linux-kernel - Re: [PATCH] cpufreq-dt: defer probing if OPP table is not ready

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Date:	Mon, 15 Dec 2014 21:32:16 -0800
From:	Dmitry Torokhov <dtor@...omium.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Dmitry Torokhov <dtor@...omium.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lucas Stach <l.stach@...gutronix.de>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Stefan Wahren <stefan.wahren@...e.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq-dt: defer probing if OPP table is not ready

On Mon, Dec 15, 2014 at 9:10 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 16 December 2014 at 05:40, Dmitry Torokhov <dtor@...omium.org> wrote:
>> cpufreq-dt driver supports mode when OPP table is provided by platform
>> code and not device tree. However on certain platforms code that fills
>> OPP table may run after cpufreq driver tries to initialize, so let's
>> report -EPROBE_DEFER if we do not find any entires in OPP table for the
>> CPU.
>>
>> Signed-off-by: Dmitry Torokhov <dtor@...omium.org>
>> ---
>>  drivers/cpufreq/cpufreq-dt.c | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/drivers/cpufreq/cpufreq-dt.c b/drivers/cpufreq/cpufreq-dt.c
>> index f56147a..4f874fa 100644
>> --- a/drivers/cpufreq/cpufreq-dt.c
>> +++ b/drivers/cpufreq/cpufreq-dt.c
>> @@ -211,6 +211,19 @@ static int cpufreq_init(struct cpufreq_policy *policy)
>>         /* OPPs might be populated at runtime, don't check for error here */
>>         of_init_opp_table(cpu_dev);
>>
>> +       /*
>> +        * But we need OPP table to function so if it is not there let's
>> +        * give platform code chance to provide it for us.
>> +        */
>> +       rcu_read_lock();
>> +       ret = dev_pm_opp_get_opp_count(cpu_dev);
>> +       rcu_read_unlock();
>> +       if (ret <= 0) {
>> +               pr_debug("OPP table is not ready, deferring probe\n");
>> +               ret = -EPROBE_DEFER;
>> +               goto out_free_opp;
>
> Hmm, so we are trying to free opps while we failed to find one.
>
> Are you sure you have tested this on one such platform where we
> fail here?

Not with the linux-next, but generally yes.

>
> Because this is what I see:
>
> void of_free_opp_table(struct device *dev)
> {
>         struct device_opp *dev_opp;
>         struct dev_pm_opp *opp, *tmp;
>
>         /* Check for existing list for 'dev' */
>         dev_opp = find_device_opp(dev);
>         if (WARN(IS_ERR(dev_opp), "%s: dev_opp: %ld\n", dev_name(dev),
>                  PTR_ERR(dev_opp)))
>                 return;
>
>         ...
> }
>
>
> And we probably will hit this WARN(), wouldn't we ?

Yes we will. Which simply means that this WARN is stupid. We also will
hit it if there is no opp table and the allocation below fails; or if
it succeeds then dev_pm_opp_init_cpufreq_table() will fail and we'll
hit this code path again.

We should either drop that WARN() or handle -ENODEV there properly.

Thanks,
Dmitry
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