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-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 <embe...@gmail.com>
---

Changes in v3:
        * add information when the bug appeared 
        * make the comment more clear
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 | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/rtc/rtc-ac100.c b/drivers/rtc/rtc-ac100.c
index 8ff9dc3fe5bf..2412aa2e8399 100644
--- a/drivers/rtc/rtc-ac100.c
+++ b/drivers/rtc/rtc-ac100.c
@@ -183,7 +183,24 @@ 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 one that is registered and
+                * returns NULL.
+                * Thus we need to check if the parent exists before
+                * we get the parent rate.
+                */
+               if (!parent)
+                       continue;
+
+               prate = clk_hw_get_rate(parent);
 
                tmp = ac100_clkout_round_rate(hw, req->rate, prate);
 
-- 
2.11.0

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