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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Stephan Gerhold <stephan@gerhold.net>,
        Saravana Kannan <saravanak@google.com>
Subject: [PATCH] driver core: Extend device_is_dependent()
Date:   Thu, 14 Jan 2021 19:41:53 +0100
Message-ID: <2073294.4OfjquceTg@kreacher>
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From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

When adding a new device link, device_is_dependent() is used to
check whether or not the prospective supplier device does not
depend on the prospective consumer one to avoid adding loops
to the graph of device dependencies.

However, device_is_dependent() does not take the ancestors of
the target device into account, so it may not detect an existing
reverse dependency if, for example, the parent of the target
device depends on the device passed as its first argument.

For this reason, extend device_is_dependent() to also check if
the device passed as its first argument is an ancestor of the
target one and return 1 if that is the case.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reported-by: Stephan Gerhold <stephan@gerhold.net> 
---
 drivers/base/core.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

Index: linux-pm/drivers/base/core.c
===================================================================
--- linux-pm.orig/drivers/base/core.c
+++ linux-pm/drivers/base/core.c
@@ -208,6 +208,16 @@ int device_links_read_lock_held(void)
 #endif
 #endif /* !CONFIG_SRCU */
 
+static bool device_is_ancestor(struct device *dev, struct device *target)
+{
+	while (target->parent) {
+		target = target->parent;
+		if (dev == target)
+			return true;
+	}
+	return false;
+}
+
 /**
  * device_is_dependent - Check if one device depends on another one
  * @dev: Device to check dependencies for.
@@ -221,7 +231,7 @@ int device_is_dependent(struct device *d
 	struct device_link *link;
 	int ret;
 
-	if (dev == target)
+	if (dev == target || device_is_ancestor(dev, target))
 		return 1;
 
 	ret = device_for_each_child(dev, target, device_is_dependent);