[ARM] omap: clk_set_parent: deny changing parent if clock is enabled

Richard Woodruff writes:
| The historic usage of this has been against single use leaf clocks
| (1st instance of gptimer).  When it was used it did:
|       clk_get()
|       clk_set_parent()
|       clk_enable()
|
| This usage was ok for that. Use on a disabled clock is needed.
|
| If there are multiple users on the clock or it is enabled there are
| problems.
|
| The call can still be unfriendly if 2 different drivers are using the
| clock with their own clock get/enable. It might be the function should
| return an error if usecount != 0 to stop surprises.  It is all around
| better if the parenting is done when the clock is off.

This is a good reason to ensure that the clock is not enabled when
clk_set_parent() is called.

Acked-by: Richard Woodruff <r-woodruff2@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/plat-omap/clock.c

@@ -144,13 +144,16 @@ int clk_set_parent(struct clk *clk, struct clk *parent)
 		return ret;
 
 	spin_lock_irqsave(&clockfw_lock, flags);
-	if (arch_clock->clk_set_parent)
-		ret = arch_clock->clk_set_parent(clk, parent);
-	if (ret == 0) {
-		if (clk->recalc)
-			clk->rate = clk->recalc(clk);
-		propagate_rate(clk);
-	}
+	if (clk->usecount == 0) {
+		if (arch_clock->clk_set_parent)
+			ret = arch_clock->clk_set_parent(clk, parent);
+		if (ret == 0) {
+			if (clk->recalc)
+				clk->rate = clk->recalc(clk);
+			propagate_rate(clk);
+		}
+	} else
+		ret = -EBUSY;
 	spin_unlock_irqrestore(&clockfw_lock, flags);
 
 	return ret;