Re: [PATCH V2] cpufreq: Disallow ->resolve_freq() for drivers providing ->target_index()

[Steve Muckle]

On Fri, Jul 22, 2016 at 01:53:13AM +0200, Rafael J. Wysocki wrote:
> On Fri, Jul 22, 2016 at 1:45 AM, Steve Muckle <steve.muckle@xxxxxxxxxx> wrote:
> > On Fri, Jul 22, 2016 at 01:32:00AM +0200, Rafael J. Wysocki wrote:
> >> On Fri, Jul 22, 2016 at 1:22 AM, Steve Muckle <steve.muckle@xxxxxxxxxx> wrote:
> >> > On Fri, Jul 22, 2016 at 01:22:22AM +0200, Rafael J. Wysocki wrote:
> >> >> OK, applied.
> >> >
> >> > FWIW I do have a concern on this patch, I think it adds unnecessary
> >> > overhead.
> >>
> >> It isn't unnecessary.  It prevents an otherwise possible kernel crash
> >> from happening.
> >
> > The logic may not be unecessary, but the overhead is. The crash could be
> > prevented in a way that doesn't require repeatedly checking a pointer
> > that doesn't change.
> 
> Well, you had the ->resolve_freq check in your patch, didn't you?
> 
> Viresh simply added a ->target_index check to it.
> 
> Now, you can argue that this is one check too many, but as long as
> drivers are allowed to implement ->target without implementing
> ->resolve_freq, the *number* of checks in this routine cannot be
> reduced.
> 
> There are three possible cases and two checks are required to
> determine which case really takes place.

My thinking was that one of these two would be preferable:

- Forcing ->target() drivers to install a ->resolve_freq callback,
  enforcing this at cpufreq driver init time. My understanding is
  ->target() drivers are deprecated anyway and theren't aren't many of
  them, though I don't know offhand exactly how many or how hard it
  would be to do for each one.

- Forcing callers (schedutil in this case) to check that either
  ->target() or ->resolve_freq() is implemented. It means
  catching and scrutinizing future callers of resolve_freq.

But even if one of these is better than it could always be done on top
of this patch I suppose. I'm also not familiar with the platforms that use
->target() style drivers. So strictly speaking for my purposes it won't
matter since the number of tests is the same for them.