Re: Re: [PATCH] media: staging: tegra-vde: fix runtime pm imbalance on error

[Rafael J. Wysocki]

On Thu, May 28, 2020 at 2:08 PM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>
> On Fri, May 22, 2020 at 04:43:12PM +0200, Thierry Reding wrote:
> > On Fri, May 22, 2020 at 04:23:18PM +0300, Dan Carpenter wrote:
> > > On Fri, May 22, 2020 at 03:10:31PM +0200, Thierry Reding wrote:
> > > > On Thu, May 21, 2020 at 08:39:02PM +0300, Dan Carpenter wrote:
> > > > > On Thu, May 21, 2020 at 05:22:05PM +0200, Rafael J. Wysocki wrote:
> > > > > > On Thu, May 21, 2020 at 11:15 AM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Thu, May 21, 2020 at 11:42:55AM +0800, dinghao.liu@xxxxxxxxxx wrote:
> > > > > > > > Hi, Dan,
> > > > > > > >
> > > > > > > > I agree the best solution is to fix __pm_runtime_resume(). But there are also
> > > > > > > > many cases that assume pm_runtime_get_sync() will change PM usage
> > > > > > > > counter on error. According to my static analysis results, the number of these
> > > > > > > > "right" cases are larger. Adjusting __pm_runtime_resume() directly will introduce
> > > > > > > > more new bugs. Therefore I think we should resolve the "bug" cases individually.
> > > > > > > >
> > > > > > >
> > > > > > > That's why I was saying that we may need to introduce a new replacement
> > > > > > > function for pm_runtime_get_sync() that works as expected.
> > > > > > >
> > > > > > > There is no reason why we have to live with the old behavior.
> > > > > >
> > > > > > What exactly do you mean by "the old behavior"?
> > > > >
> > > > > I'm suggesting we leave pm_runtime_get_sync() alone but we add a new
> > > > > function which called pm_runtime_get_sync_resume() which does something
> > > > > like this:
> > > > >
> > > > > static inline int pm_runtime_get_sync_resume(struct device *dev)
> > > > > {
> > > > >         int ret;
> > > > >
> > > > >         ret = __pm_runtime_resume(dev, RPM_GET_PUT);
> > > > >         if (ret < 0) {
> > > > >                 pm_runtime_put(dev);
> > > > >                 return ret;
> > > > >         }
> > > > >         return 0;
> > > > > }
> > > > >
> > > > > I'm not sure if pm_runtime_put() is the correct thing to do?  The other
> > > > > thing is that this always returns zero on success.  I don't know that
> > > > > drivers ever care to differentiate between one and zero returns.
> > > > >
> > > > > Then if any of the caller expect that behavior we update them to use the
> > > > > new function.
> > > >
> > > > Does that really have many benefits, though? I understand that this
> > > > would perhaps be easier to use because it is more in line with how other
> > > > functions operate. On the other hand, in some cases you may want to call
> > > > a different version of pm_runtime_put() on failure, as discussed in
> > > > other threads.
> > >
> > > I wasn't CC'd on the other threads so I don't know.  :/
> >
> > It was actually earlier in this thread, see here for example:
> >
> >       http://patchwork.ozlabs.org/project/linux-tegra/patch/20200520095148.10995-1-dinghao.liu@xxxxxxxxxx/#2438776
>
> I'm not seeing what you're talking about.
>
> The only thing I see in this thread is that we don't want to call
> pm_runtime_mark_last_busy(dev) which updates the last_busy time that is
> used for autosuspend.

That shouldn't be a problem, though, because if pm_runtime_get_sync()
returns an error, PM-runtime is not going to work for this device
until it is explicitly disabled for it and fixed up.

> The other thing that was discussed was pm_runtime_put_noidle() vs
> pm_runtime_put_autosuspend().  "The pm_runtime_put_noidle() should have
> the same effect as yours variant".  So apparently they are equivalent
> in this situation.  How should we choose one vs the other?

The point is that pm_runtime_put_noidle() is *sufficient* to drop the
reference and nothing more is needed in the error path.

So you can always do something like this:

ret = pm_runtime_get_sync(dev);
if (ret < 0) {
        pm_runtime_put_noidle(dev);
        return ret;
}

However, it would not be a bug to do something like this:

        ret = pm_runtime_get_sync(dev);
        if (ret < 0)
                goto rpm_put;

        ...

rpm_put:
        pm_runtime_put_autosuspend(dev);

> I'm not trying to be obtuse.  I understand that probably if I worked in
> PM then I wouldn't need documentation...  :/

So Documentation/power/runtime_pm.rst says this:

  `int pm_runtime_get_sync(struct device *dev);`
    - increment the device's usage counter, run pm_runtime_resume(dev) and
      return its result

In particular, it doesn't say "decrement the device's usage counter on
errors returned by pm_runtime_resume(dev)", so I'm not sure where that
expectation comes from.