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[PATCH v2] rtc: ac100: Fix ac100 determine rate bug

This patch fixes a bug, that prevents the Allwinner A83T and the A80
from a successful boot. You can find the shortend trace below:

Unable to handle kernel NULL pointer dereference at virtual address
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-gb89e32ccd1be #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: 04940631b8d2 ("rtc: ac100: Add clk output support")

Signed-off-by: Philipp Rossak <embed3d@xxxxxxxxx>

Changes in v2:
	* add tag Fixes: ... to commit message
	* add comment to if statement why we are doing this check

 drivers/rtc/rtc-ac100.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/rtc/rtc-ac100.c b/drivers/rtc/rtc-ac100.c
index 8ff9dc3fe5bf..ba73201d8cc1 100644
--- a/drivers/rtc/rtc-ac100.c
+++ b/drivers/rtc/rtc-ac100.c
@@ -183,7 +183,17 @@ 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;
+		/*
+		 * We purposefully left open the possibility to use the clock
+		 * from the codec side but it is not implemented right now.
+		 * Thus we need to check if the parent exists.
+		 */
+		if (!parent)
+			continue;
+		prate = clk_hw_get_rate(parent);
 		tmp = ac100_clkout_round_rate(hw, req->rate, prate);