linux-kernel - Re: [PATCH v3] rtc: ac100: Fix ac100 determine rate bug

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Date:   Thu, 15 Feb 2018 15:11:54 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Philipp Rossak <embed3d@...il.com>
Cc:     wens@...e.org, a.zummo@...ertech.it, alexandre.belloni@...tlin.com,
        linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com,
        linux-rtc@...r.kernel.org
Subject: Re: [PATCH v3] rtc: ac100: Fix ac100 determine rate bug

On Wed, Feb 14, 2018 at 02:56:12PM +0100, Philipp Rossak wrote:
> 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.

Out of curiosity, which patch has introduced this? I couldn't find any
obvious match.

> 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 <embed3d@...il.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;

I'm sorry, but I still don't get it. When you register that clock, you
will give it two parents. Why would that change during the life of the
clock?

This really looks like a workaround rather than an actual fix.

Maxime

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

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