rtc: ac100: Fix ac100 determine rate bug

This patch fixes a bug, that prevents the Allwinner A83T and the A80
from a successful boot.

The bug is there since v4.16-rc1 and appeared after the clk branch was
merged.

You can find the shortend trace below:

Unable to handle kernel NULL pointer dereference at virtual address
00000000
pgd = (ptrval)
[00000000] *pgd=00000000
Internal error: Oops: 5 [#1] SMP ARM
Modules linked in:
CPU: 0 PID: 49 Comm: kworker/0:1 Not tainted 4.15.0-10190-gb89e32cc #2
Hardware name: Allwinner sun8i Family
Workqueue: events deferred_probe_work_func
PC is at clk_hw_get_rate+0x0/0x34
LR is at ac100_clkout_determine_rate+0x48/0x19c

[ ... ]

(clk_hw_get_rate) from (ac100_clkout_determine_rate+0x48/0x19c)
(ac100_clkout_determine_rate) from  (clk_core_set_rate_nolock+0x3c/0x1a0)
(clk_core_set_rate_nolock) from (clk_set_rate+0x30/0x88)
(clk_set_rate) from (of_clk_set_defaults+0x200/0x364)
(of_clk_set_defaults) from (platform_drv_probe+0x18/0xb0)

To fix that bug, we first check if the return of the
clk_hw_get_parent_by_index is non zero. If it is zero we skip that
clock parent.

The BUG report could be found here: https://lkml.org/lkml/2018/2/10/198

Fixes: 04940631 ("rtc: ac100: Add clk output support")
Signed-off-by: Philipp Rossak <embed3d@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-ac100.c

@@ -183,7 +183,29 @@ static int ac100_clkout_determine_rate(struct clk_hw *hw,
 
 	for (i = 0; i < num_parents; i++) {
 		struct clk_hw *parent = clk_hw_get_parent_by_index(hw, i);
-		unsigned long tmp, prate = clk_hw_get_rate(parent);
+		unsigned long tmp, prate;
+
+		/*
+		 * The clock has two parents, one is a fixed clock which is
+		 * internally registered by the ac100 driver. The other parent
+		 * is a clock from the codec side of the chip, which we
+		 * properly declare and reference in the devicetree and is
+		 * not implemented in any driver right now.
+		 * If the clock core looks for the parent of that second
+		 * missing clock, it can't find one that is registered and
+		 * returns NULL.
+		 * So we end up in a situation where clk_hw_get_num_parents
+		 * returns the amount of clocks we can be parented to, but
+		 * clk_hw_get_parent_by_index will not return the orphan
+		 * clocks.
+		 * Thus we need to check if the parent exists before
+		 * we get the parent rate, so we could use the RTC
+		 * without waiting for the codec to be supported.
+		 */
+		if (!parent)
+			continue;
+
+		prate = clk_hw_get_rate(parent);
 
 		tmp = ac100_clkout_round_rate(hw, req->rate, prate);