misc: atmel_tclib: get and use slow clock

Commit dca1a4b5ff ("clk: at91: keep slow clk enabled to prevent system
hang") added a workaround for the slow clock as it is not properly handled
by its users.

Get and use the slow clock as it is necessary for the timer counters.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Thierry Reding <thierry.reding@gmail.com>

drivers/clocksource/tcb_clksrc.c

@@ -193,10 +193,17 @@ static int __init setup_clkevents(struct atmel_tc *tc, int clk32k_divisor_idx)
 	struct clk *t2_clk = tc->clk[2];
 	int irq = tc->irq[2];
 
-	/* try to enable t2 clk to avoid future errors in mode change */
-	ret = clk_prepare_enable(t2_clk);
+	ret = clk_prepare_enable(tc->slow_clk);
 	if (ret)
 		return ret;
+
+	/* try to enable t2 clk to avoid future errors in mode change */
+	ret = clk_prepare_enable(t2_clk);
+	if (ret) {
+		clk_disable_unprepare(tc->slow_clk);
+		return ret;
+	}
+
 	clk_disable(t2_clk);
 
 	clkevt.regs = tc->regs;
@@ -209,6 +216,7 @@ static int __init setup_clkevents(struct atmel_tc *tc, int clk32k_divisor_idx)
 	ret = request_irq(irq, ch2_irq, IRQF_TIMER, "tc_clkevt", &clkevt);
 	if (ret) {
 		clk_unprepare(t2_clk);
+		clk_disable_unprepare(tc->slow_clk);
 		return ret;
 	}