Re: [PATCH] clk: prevent erronous parsing of children during rate change

From: Stephen Boyd
Date: Mon Sep 22 2014 - 15:18:22 EST


On 08/21, Tero Kristo wrote:
> In some cases, clocks can switch their parent with clk_set_rate, for
> example clk_mux can do this in some cases. Current implementation of
> clk_change_rate uses un-safe list iteration on the clock children, which
> will cause wrong clocks to be parsed in case any of the clock children
> change their parents during the change rate operation. Fixed by using
> the safe list iterator instead.
>
> The problem was detected due to some divide by zero errors generated
> by clock init on dra7-evm board, see discussion under
> http://article.gmane.org/gmane.linux.ports.arm.kernel/349180 for details.
>
> Signed-off-by: Tero Kristo <t-kristo@xxxxxx>
> To: Mike Turquette <mturquette@xxxxxxxxxx>
> Reported-by: Nishanth Menon <nm@xxxxxx>
> ---
> drivers/clk/clk.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index b76fa69..bacc06f 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -1467,6 +1467,7 @@ static struct clk *clk_propagate_rate_change(struct clk *clk, unsigned long even
> static void clk_change_rate(struct clk *clk)
> {
> struct clk *child;
> + struct hlist_node *tmp;
> unsigned long old_rate;
> unsigned long best_parent_rate = 0;
> bool skip_set_rate = false;
> @@ -1502,7 +1503,11 @@ static void clk_change_rate(struct clk *clk)
> if (clk->notifier_count && old_rate != clk->rate)
> __clk_notify(clk, POST_RATE_CHANGE, old_rate, clk->rate);
>
> - hlist_for_each_entry(child, &clk->children, child_node) {
> + /*
> + * Use safe iteration, as change_rate can actually swap parents
> + * for certain clock types.
> + */
> + hlist_for_each_entry_safe(child, tmp, &clk->children, child_node) {
> /* Skip children who will be reparented to another clock */
> if (child->new_parent && child->new_parent != clk)
> continue;

Are we not hitting the new_parent check here? I don't understand
how we can be changing parents here unless the check is being
avoided, in which case I wonder why determine_rate isn't being
used.

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