Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Fri, 22 Jul 2016 02:18:54 +0200 | Subject | Re: [PATCH V2] cpufreq: Disallow ->resolve_freq() for drivers providing ->target_index() |
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On Fri, Jul 22, 2016 at 2:09 AM, Steve Muckle <steve.muckle@linaro.org> wrote: > 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@linaro.org> 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@linaro.org> 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.
That would have been possible, but your series didn't do that.
> My understanding is > ->target() drivers are deprecated anyway
No, they aren't.
There simply are cases in which frequency tables are not workable (like the ACPI CPPC one).
> 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 that doesn't reduce the number of checks in cpufreq_driver_resolve_freq().
There still are three choices in there: return a frequency from the table (if present), or call ->resolve_freq (if implemented), or return target_freq (as the last resort).
> But even if one of these is better than it could always be done on top > of this patch I suppose.
Right.
> 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.
OK
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