Re: [PATCH v3 1/3] PM: domains: Make set_performance_state() callback optional

[Viresh Kumar]

On 18-01-21, 13:46, Ulf Hansson wrote:
> You seem to be worried about latency/overhead while doing the
> propagation upwards in the hierarchy. That seems like a reasonable
> concern to me, especially as the genpd lock is taken at each level.

I am not sure how many levels of domains we have normally, but unless the number
is big it won't be a very big problem.

> However, to mitigate this can be rather messy. In principle, we would
> need to walk the hierarchy upwards, each time a new subdomain is added
> in genpd_add_subdomain(). While doing this, we would also need to keep
> track on what level we set to continue the propagation of the
> performance states for. Even if this can be done in non-latency
> sensitive paths, I don't think it's worth it because of complexity (I
> haven't even thought of what happens when removing a subdomain).

What about a new field in the domain structure like 'can-handle-pstates', and
then whenever sub-domain gets added it just needs to check its parent's field
and set his own.

> So, maybe we should simply just stick to the existing code, forcing
> the parent to have a ->set_performance() callback assigned if
> propagation should continue?

I think it would be better to fix the issue even if we aren't fully optimized
and making the change to make sure we keep propagating is rather important.

-- 
viresh