Commit 858d5881 authored by Masahiro Yamada's avatar Masahiro Yamada Committed by Stephen Boyd

clk: avoid circular clock topology

Currently, clk_register() never checks a circular parent looping,
but clock providers could register such an insane clock topology.
For example, "clk_a" could have "clk_b" as a parent, and vice versa.
In this case, clk_core_reparent() creates a circular parent list
and __clk_recalc_accuracies() calls itself recursively forever.

The core infrastructure should be kind enough to bail out, showing
an appropriate error message in such a case.  This helps to easily
find a bug in clock providers.  (uh, I made such a silly mistake
when I was implementing my clock providers first.  I was upset
because the kernel did not respond, without any error message.)

This commit adds a new helper function, __clk_is_ancestor().  It
returns true if the second argument is a possible ancestor of the
first one.  If a clock core is a possible ancestor of itself, it
would make a loop when it were registered.  That should be detected
as an error.
Signed-off-by: default avatarMasahiro Yamada <>
Reviewed-by: default avatarVladimir Zapolskiy <>
Signed-off-by: default avatarStephen Boyd <>
parent 5146e0b0
......@@ -2252,6 +2252,38 @@ static inline void clk_debug_unregister(struct clk_core *core)
* __clk_is_ancestor - check if a clk_core is a possible ancestor of another
* @core: clock core
* @ancestor: ancestor clock core
* Returns true if there is a possibility that @ancestor can be an ancestor
* of @core, false otherwise.
* This function can be used against @core or @ancestor that has not been
* registered yet.
static bool __clk_is_ancestor(struct clk_core *core, struct clk_core *ancestor)
struct clk_core *parent;
int i;
for (i = 0; i < core->num_parents; i++) {
parent = clk_core_get_parent_by_index(core, i);
* If ancestor has not been added to clk_{root,orphan}_list
* yet, clk_core_lookup() cannot find it. If parent is NULL,
* compare the name strings, too.
if ((parent && (parent == ancestor ||
__clk_is_ancestor(parent, ancestor))) ||
(!parent && !strcmp(core->parent_names[i], ancestor->name)))
return true;
return false;
* __clk_core_init - initialize the data structures in a struct clk_core
* @core: clk_core being initialized
......@@ -2317,6 +2349,14 @@ static int __clk_core_init(struct clk_core *core)
"%s: invalid NULL in %s's .parent_names\n",
__func__, core->name);
/* If core is an ancestor of itself, it would make a loop. */
if (__clk_is_ancestor(core, core)) {
pr_err("%s: %s would create circular parent\n", __func__,
ret = -EINVAL;
goto out;
core->parent = __clk_init_parent(core);
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