linux-kernel - Re: [PATCH v3] PM / OPP: discard duplicate OPPs

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Date:	Tue, 20 May 2014 07:32:48 -0500
From:	Nishanth Menon <nm@...com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Chander Kashyap <chander.kashyap@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>,
	"Brown, Len" <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Chander Kashyap <k.chander@...sung.com>,
	Inderpal Singh <inderpal.s@...sung.com>
Subject: Re: [PATCH v3] PM / OPP: discard duplicate OPPs

On Tue, May 20, 2014 at 7:05 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 20 May 2014 16:56, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> But we aren't talking about failure here. Its not failure. The operation
>> we are trying to do is already done and nothing should break if the
>> OPP was already there or its added now. Its all the same.
>
> Though after more thought into this I feel this must also be done:
>
> diff --git a/drivers/base/power/opp.c b/drivers/base/power/opp.c
> index bdf09f5..3f540d8 100644
> --- a/drivers/base/power/opp.c
> +++ b/drivers/base/power/opp.c
> @@ -453,9 +453,13 @@ int dev_pm_opp_add(struct device *dev, unsigned
> long freq, unsigned long u_volt)
>         }
>
>         if (new_opp->rate == opp->rate) {
> +               int ret = 0;
> +
> +               if (new_opp->u_volt == opp->u_volt)
> +                       ret = -EEXIST;
Else -> we now have two OPPs with the same key (same frequency, but
different voltage) -> That does not make sense.
Example: why would you add:
If you already had {1GHz, 1.2V}
and you attempted:
{1GHz, 1.1V} (if you could do that, then you should added {1GHz, 1.1V}
in the first place)
OR
{1GHz, 1.3V} (if you could do that, then you should add {1GHz, 1.3V}
and the {1GHz, 1.2V} is wrong)


In addition, if old OPP was disabled, that new OPP would be in enabled
state -> which also does not make sense either - since we disabled
that frequency for some reason.

>                 mutex_unlock(&dev_opp_list_lock);
>                 kfree(new_opp);
> -               return 0;
> +               return ret;
>         }
>
>         list_add_rcu(&new_opp->node, head);

The reason I ask to return error is because if we attempt to add two
OPPs with the same key (frequency), then it breaks the logic in
remaining search logic.


Regards,
Nishanth Menon
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