clk: Fix race condition between clk_set_parent and clk_enable()

Without this patch, the following race condition is possible.
* clk-A has two parents - clk-X and clk-Y.
* All three are disabled and clk-X is current parent.
* Thread A: clk_set_parent(clk-A, clk-Y).
* Thread A: <snip execution flow>
* Thread A: Grabs enable lock.
* Thread A: Sees enable count of clk-A is 0, so doesn't enable clk-Y.
* Thread A: Updates clk-A SW parent to clk-Y
* Thread A: Releases enable lock.
* Thread B: clk_enable(clk-A).
* Thread B: clk_enable() enables clk-Y, then enabled clk-A and returns.

clk-A is now enabled in software, but not clocking in hardware since the
hardware parent is still clk-X.

The only way to avoid race conditions between clk_set_parent() and
clk_enable/disable() is to ensure that clk_enable/disable() calls don't
require changes to hardware enable state between changes to software clock
topology and hardware clock topology.

The options to achieve the above are:
1. Grab the enable lock before changing software/hardware topology and
   release it afterwards.
2. Keep the clock enabled for the duration of software/hardware topology
   change so that any additional enable/disable calls don't try to change
   the hardware state. Once the topology change is complete, the clock can
   be put back in its original enable state.

Option (1) is not an acceptable solution since the set_parent() ops might
need to sleep.

Therefore, this patch implements option (2).

This patch doesn't violate any API semantics. clk_disable() doesn't
guarantee that the clock is actually disabled. So, no clients of a clock
can assume that a clock is disabled after their last call to clk_disable().
So, enabling the clock during a parent change is not a violation of any API
semantics.

This also has the nice side effect of simplifying the error handling code.

Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: fixed up whitespace issue]

drivers/clk/clk.c

@@ -1377,23 +1377,33 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent, u8 p_index)
 	unsigned long flags;
 	int ret = 0;
 	struct clk *old_parent = clk->parent;
-	bool migrated_enable = false;
 
-	/* migrate prepare */
-	if (clk->prepare_count)
+	/*
+	 * Migrate prepare state between parents and prevent race with
+	 * clk_enable().
+	 *
+	 * If the clock is not prepared, then a race with
+	 * clk_enable/disable() is impossible since we already have the
+	 * prepare lock (future calls to clk_enable() need to be preceded by
+	 * a clk_prepare()).
+	 *
+	 * If the clock is prepared, migrate the prepared state to the new
+	 * parent and also protect against a race with clk_enable() by
+	 * forcing the clock and the new parent on.  This ensures that all
+	 * future calls to clk_enable() are practically NOPs with respect to
+	 * hardware and software states.
+	 *
+	 * See also: Comment for clk_set_parent() below.
+	 */
+	if (clk->prepare_count) {
 		__clk_prepare(parent);
-
-	flags = clk_enable_lock();
-
-	/* migrate enable */
-	if (clk->enable_count) {
-		__clk_enable(parent);
-		migrated_enable = true;
+		clk_enable(parent);
+		clk_enable(clk);
 	}
 
 	/* update the clk tree topology */
+	flags = clk_enable_lock();
 	clk_reparent(clk, parent);
-
 	clk_enable_unlock(flags);
 
 	/* change clock input source */
@@ -1401,43 +1411,27 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent, u8 p_index)
 		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.
-		 */
 		flags = clk_enable_lock();
-
 		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);
-		}
-
 		clk_enable_unlock(flags);
 
-		if (clk->prepare_count)
+		if (clk->prepare_count) {
+			clk_disable(clk);
+			clk_disable(parent);
 			__clk_unprepare(parent);
-
+		}
 		return ret;
 	}
 
-	/* clean up enable for old parent if migration was done */
-	if (migrated_enable) {
-		flags = clk_enable_lock();
-		__clk_disable(old_parent);
-		clk_enable_unlock(flags);
-	}
-
-	/* clean up prepare for old parent if migration was done */
-	if (clk->prepare_count)
+	/*
+	 * Finish the migration of prepare state and undo the changes done
+	 * for preventing a race with clk_enable().
+	 */
+	if (clk->prepare_count) {
+		clk_disable(clk);
+		clk_disable(old_parent);
 		__clk_unprepare(old_parent);
+	}
 
 	/* update debugfs with new clk tree topology */
 	clk_debug_reparent(clk, parent);
@@ -1449,12 +1443,17 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent, u8 p_index)
  * @clk: the mux clk whose input we are switching
  * @parent: the new input to clk
  *
- * Re-parent clk to use parent as it's new input source.  If clk has the
- * CLK_SET_PARENT_GATE flag set then clk must be gated for this
- * operation to succeed.  After successfully changing clk's parent
- * clk_set_parent will update the clk topology, sysfs topology and
- * propagate rate recalculation via __clk_recalc_rates.  Returns 0 on
- * success, -EERROR otherwise.
+ * Re-parent clk to use parent as its new input source.  If clk is in
+ * prepared state, the clk will get enabled for the duration of this call. If
+ * that's not acceptable for a specific clk (Eg: the consumer can't handle
+ * that, the reparenting is glitchy in hardware, etc), use the
+ * CLK_SET_PARENT_GATE flag to allow reparenting only when clk is unprepared.
+ *
+ * After successfully changing clk's parent clk_set_parent will update the
+ * clk topology, sysfs topology and propagate rate recalculation via
+ * __clk_recalc_rates.
+ *
+ * Returns 0 on success, -EERROR otherwise.
  */
 int clk_set_parent(struct clk *clk, struct clk *parent)
 {