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