linux-kernel - Re: [PATCH v2 4/8] clk: migrate the count of orphaned clocks at init

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Message-id: <7c9b0ac1-582c-9b25-4d62-844667e80507@samsung.com>
Date:   Thu, 15 Feb 2018 13:55:41 +0100
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Jerome Brunet <jbrunet@...libre.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        Stephen Boyd <sboyd@...eaurora.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Dong Aisheng <aisheng.dong@....com>,
        Krzysztof Kozlowski <krzk@...nel.org>
Subject: Re: [PATCH v2 4/8] clk: migrate the count of orphaned clocks at
 init

Hi Jerome,

On 2018-02-14 14:43, Jerome Brunet wrote:
> The orphan clocks reparents should migrate any existing count from the
> orphan clock to its new acestor clocks, otherwise we may have
> inconsistent counts in the tree and end-up with gated critical clocks
>
> Assuming we have two clocks, A and B.
> * Clock A has CLK_IS_CRITICAL flag set.
> * Clock B is an ancestor of A which can gate. Clock B gate is left
>    enabled by the bootloader.
>
> Step 1: Clock A is registered. Since it is a critical clock, it is
> enabled. The clock being still an orphan, no parent are enabled.
>
> Step 2: Clock B is registered and reparented to clock A (potentially
> through several other clocks). We are now in situation where the enable
> count of clock A is 1 while the enable count of its ancestors is 0, which
> is not good.
>
> Step 3: in lateinit, clk_disable_unused() is called, the enable_count of
> clock B being 0, clock B is gated and and critical clock A actually gets
> disabled.
>
> This situation was found while adding fdiv_clk gates to the meson8b
> platform.  These clocks parent clk81 critical clock, which is the mother
> of all peripheral clocks in this system. Because of the issue described
> here, the system is crashing when clk_disable_unused() is called.
>
> The situation is solved by reverting
> commit f8f8f1d04494 ("clk: Don't touch hardware when reparenting during registration").
> To avoid breaking again the situation described in this commit
> description, enabling critical clock should be done before walking the
> orphan list. This way, a parent critical clock may not be accidentally
> disabled due to the CLK_OPS_PARENT_ENABLE mechanism.
>
> Fixes: f8f8f1d04494 ("clk: Don't touch hardware when reparenting during registration")
> Cc: Stephen Boyd <sboyd@...eaurora.org>
> Cc: Shawn Guo <shawnguo@...nel.org>
> Cc: Dong Aisheng <aisheng.dong@....com>
> Signed-off-by: Jerome Brunet <jbrunet@...libre.com>

Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>

This patch fixes kernel oops on Exynos5422-based boards (Odroid XU3/XU4)
mentioned in the following threads:
https://patchwork.kernel.org/patch/10185357/
https://www.spinics.net/lists/linux-samsung-soc/msg61766.html

> ---
>   drivers/clk/clk.c | 37 +++++++++++++++++++++----------------
>   1 file changed, 21 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index a4b4e4d6df5e..cca05ea2c058 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -2969,23 +2969,38 @@ static int __clk_core_init(struct clk_core *core)
>   		rate = 0;
>   	core->rate = core->req_rate = rate;
>   
> +	/*
> +	 * Enable CLK_IS_CRITICAL clocks so newly added critical clocks
> +	 * don't get accidentally disabled when walking the orphan tree and
> +	 * reparenting clocks
> +	 */
> +	if (core->flags & CLK_IS_CRITICAL) {
> +		unsigned long flags;
> +
> +		clk_core_prepare(core);
> +
> +		flags = clk_enable_lock();
> +		clk_core_enable(core);
> +		clk_enable_unlock(flags);
> +	}
> +
>   	/*
>   	 * walk the list of orphan clocks and reparent any that newly finds a
>   	 * parent.
>   	 */
>   	hlist_for_each_entry_safe(orphan, tmp2, &clk_orphan_list, child_node) {
>   		struct clk_core *parent = __clk_init_parent(orphan);
> -		unsigned long flags;
>   
>   		/*
> -		 * we could call __clk_set_parent, but that would result in a
> -		 * redundant call to the .set_rate op, if it exists
> +		 * We need to use __clk_set_parent_before() and _after() to
> +		 * to properly migrate any prepare/enable count of the orphan
> +		 * clock. This is important for CLK_IS_CRITICAL clocks, which
> +		 * are enabled during init but might not have a parent yet.
>   		 */
>   		if (parent) {
>   			/* update the clk tree topology */
> -			flags = clk_enable_lock();
> -			clk_reparent(orphan, parent);
> -			clk_enable_unlock(flags);
> +			__clk_set_parent_before(orphan, parent);
> +			__clk_set_parent_after(orphan, parent, NULL);
>   			__clk_recalc_accuracies(orphan);
>   			__clk_recalc_rates(orphan, 0);
>   		}
> @@ -3002,16 +3017,6 @@ static int __clk_core_init(struct clk_core *core)
>   	if (core->ops->init)
>   		core->ops->init(core->hw);
>   
> -	if (core->flags & CLK_IS_CRITICAL) {
> -		unsigned long flags;
> -
> -		clk_core_prepare(core);
> -
> -		flags = clk_enable_lock();
> -		clk_core_enable(core);
> -		clk_enable_unlock(flags);
> -	}
> -
>   	kref_init(&core->ref);
>   out:
>   	clk_pm_runtime_put(core);

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland