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

[Saravana Kannan]

On 11/9/16 4:10 PM, Chanwoo Choi wrote:
> Hi,
>
> On 2016ë 11ì 10ì 05:34, Saravana Kannan wrote:
> > On 11/08/2016 06:38 PM, Chanwoo Choi wrote:
> > > On 2016ë 11ì 09ì 11:36, Chanwoo Choi wrote:
> > > > Hi,
> > > >
> > > > On 2016ë 11ì 09ì 10:33, Chanwoo Choi wrote:
> > > > > On 2016ë 11ì 09ì 05:52, Saravana Kannan wrote:
> > > > > > 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.
> > > > > If governor is not present during devfreq_add_device(),
> > > > > the devfreq instance is not able to register the devfreq framework.
> > > > > So, the user never touch the sysfs[1] to change the governor
> > > > > because the sysfs[1] is not created by devfreq framework.
> > > > > [1] /sys/class/devfreq/[device name]/governor
> > > > >
> > > > > In summary, if governor is not present during devfreq_add_device(),
> > > > > the devfreq framework doesn't create tye sysfs[1] for governor.
> > > > >
> > > > > The user-space never change the governor through sysfs[1]
> > > > > because there is no any sysfs entry[1].
> > > > I checked the code of devfreq_add_device().
> > > > As you mentioned, if there is no governor,
> > > > devfreq_add_device() don't pass wihtout calling the devfreq->governor->even_handler().
> > > Wrong expression / don't pass -> pass
> > I think it's correct as is. Just because the governor isn't there (or hasn't registered yet) doesn't mean the device add should fail.
> >
> > That aside, do you care to ack this patch for the way the code is currently?
> Reviewed-by: Chanwoo Choi <cw00.choi@xxxxxxxxxxx>
>
> After fixing the bug of devfreq_add_device(),
> I'll remove the unneeded 'if statement' of prev_governor in governor_store.

It's been 2 years and this patch still hasn't been picked up. Can we 
please pick this up and get this into the next kernel release?

Thanks,

Saravana

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