clk: introduce clk_is_match

Some drivers compare struct clk pointers as a means of knowing
if the two pointers reference the same clock hardware. This behavior is
dubious (drivers must not dereference struct clk), but did not cause any
regressions until the per-user struct clk patch was merged. Now the test
for matching clk's will always fail with per-user struct clk's.

clk_is_match is introduced to fix the regression and prevent drivers
from comparing the pointers manually.

Fixes: 035a61c314eb ("clk: Make clk API return per-user struct clk instances")
Cc: Russell King <[e-mail]>
Cc: Shawn Guo <[e-mail]>
Cc: Tomeu Vizoso <[e-mail]>
Signed-off-by: Michael Turquette <[e-mail]>
[[e-mail]: Fix COMMON_CLK=N && HAS_CLK=Y config]
Signed-off-by: Arnd Bergmann <[e-mail]>
[[e-mail]: const arguments to clk_is_match() and
remove unnecessary ternary operation]
Signed-off-by: Stephen Boyd <[e-mail]>

drivers/clk/clk.c

···
 }
 
 /**
+ * clk_is_match - check if two clk's point to the same hardware clock
+ * @p: clk compared against q
+ * @q: clk compared against p
+ *
+ * Returns true if the two struct clk pointers both point to the same hardware
+ * clock node. Put differently, returns true if struct clk *p and struct clk *q
+ * share the same struct clk_core object.
+ *
+ * Returns false otherwise. Note that two NULL clks are treated as matching.
+ */
+bool clk_is_match(const struct clk *p, const struct clk *q)
+{
+	/* trivial case: identical struct clk's or both NULL */
+	if (p == q)
+		return true;
+
+	/* true if clk->core pointers match. Avoid derefing garbage */
+	if (!IS_ERR_OR_NULL(p) && !IS_ERR_OR_NULL(q))
+		if (p->core == q->core)
+			return true;
+
+	return false;
+}
+EXPORT_SYMBOL_GPL(clk_is_match);
+
+/**
  * __clk_init - initialize the data structures in a struct clk
  * @dev:	device initializing this clk, placeholder for now
  * @clk:	clk being initialized

include/linux/clk.h

···
  */
 int clk_get_phase(struct clk *clk);
 
+/**
+ * clk_is_match - check if two clk's point to the same hardware clock
+ * @p: clk compared against q
+ * @q: clk compared against p
+ *
+ * Returns true if the two struct clk pointers both point to the same hardware
+ * clock node. Put differently, returns true if struct clk *p and struct clk *q
+ * share the same struct clk_core object.
+ *
+ * Returns false otherwise. Note that two NULL clks are treated as matching.
+ */
+bool clk_is_match(const struct clk *p, const struct clk *q);
+
 #else
 
 static inline long clk_get_accuracy(struct clk *clk)
···
 static inline long clk_get_phase(struct clk *clk)
 {
 	return -ENOTSUPP;
+}
+
+static inline bool clk_is_match(const struct clk *p, const struct clk *q)
+{
+	return p == q;
 }
 
 #endif