Re: [PATCH v3] PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.

[Akhil P Oommen]

On 6/22/2018 1:52 PM, Enric Balletbo i Serra wrote:
> Hi Ezequiel and Akhil,
>
> On 22/06/18 09:03, Akhil P Oommen wrote:
> > On 6/22/2018 6:41 AM, Ezequiel Garcia wrote:
> > > Hey Enric,
> > >
> > > On Fri, 2018-06-22 at 00:04 +0200, Enric Balletbo i Serra wrote:
> > > > When the devfreq driver and the governor driver are built as modules,
> > > > the call to devfreq_add_device() or governor_store() fails because
> > > > the
> > > > governor driver is not loaded at the time the devfreq driver loads.
> > > > The
> > > > devfreq driver has a build dependency on the governor but also should
> > > > have a runtime dependency. We need to make sure that the governor
> > > > driver
> > > > is loaded before the devfreq driver.
> > > >
> > > > This patch fixes this bug by adding a try_then_request_governor()
> > > > function. First tries to find the governor, and then, if it is not
> > > > found,
> > > > it requests the module and tries again.
> > > >
> > > > Fixes: 1b5c1be2c88e (PM / devfreq: map devfreq drivers to governor
> > > > using name)
> > > > Signed-off-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
> > > > ---
> > > >
> > > > Changes in v3:
> > > > - Remove unneded change in dev_err message.
> > > > - Fix err returned value in case to not find the governor.
> > > >
> > > > Changes in v2:
> > > > - Add a new function to request the module and call that function
> > > > from
> > > >  ÂÂ devfreq_add_device and governor_store.
> > > >
> > > >  Â drivers/devfreq/devfreq.c | 65 ++++++++++++++++++++++++++++++++-----
> > > > --
> > > [snip snip]
> > > > -ÂÂÂ governor = find_devfreq_governor(devfreq->governor_name);
> > > > +ÂÂÂ governor = try_then_request_governor(devfreq-
> > > > > governor_name);
> > > >  ÂÂÂÂÂ if (IS_ERR(governor)) {
> > > >  ÂÂÂÂÂÂÂÂÂ dev_err(dev, "%s: Unable to find governor for the
> > > > device\n",
> > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂ __func__);
> > > >  ÂÂÂÂÂÂÂÂÂ err = PTR_ERR(governor);
> > > > -ÂÂÂÂÂÂÂ goto err_init;
> > > > +ÂÂÂÂÂÂÂ goto err_unregister;
> > > >  ÂÂÂÂÂ }
> > > >  Â +ÂÂÂ mutex_lock(&devfreq_list_lock);
> > > > +
> > > I know it's not something we are introducing in this patch,
> > > but still... calling a hook with a mutex held looks
> > > fishy to me.
> > >
> > > This lock should only protect the list, unless I am missing
> > > something.
> I think so too.
>
> > > >  ÂÂÂÂÂ devfreq->governor = governor;
> > > >  ÂÂÂÂÂ err = devfreq->governor->event_handler(devfreq,
> > > > DEVFREQ_GOV_START,
> > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ NULL);
> > > > @@ -663,14 +703,16 @@ struct devfreq *devfreq_add_device(struct
> > > > device *dev,
> > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂ __func__);
> > > >  ÂÂÂÂÂÂÂÂÂ goto err_init;
> > > >  ÂÂÂÂÂ }
> > > > +
> > > > +ÂÂÂ list_add(&devfreq->node, &devfreq_list);
> > > > +
> > > >  ÂÂÂÂÂ mutex_unlock(&devfreq_list_lock);
> > > >  Â ÂÂÂÂÂ return devfreq;
> > > >  Â Â err_init:
> > > > -ÂÂÂ list_del(&devfreq->node);
> > > >  ÂÂÂÂÂ mutex_unlock(&devfreq_list_lock);
> > > > -
> > > > +err_unregister:
> > > >  ÂÂÂÂÂ device_unregister(&devfreq->dev);
> > > >  Â err_dev:
> > > >  ÂÂÂÂÂ if (devfreq)
> > > > @@ -988,12 +1030,13 @@ static ssize_t governor_store(struct device
> > > > *dev, struct device_attribute *attr,
> > > >  ÂÂÂÂÂ if (ret != 1)
> > > >  ÂÂÂÂÂÂÂÂÂ return -EINVAL;
> > > >  Â -ÂÂÂ mutex_lock(&devfreq_list_lock);
> > > > -ÂÂÂ governor = find_devfreq_governor(str_governor);
> > > > +ÂÂÂ governor = try_then_request_governor(str_governor);
> > > >  ÂÂÂÂÂ if (IS_ERR(governor)) {
> > > > -ÂÂÂÂÂÂÂ ret = PTR_ERR(governor);
> > > > -ÂÂÂÂÂÂÂ goto out;
> > > > +ÂÂÂÂÂÂÂ return PTR_ERR(governor);
> > > >  ÂÂÂÂÂ }
> > > > +
> > > > +ÂÂÂ mutex_lock(&devfreq_list_lock);
> > > > +
> > > >  ÂÂÂÂÂ if (df->governor == governor) {
> > > >  ÂÂÂÂÂÂÂÂÂ ret = 0;
> > > >  ÂÂÂÂÂÂÂÂÂ goto out;
> > > > --
> > > > 2.17.1
> > > Regards,
> > > Eze
> > Adding to Ezequiel's point, shouldn't we take more granular lock (devfreq->lock)
> > first and then call devfreq_list_lock at the time of adding to the list?
> Yes, I think so. I think, though, that this should be a separate patch, not sure
> if a pre or post patch to this one, but for sure it's another topic. Current
> patch tries to solve different problem an only tries to follow the current
> locking/unlocking. Anyway this is a maintainer decision I guess.
>
> Thanks,
>   Enric
>
> > -Akhil.
I agree.
-Akhil.