[PATCH 3/3] clk: Fixup locking issues for clk_set_parent

[Ulf Hansson]

From: Ulf Hansson <ulf.hansson at linaro.org>

Updating the clock tree topology must be protected with the spinlock
when doing clk_set_parent, otherwise we can not handle the migration
of the enable_count in a safe manner.

While issuing the .set_parent callback to make the clk-hw perform the
switch to the new parent, we can not hold the spinlock since it is must
be allowed to be slow path. This complicates error handling, but is still
possible to achieve.

Signed-off-by: Ulf Hansson <ulf.hansson at linaro.org>
---
 drivers/clk/clk.c |   68 +++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 51 insertions(+), 17 deletions(-)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index a61cf6d..0c19420 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -1258,30 +1258,69 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent, u8 p_index)
 	unsigned long flags;
 	int ret;
 	struct clk *old_parent = clk->parent;
+	bool migrated_enable = false;
 
-	/* migrate prepare and enable */
+	/* migrate prepare */
 	if (clk->prepare_count)
 		__clk_prepare(parent);
 
-	/* FIXME replace with clk_is_enabled(clk) someday */
 	spin_lock_irqsave(&enable_lock, flags);
-	if (clk->enable_count)
+
+	/* migrate enable */
+	if (clk->enable_count) {
 		__clk_enable(parent);
+		migrated_enable = true;
+	}
+
+	/* update the clk tree topology */
+	clk_reparent(clk, parent);
+
 	spin_unlock_irqrestore(&enable_lock, flags);
 
 	/* change clock input source */
 	ret = clk->ops->set_parent(clk->hw, p_index);
+	if (ret) {
+		/*
+		 * The error handling is tricky due to that we need to release
+		 * the spinlock while issuing the .set_parent callback. This
+		 * means the new parent might have been enabled/disabled in
+		 * between, which must be considered when doing rollback.
+		 */
+		spin_lock_irqsave(&enable_lock, flags);
 
-	/* clean up old prepare and enable */
-	spin_lock_irqsave(&enable_lock, flags);
-	if (clk->enable_count)
+		clk_reparent(clk, old_parent);
+
+		if (migrated_enable && clk->enable_count) {
+			__clk_disable(parent);
+		} else if (migrated_enable && (clk->enable_count == 0)) {
+			__clk_disable(old_parent);
+		} else if (!migrated_enable && clk->enable_count) {
+			__clk_disable(parent);
+			__clk_enable(old_parent);
+		}
+
+		spin_unlock_irqrestore(&enable_lock, flags);
+
+		if (clk->prepare_count)
+			__clk_unprepare(parent);
+
+		return ret;
+	}
+
+	/* clean up enable for old parent if migration was done */
+	if (migrated_enable) {
+		spin_lock_irqsave(&enable_lock, flags);
 		__clk_disable(old_parent);
-	spin_unlock_irqrestore(&enable_lock, flags);
+		spin_unlock_irqrestore(&enable_lock, flags);
+	}
 
+	/* clean up prepare for old parent if migration was done */
 	if (clk->prepare_count)
 		__clk_unprepare(old_parent);
 
-	return ret;
+	/* update debugfs with new clk tree topology */
+	clk_debug_reparent(clk, parent);
+	return 0;
 }
 
 /**
@@ -1339,16 +1378,11 @@ int clk_set_parent(struct clk *clk, struct clk *parent)
 	/* do the re-parent */
 	ret = __clk_set_parent(clk, parent, p_index);
 
-	/* propagate ABORT_RATE_CHANGE if .set_parent failed */
-	if (ret) {
+	/* propagate rate recalculation accordingly */
+	if (ret)
 		__clk_recalc_rates(clk, ABORT_RATE_CHANGE);
-		goto out;
-	}
-
-	/* propagate rate recalculation downstream */
-	clk_reparent(clk, parent);
-	clk_debug_reparent(clk, parent);
-	__clk_recalc_rates(clk, POST_RATE_CHANGE);
+	else
+		__clk_recalc_rates(clk, POST_RATE_CHANGE);
 
 out:
 	mutex_unlock(&prepare_lock);
-- 
1.7.10