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Subject[PATCH v4] 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

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:

Fixes: 04940631b8d2 ("rtc: ac100: Add clk output support")

Signed-off-by: Philipp Rossak <>

Changes in v4:
* add more information to the comment
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 | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/rtc/rtc-ac100.c b/drivers/rtc/rtc-ac100.c
index 8ff9dc3fe5bf..08ca8c46a8ff 100644
--- a/drivers/rtc/rtc-ac100.c
+++ b/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);

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