Hey,

On Thursday 09 Jul 2020 at 16:10:48 (+0530), Viresh Kumar wrote:
[..]

quoted

I agree that this happening is a cornercase and a reason for which
cpufreq_get_hw_max_freq() was made weak. If some platform has entirely
firmware driven frequency control, but it enables CONFIG_CPU_FREQ
(as is the default) and it defines its own cpufreq_get_hw_max_freq(),
it could benefit from AMU use.

So I did believe it was best for these checks to be decoupled, for this
reason, and potential other reasons in the future, involving more
decoupling from cpufreq.

I do have code in progress to clean the overall interaction between
cpufreq and AMUs, started at [1]. Bear with me on this, it is all
connected :).

Of course I missed few things here.

- I didn't realize that cpufreq_get_hw_max_freq() is defined weak :(

  I understand that we want to support everything that is possible,
  but there is no need to support cases which we may never have
  actually. We have seen code going in the kernel, which no one ever
  ends up using.

  Do we see a case in near future where someone is going to override
  this weak implementation ? If we don't have an actual target for it
  at the moment, then we should probably remove the weak attribute and
  simplify the code.

I saw this case during FVP testing, although I acknowledge the 'virtual'
part of that platform [1]. But allowing this does enable AMU testing on
an AEM FVP.

While I completely understand the reasoning behind avoiding to introduce
large changes for small corner-case gains, the arguments for this
support was:
 - (1) AMUs are a new feature and it will take some time until we see the
   real usecases. That's always the case with early support for a
   feature - we want to add it early to enable its use and testing, but
   it will take some time to establish the true usecases.
 - (2) It literally needed 2 lines of code + the weak cpufreq function
   to support this.

- I understood earlier that, we don't pick up AMU support unless all
  CPUs of a policy are supported by AMUs, but forgot that later while
  writing the patch. What is the thing with AMUs? Why would some
  platform add it only for some CPUs out of a policy ? Do we have such
  platforms already or in queue ?

Given that I can't guarantee what hardware will or won't do, and given
that AMUs are an optional feature, I controlled the only thing I could:
the software :). By not making assumptions about the hardware, I ensured
that the code does not break the interaction between cpufreq use or AMU
use for frequency invariance.

This will be nicer in the new code as the control will be at CPU level,
rather than policy level.

[1]
https://developer.arm.com/tools-and-software/simulation-models/fixed-virtual-platforms

Regards,
Ionela.

Lets discuss more after we have settled on the first point here.

Thanks for review Ionela.

-- 
viresh

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