Re: [PATCH v3] power: introduce library for device-specific OPPs

[Nishanth Menon]

Rafael J. Wysocki had written, on 09/22/2010 07:03 PM, the following:
> [Trimming the CC list slightly.]
[...]

> ...
>
> First, thanks for addressing the previous comments, things look much better
> now.  In particular the documentation has been improved a lot in my view.
Thanks for the excellent reviews :)

[...]

> > +
> > +WARNING on OPP List Modification Vs Query operations:
> > +----------------------------------------------------
> > +The OPP layer's query functions are expected to be used in multiple contexts
> > +(including calls from interrupt locked context) based on SoC framework
> > +implementation. Only OPP modification functions are guaranteed exclusivity by
> > +the OPP library. Exclusivity between query functions and modification functions
> > +should be handled by the users such as the SoC framework appropriately; else,
> > +there is a risk for the query functions to retrieve stale data.
>
> Well, this sounds like a good use case for RCU.
Kevin did point out rwlock but am I confusing with
http://lwn.net/Articles/364583/
If I get the message right, rwlock is more or less on it's way out?

[...]

> > +static struct device_opp *find_device_opp(struct device *dev)
> > +{
> > +     struct device_opp *tmp_dev_opp, *dev_opp = ERR_PTR(-ENODEV);
> > +
> > +     if (unlikely(!dev || IS_ERR(dev))) {
> > +             pr_err("%s: Invalid parameters being passed\n", __func__);
> > +             return ERR_PTR(-EINVAL);
> > +     }
> > +
> > +     list_for_each_entry(tmp_dev_opp, &dev_opp_list, node) {
> > +             if (tmp_dev_opp->dev == dev) {
> > +                     dev_opp = tmp_dev_opp;
> > +                     break;
> > +             }
> > +     }
>
> As I said, it seems you can use RCU read locking around the list traversal
> to protect it from concurrent modification.
>
> > +     return dev_opp;
> > +}
> > +
[...]

> > +struct opp *opp_find_freq_exact(struct device *dev,
> > +                                  unsigned long freq, bool available)
> > +{
> > +     struct device_opp *dev_opp;
> > +     struct opp *temp_opp, *opp = ERR_PTR(-ENODEV);
> > +
> > +     dev_opp = find_device_opp(dev);
> > +     if (IS_ERR(dev_opp))
> > +             return opp;
> > +
> > +     list_for_each_entry(temp_opp, &dev_opp->opp_list, node) {
> > +             if (temp_opp->available == available &&
> > +                             temp_opp->rate == freq) {
> > +                     opp = temp_opp;
> > +                     break;
> > +             }
> > +     }
> > +
>
> Same here.
>
> > +     return opp;
> > +}
> > +
> > +/**
> > + * opp_find_freq_ceil() - Search for an rounded ceil freq
> > + * @dev:     device for which we do this operation
> > + * @freq:    Start frequency
> > + *
> > + * Search for the matching ceil *available* OPP from a starting freq
> > + * for a device.
> > + *
> > + * Returns matching *opp and refreshes *freq accordingly, else returns
> > + * ERR_PTR in case of error and should be handled using IS_ERR.
> > + *
> > + * Example usages:
> > + *   * find match/next highest available frequency *
> > + *   freq = 350000;
> > + *   opp = opp_find_freq_ceil(dev, &freq))
> > + *   if (IS_ERR(opp))
> > + *           pr_err("unable to find a higher frequency\n");
> > + *   else
> > + *           pr_info("match freq = %ld\n", freq);
> > + *
> > + *   * print all supported frequencies in ascending order *
> > + *   freq = 0; * Search for the lowest available frequency *
> > + *   while (!IS_ERR(opp = opp_find_freq_ceil(OPP_MPU, &freq)) {
> > + *           pr_info("freq = %ld\n", freq);
> > + *           freq++; * for next higher match *
> > + *   }
>
> I think it's sufficient to put the examples into the doc (the ones below too).
Ack. thanks for pointing it out.. will fix in v4.

> > +     freq_table[i].index = i;
> > +     freq_table[i].frequency = CPUFREQ_TABLE_END;
> > +
> > +     *table = &freq_table[0];
> > +}
> > +#endif               /* CONFIG_CPU_FREQ */
>
> The rest looks fine to me.
thx.

--
Regards,
Nishanth Menon
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