.states = {
+		{ /* entry 0 is for polling */},
+		{
+			.enter			= s390_enter_idle,
+			.name			= "IDLE",
+			.desc			= "ENABLED WAIT",
+		},
+	},
+	.safe_state_index = 0,
+	.state_count = 2,
+};
+
+static int s390_cpuidle_cpu_online(unsigned int cpu)
+{
+	struct cpuidle_device *dev = &per_cpu(cpuidle_dev, cpu);
+	int rc;
+
+	if (dev->registered) {
+		cpuidle_pause_and_lock();
+		rc = cpuidle_enable_device(dev);
+		cpuidle_resume_and_unlock();
+		if (rc)
+			pr_err("Failed to enable cpuidle device on cpu %u\n", cpu);
+	} else {
+		dev->cpu = cpu;
+		rc = cpuidle_register_device(dev);
+		if (rc)
+			pr_err("Failed to register cpuidle driver on cpu %u\n", cpu);
+	}
+	return rc;
+}
+
+static int s390_cpuidle_cpu_dead(unsigned int cpu)
+{
+	struct cpuidle_device *dev = &per_cpu(cpuidle_dev, cpu);
+
+	if (!dev->registered)
+		return 0;
+	cpuidle_pause_and_lock();
+	cpuidle_disable_device(dev);
+	cpuidle_resume_and_unlock();
+	return 0;
+}
+
+/*
+ * The target_residency and exit_latency values are benchmark-derived estimates
+ * that remain non-deterministic due to s390's virtualized architecture.
+ *
+ * Configuration strategy:
+ * - Poll idle state: Values derived from the next enabled idle state (EW)
+ * - Enabled Wait state: Values selected based on idle behavior and empirical
+ *   measurement data
+ *
+ * Goal is to improve responsiveness for workloads with frequent sleep/wakeup
+ * cycles while minimizing any side effects.
+ */
+static void __init s390_cpuidle_ew_tune(void)
+{
+	struct cpuidle_state *state = &s390_cpuidle_driver.states[1];
+
+	if (machine_is_lpar()) {
+		state->target_residency = 5;
+		state->exit_latency = 5;
+	} else {
+		state->target_residency = 1;
+		state->exit_latency = 1;
+	}
+}
+
+static int __init s390_cpuidle_init(void)
+{
+	int rc;
+
+	s390_cpuidle_ew_tune();
+	cpuidle_poll_state_init(&s390_cpuidle_driver);
+	rc = cpuidle_register(&s390_cpuidle_driver, NULL);
+	if (rc)
+		return rc;
+	rc = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
+				       "cpuidle/s390:online",
+				       s390_cpuidle_cpu_online,
+				       s390_cpuidle_cpu_dead);
+	if (rc < 0) {
+		cpuidle_unregister(&s390_cpuidle_driver);
+		pr_err("Failed to allocate hotplug state: cpuidle/s390:online\n");
+		return rc;
+	}
+	return 0;
+}
+device_initcall(s390_cpuidle_init);