On Tue, Feb 13, 2018 at 01:14:14PM +0100, Philipp Rossak wrote:
> This patch fixes a bug, that prevents the Allwinner A83T and the A80
> from a successful boot. You can find the shortend trace below:

Since when is it there?

> 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")

Should it be sent to stable?

> Signed-off-by: Philipp Rossak <embe...@gmail.com>
> ---
> 
> 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);

clk_hw_get_num_parents should return the exact number of parents,
which is going to be 1 if you only have one parent, like all DTS seems
to have.

If not, then it should be explained in the comment and / or fixed
properly.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com

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