[PATCH] clk: Introduce clk_try_parent()

[Thierry Reding <thierry.reding@gmail.com>]

From: Thierry Reding <treding@nvidia.com>

This new function is similar to clk_set_parent(), except that it doesn't
actually change the parent. It merely checks that the given parent clock
can be a parent for the given clock.

A situation where this is useful is to check that a particular setup is
valid before switching to it. One specific use-case for this is atomic
modesetting in the DRM framework where setting a mode is divided into a
check phase where a given configuration is validated before applying
changes to the hardware.

Signed-off-by: Thierry Reding <treding@nvidia.com>
---
 drivers/clk/clk.c   | 36 ++++++++++++++++++++++++++++++++++++
 include/linux/clk.h |  9 +++++++++
 2 files changed, 45 insertions(+)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 01e5009c3e86..7e1c819aa6fc 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -1673,6 +1673,42 @@ void __clk_reparent(struct clk *clk, struct clk *new_parent)
 }
 
 /**
+ * clk_try_parent - check if a clock can be the parent clock source of another
+ * @clk: clock source
+ * @parent: parent clock source
+ *
+ * This is like clk_set_parent(), except that it only checks that parent can
+ * be the parent clock source for clock.
+ *
+ * Returns success (0) or negative errno.
+ */
+int clk_try_parent(struct clk *clk, struct clk *parent)
+{
+	int err = 0;
+
+	if (!clk || !parent)
+		return -EINVAL;
+
+	if ((clk->num_parents > 1) && !clk->ops->set_parent)
+		return -ENOSYS;
+
+	clk_prepare_lock();
+
+	if (clk->parent == parent)
+		goto unlock;
+
+	err = clk_fetch_parent_index(clk, parent);
+	if (err > 0)
+		err = 0;
+
+unlock:
+	clk_prepare_unlock();
+
+	return err;
+}
+EXPORT_SYMBOL_GPL(clk_try_parent);
+
+/**
  * clk_set_parent - switch the parent of a mux clk
  * @clk: the mux clk whose input we are switching
  * @parent: the new input to clk
diff --git a/include/linux/clk.h b/include/linux/clk.h
index fb1ac65f127c..d8e9b8865d38 100644
--- a/include/linux/clk.h
+++ b/include/linux/clk.h
@@ -328,6 +328,15 @@ long clk_round_rate(struct clk *clk, unsigned long rate);
 int clk_set_rate(struct clk *clk, unsigned long rate);
 
 /**
+ * clk_try_parent - check if a clock can be the parent clock source of another
+ * @clk: clock source
+ * @parent: parent clock source
+ *
+ * Returns success (0) or negative errno.
+ */
+int clk_try_parent(struct clk *clk, struct clk *parent);
+
+/**
  * clk_set_parent - set the parent clock source for this clock
  * @clk: clock source
  * @parent: parent clock source
-- 
2.1.3