Re: [PATCH v3] PM / devfreq: Restart previous governor if new governor fails to start

[Saravana Kannan]

On 11/08/2016 01:02 AM, Chanwoo Choi wrote:
> Hi,
>
> On 2016ë 11ì 08ì 03:57, Saravana Kannan wrote:
> > On 10/26/2016 05:06 PM, Chanwoo Choi wrote:
> > > Hi,
> > >
> > > On 2016ë 10ì 27ì 04:17, Saravana Kannan wrote:
> > > > If the new governor fails to start, switch back to old governor so that the
> > > > devfreq state is not left in some weird limbo.
> > > >
> > > > Signed-off-by: Saravana Kannan <skannan@xxxxxxxxxxxxxx>
> > > > ---
> > > > * Fix NULL deref for real this time.
> > > > * Addressed some style preferences.
> > > >
> > > >    drivers/devfreq/devfreq.c | 13 +++++++++++--
> > > >    1 file changed, 11 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> > > > index bf3ea76..77c41a5 100644
> > > > --- a/drivers/devfreq/devfreq.c
> > > > +++ b/drivers/devfreq/devfreq.c
> > > > @@ -919,7 +919,7 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr,
> > > >        struct devfreq *df = to_devfreq(dev);
> > > >        int ret;
> > > >        char str_governor[DEVFREQ_NAME_LEN + 1];
> > > > -    struct devfreq_governor *governor;
> > > > +    const struct devfreq_governor *governor, *prev_governor;
> > > >
> > > >        ret = sscanf(buf, "%" __stringify(DEVFREQ_NAME_LEN) "s", str_governor);
> > > >        if (ret != 1)
> > > > @@ -944,12 +944,21 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr,
> > > >                goto out;
> > > >            }
> > > >        }
> > > > +    prev_governor = df->governor;
> > > >        df->governor = governor;
> > > >        strncpy(df->governor_name, governor->name, DEVFREQ_NAME_LEN);
> > > >        ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL);
> > > > -    if (ret)
> > > > +    if (ret) {
> > > >            dev_warn(dev, "%s: Governor %s not started(%d)\n",
> > > >                 __func__, df->governor->name, ret);
> > > > +        if (prev_governor) {
> > >
> > > I think that prev_governor is always set. You don't need to check it.
> > > Why do check it?
> >
> > Not true. Even higher up in this function, we check if df->governor != NULL. Simple example would be that the default governor passed in while adding the device isn't compiled in.
>
> I don't understand. If device is not registered by devfreq_add_device(),
> you don't change the governor by using governor_store().
>
> If you can change the governor through governor_store(),
> it means that the devfreq instance was added without any problem.
> The added devfreq instance must have the sepecific governor
> on df->governor variable.
>
> So, I think that df->governor is always set and then prev_governor is always set.

Read the code more closely. add_device doesn't (and shouldn't) fail if 
the governor isn't present. After that, the governor could be changed 
from user space.

-Saravana

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