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SubjectRe: [PATCH] clk: prevent erronous parsing of children during rate change
On 09/22/2014 10:18 PM, Stephen Boyd wrote:
> 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@ti.com>
>> To: Mike Turquette <mturquette@linaro.org>
>> Reported-by: Nishanth Menon <nm@ti.com>
>> ---
>> 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.
>

It depends how the clock underneath handles the situation. The error I
am seeing actually happens with a SoC specific compound clock (DPLL)
which integrates set_rate + mux functionality into a single clock node.
A call to the clk_set_rate changes the parent of this clock (from bypass
clock to reference clock), in addition to changing the rate (tune the
mul+div.) I looked at using the determine rate call with this type but
it breaks everything up... the parent gets changed but not the clock
rate, in addition to some other issues.

-Tero


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