PM: QoS: Introduce a CPU system wakeup QoS limit

Some platforms supports multiple low power states for CPUs that can be used
when entering system-wide suspend. Currently we are always selecting the
deepest possible state for the CPUs, which can break the system wakeup
latency constraint that may be required for a use case.

Let's take the first step towards addressing this problem, by introducing
an interface for user space, that allows us to specify the CPU system
wakeup QoS limit. Subsequent changes will start taking into account the new
QoS limit.

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/20251125112650.329269-2-ulf.hansson@linaro.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/linux/pm_qos.h

@@ -162,6 +162,15 @@ static inline void cpu_latency_qos_update_request(struct pm_qos_request *req,
 static inline void cpu_latency_qos_remove_request(struct pm_qos_request *req) {}
 #endif
 
+#ifdef CONFIG_PM_QOS_CPU_SYSTEM_WAKEUP
+s32 cpu_wakeup_latency_qos_limit(void);
+#else
+static inline s32 cpu_wakeup_latency_qos_limit(void)
+{
+	return PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
+}
+#endif
+
 #ifdef CONFIG_PM
 enum pm_qos_flags_status __dev_pm_qos_flags(struct device *dev, s32 mask);
 enum pm_qos_flags_status dev_pm_qos_flags(struct device *dev, s32 mask);

kernel/power/Kconfig

@@ -202,6 +202,17 @@ config PM_WAKELOCKS_GC
 	depends on PM_WAKELOCKS
 	default y
 
+config PM_QOS_CPU_SYSTEM_WAKEUP
+	bool "User space interface for CPU system wakeup QoS"
+	depends on CPU_IDLE
+	help
+	  Enable this to allow user space via the cpu_wakeup_latency file to
+	  specify a CPU system wakeup latency limit.
+
+	  This may be particularly useful for platforms supporting multiple low
+	  power states for CPUs during system-wide suspend and s2idle in
+	  particular.
+
 config PM
 	bool "Device power management core functionality"
 	help

kernel/power/qos.c

@@ -415,6 +415,105 @@ static struct miscdevice cpu_latency_qos_miscdev = {
 	.fops = &cpu_latency_qos_fops,
 };
 
+#ifdef CONFIG_PM_QOS_CPU_SYSTEM_WAKEUP
+/* The CPU system wakeup latency QoS. */
+static struct pm_qos_constraints cpu_wakeup_latency_constraints = {
+	.list = PLIST_HEAD_INIT(cpu_wakeup_latency_constraints.list),
+	.target_value = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT,
+	.default_value = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT,
+	.no_constraint_value = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT,
+	.type = PM_QOS_MIN,
+};
+
+/**
+ * cpu_wakeup_latency_qos_limit - Current CPU system wakeup latency QoS limit.
+ *
+ * Returns the current CPU system wakeup latency QoS limit that may have been
+ * requested by user space.
+ */
+s32 cpu_wakeup_latency_qos_limit(void)
+{
+	return pm_qos_read_value(&cpu_wakeup_latency_constraints);
+}
+
+static int cpu_wakeup_latency_qos_open(struct inode *inode, struct file *filp)
+{
+	struct pm_qos_request *req;
+
+	req = kzalloc(sizeof(*req), GFP_KERNEL);
+	if (!req)
+		return -ENOMEM;
+
+	req->qos = &cpu_wakeup_latency_constraints;
+	pm_qos_update_target(req->qos, &req->node, PM_QOS_ADD_REQ,
+			     PM_QOS_RESUME_LATENCY_NO_CONSTRAINT);
+	filp->private_data = req;
+
+	return 0;
+}
+
+static int cpu_wakeup_latency_qos_release(struct inode *inode,
+					  struct file *filp)
+{
+	struct pm_qos_request *req = filp->private_data;
+
+	filp->private_data = NULL;
+	pm_qos_update_target(req->qos, &req->node, PM_QOS_REMOVE_REQ,
+			     PM_QOS_RESUME_LATENCY_NO_CONSTRAINT);
+	kfree(req);
+
+	return 0;
+}
+
+static ssize_t cpu_wakeup_latency_qos_read(struct file *filp, char __user *buf,
+					   size_t count, loff_t *f_pos)
+{
+	s32 value = pm_qos_read_value(&cpu_wakeup_latency_constraints);
+
+	return simple_read_from_buffer(buf, count, f_pos, &value, sizeof(s32));
+}
+
+static ssize_t cpu_wakeup_latency_qos_write(struct file *filp,
+					    const char __user *buf,
+					    size_t count, loff_t *f_pos)
+{
+	struct pm_qos_request *req = filp->private_data;
+	s32 value;
+
+	if (count == sizeof(s32)) {
+		if (copy_from_user(&value, buf, sizeof(s32)))
+			return -EFAULT;
+	} else {
+		int ret;
+
+		ret = kstrtos32_from_user(buf, count, 16, &value);
+		if (ret)
+			return ret;
+	}
+
+	if (value < 0)
+		return -EINVAL;
+
+	pm_qos_update_target(req->qos, &req->node, PM_QOS_UPDATE_REQ, value);
+
+	return count;
+}
+
+static const struct file_operations cpu_wakeup_latency_qos_fops = {
+	.open = cpu_wakeup_latency_qos_open,
+	.release = cpu_wakeup_latency_qos_release,
+	.read = cpu_wakeup_latency_qos_read,
+	.write = cpu_wakeup_latency_qos_write,
+	.llseek = noop_llseek,
+};
+
+static struct miscdevice cpu_wakeup_latency_qos_miscdev = {
+	.minor = MISC_DYNAMIC_MINOR,
+	.name = "cpu_wakeup_latency",
+	.fops = &cpu_wakeup_latency_qos_fops,
+};
+#endif /* CONFIG_PM_QOS_CPU_SYSTEM_WAKEUP */
+
 static int __init cpu_latency_qos_init(void)
 {
 	int ret;
@@ -424,6 +523,13 @@ static int __init cpu_latency_qos_init(void)
 		pr_err("%s: %s setup failed\n", __func__,
 		       cpu_latency_qos_miscdev.name);
 
+#ifdef CONFIG_PM_QOS_CPU_SYSTEM_WAKEUP
+	ret = misc_register(&cpu_wakeup_latency_qos_miscdev);
+	if (ret < 0)
+		pr_err("%s: %s setup failed\n", __func__,
+		       cpu_wakeup_latency_qos_miscdev.name);
+#endif
+
 	return ret;
 }
 late_initcall(cpu_latency_qos_init);