[PATCH v2] PM: domains: Prevent power off for parent unless child is in deepest state

[Ulf Hansson]

A PM domain managed by genpd may support multiple idlestates (power-off
states). During genpd_power_off() a genpd governor may be asked to select
one of the idlestates based upon the dev PM QoS constraints, for example.

However, there is a problem with the behaviour around this in genpd. More
precisely, a parent-domain is allowed to be powered off, no matter of what
idlestate that has been selected for the child-domain.

For the stm32mp1 platform from STMicro, this behaviour doesn't play well.
Instead, the parent-domain must not be powered off, unless the deepest
idlestate has been selected for the child-domain. As the current behaviour
in genpd is quite questionable anyway, let's simply change it into what is
needed by the stm32mp1 platform.

If it surprisingly turns out that other platforms may need a different
behaviour from genpd, then we will have to revisit this to find a way to
make it configurable.

Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
---

Changes in v2:
	- Clarified commit message - based upon discussions with Dmitry.
	- Updated a comment in the code, suggested by Dmitry.

---
 drivers/base/power/domain.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
index 5db704f02e71..c87588c21700 100644
--- a/drivers/base/power/domain.c
+++ b/drivers/base/power/domain.c
@@ -636,6 +636,18 @@ static int genpd_power_off(struct generic_pm_domain *genpd, bool one_dev_on,
 			atomic_read(&genpd->sd_count) > 0)
 		return -EBUSY;
 
+	/*
+	 * The children must be in their deepest (powered-off) states to allow
+	 * the parent to be powered off. Note that, there's no need for
+	 * additional locking, as powering on a child, requires the parent's
+	 * lock to be acquired first.
+	 */
+	list_for_each_entry(link, &genpd->parent_links, parent_node) {
+		struct generic_pm_domain *child = link->child;
+		if (child->state_idx < child->state_count - 1)
+			return -EBUSY;
+	}
+
 	list_for_each_entry(pdd, &genpd->dev_list, list_node) {
 		enum pm_qos_flags_status stat;
 
@@ -1073,6 +1085,13 @@ static void genpd_sync_power_off(struct generic_pm_domain *genpd, bool use_lock,
 	    || atomic_read(&genpd->sd_count) > 0)
 		return;
 
+	/* Check that the children are in their deepest (powered-off) state. */
+	list_for_each_entry(link, &genpd->parent_links, parent_node) {
+		struct generic_pm_domain *child = link->child;
+		if (child->state_idx < child->state_count - 1)
+			return;
+	}
+
 	/* Choose the deepest state when suspending */
 	genpd->state_idx = genpd->state_count - 1;
 	if (_genpd_power_off(genpd, false))
-- 
2.25.1