[RFC v1 8/8] PM: standby: Add sysfs attribute for runtime standby transitions

[Antheas Kapenekakis <lkml@antheas.dev>]

Add a sysfs attribute to allow informing the kernel about the current
standby state of the device depending on user involvement, those being:
"active", "inactive", "sleep", and "resume" (in "sleep" but preparing
for presenting to the user faster).

Signed-off-by: Antheas Kapenekakis <lkml@antheas.dev>
---
 kernel/power/main.c  | 84 ++++++++++++++++++++++++++++++++++++++++++++
 kernel/power/power.h |  1 +
 2 files changed, 85 insertions(+)

diff --git a/kernel/power/main.c b/kernel/power/main.c
index 03b2c5495c77..30494be41557 100644
--- a/kernel/power/main.c
+++ b/kernel/power/main.c
@@ -830,6 +830,89 @@ static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
 
 power_attr(state);
 
+#ifdef CONFIG_SUSPEND
+/*
+ * standby - control system s2idle standby state.
+ *
+ * show() returns available standby states, which may be "active", "screen_off",
+ * "sleep" and "resume" (still in sleep but preparing to present to user).
+ * See Documentation/admin-guide/pm/standby-states.rst for a description of
+ * what they mean.
+ *
+ * store() accepts one of those strings and initiates a transition to that
+ * standby state.
+ *
+ * For backwards compatibility, when the system suspends, it first enters the
+ * state "sleep", regardless of what was written into store() and then during
+ * resume restores the previous value.
+ */
+static ssize_t standby_show(struct kobject *kobj, struct kobj_attribute *attr,
+			    char *buf)
+{
+	unsigned int sleep_flags;
+	standby_state_t i, curr;
+	char *s = buf;
+
+	sleep_flags = lock_system_sleep();
+	pm_standby_refresh_states();
+	curr = pm_standby_get_state();
+	unlock_system_sleep(sleep_flags);
+
+	if (curr < 0)
+		return -EBUSY;
+
+	for (i = PM_STANDBY_MIN; i < PM_STANDBY_MAX; i++)
+		if (standby_states[i])
+			s += sprintf(s, curr == i ? "[%s] " : "%s ", standby_states[i]);
+
+	if (s != buf)
+		/* convert the last space to a newline */
+		*(s - 1) = '\n';
+	return (s - buf);
+}
+
+static standby_state_t decode_standby_state(const char *buf, size_t n)
+{
+	standby_state_t state;
+	char *p;
+	int len;
+
+	p = memchr(buf, '\n', n);
+	len = p ? p - buf : n;
+
+	for (state = PM_STANDBY_MIN; state < PM_STANDBY_MAX; state++) {
+		const char *label = standby_states[state];
+
+		if (label && len == strlen(label) && !strncmp(buf, label, len))
+			return state;
+	}
+
+	return PM_STANDBY_MAX;
+}
+
+static ssize_t standby_store(struct kobject *kobj, struct kobj_attribute *attr,
+			     const char *buf, size_t n)
+{
+	unsigned int sleep_flags;
+	standby_state_t state;
+	int error;
+
+	sleep_flags = lock_system_sleep();
+	pm_standby_refresh_states();
+	state = decode_standby_state(buf, n);
+
+	if (state >= PM_STANDBY_MAX)
+		return -EINVAL;
+
+	error = pm_standby_transition(state);
+	unlock_system_sleep(sleep_flags);
+
+	return error ? error : n;
+}
+
+power_attr(standby);
+#endif
+
 #ifdef CONFIG_PM_SLEEP
 /*
  * The 'wakeup_count' attribute, along with the functions defined in
@@ -1084,6 +1167,7 @@ static struct attribute * g[] = {
 #ifdef CONFIG_SUSPEND
 	&mem_sleep_attr.attr,
 	&sync_on_suspend_attr.attr,
+	&standby_attr.attr,
 #endif
 #ifdef CONFIG_PM_AUTOSLEEP
 	&autosleep_attr.attr,
diff --git a/kernel/power/power.h b/kernel/power/power.h
index 75b63843886e..2327a1ce2b05 100644
--- a/kernel/power/power.h
+++ b/kernel/power/power.h
@@ -216,6 +216,7 @@ extern void swsusp_show_speed(ktime_t, ktime_t, unsigned int, char *);
 extern const char * const pm_labels[];
 extern const char *pm_states[];
 extern const char *mem_sleep_states[];
+extern const char *standby_states[];
 
 extern int suspend_devices_and_enter(suspend_state_t state);
 #else /* !CONFIG_SUSPEND */
-- 
2.52.0