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

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Date:   Mon, 31 Jan 2022 12:37:43 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J . Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org
Cc:     Kevin Hilman <khilman@...nel.org>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Dmitry Osipenko <digetx@...il.com>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Dong Aisheng <aisheng.dong@....com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] PM: domains: Prevent power off for parent unless child is in deepest state

A PM domain managed by genpd may support multiple idlestates. 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.

So far, we have not received any reports about errors, possibly because
there might not be platform with this hierarchical configuration, yet.
Nevertheless, it seems reasonable to change the behaviour into preventing
the parent-domain from being powered off, unless the deepest idlestate has
been selected for the child-domain, so let's do that.

Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
---
 drivers/base/power/domain.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
index 5db704f02e71..7f97c5cabdc2 100644
--- a/drivers/base/power/domain.c
+++ b/drivers/base/power/domain.c
@@ -636,6 +636,17 @@ 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 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 +1084,13 @@ 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 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