linux-kernel - Re: [PATCH V2 5/5] PM / Domains: Propagate performance state updates

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Date:   Fri, 30 Nov 2018 16:36:50 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>, Pavel Machek <pavel@....cz>,
        Len Brown <len.brown@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Niklas Cassel <niklas.cassel@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 5/5] PM / Domains: Propagate performance state updates

On 30-11-18, 11:18, Ulf Hansson wrote:
> On Fri, 30 Nov 2018 at 10:59, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > Sure, but the ordering of locks is always subdomain first and then master.
> > Considering the case of Qcom, we have two domains Cx (sub-domain) and Mx (master).
> >
> > On first genpd_power_on(Cx) call, we will first call genpd_power_on(Mx) which
> > will just power it on as none of its master will have perf-state support. We
> > then call _genpd_power_on(Cx) which will also not do anything with Mx as its own
> > (Cx's) pstate would be 0 at that time. But even if it had a valid value, it will
> > propagate just fine with all proper locking in place.
> 
> Can you explain that, it's not super easy to follow the flow.

Sorry, I somehow assumed you would know it already :)

> So what will happen if Cx has a value that needs to be propagated?
> What locks will be taken, and in what order?
> 
> Following, what if we had a Bx domain, being the subdomain of Cx, and
> it too had a value that needs to be propagated.

Lets take the worst example, we have Bx (sub-domain of Cx), Cx (sub-domain of
Mx) and Dx (master). Normal power-on/off will always have the values 0, so lets
consider resume sequence where all the domains will have a value pstate value.
And please forgive me for any bugs I have introduced in the following
super-complex sequence :)

genpd_runtime_resume(dev) //domain Bx
    -> genpd_lock(Bx)
    -> genpd_power_on(Bx)
 
        -> genpd_lock(Cx)
        -> genpd_power_on(Cx)
 
            -> genpd_lock(Dx)
            -> genpd_power_on(Dx)
 
                -> _genpd_power_on(Dx)
                    -> _genpd_set_performance_state(Dx, Dxstate) {
                        //Doesn't have any masters
                        -> genpd->set_performance_state(Dx, Dxstate);
                    }
 
            -> genpd_unlock(Dx)
 
            -> _genpd_power_on(Cx)
                -> _genpd_set_performance_state(Cx, Cxstate) {
                    //have one master, Dx
                    -> genpd_lock(Dx)
                        -> _genpd_set_performance_state(Dx, Dxstate) {
                            //Doesn't have any masters
                            -> genpd->set_performance_state(Dx, Dxstate);
                        }
 
                    -> genpd_unlock(Dx)
 
                    // Change Cx state
                    -> genpd->set_performance_state(Cx, Cxstate);
                }
 
        -> genpd_unlock(Cx)
 
        -> _genpd_power_on(Bx)
            -> _genpd_set_performance_state(Bx, Bxstate) {
                //have one master, Cx
                -> genpd_lock(Cx)
                -> _genpd_set_performance_state(Cx, Cxstate) {
                    //have one master, Dx
                    -> genpd_lock(Dx)
                        -> _genpd_set_performance_state(Dx, Dxstate) {
                            //Doesn't have any masters
                            -> genpd->set_performance_state(Dx, Dxstate);
                        }
 
                    -> genpd_unlock(Dx)
 
                    // Change Cx state
                    -> genpd->set_performance_state(Cx, Cxstate);
                }
                -> genpd_unlock(Cx)
 
                -> genpd->set_performance_state(Bx, Bxstate);
            }
 
    -> genpd_unlock(Bx)



> It sounds like we will
> do the propagation one time per level. Is that really necessary,
> couldn't we just do it once, after the power on sequence have been
> completed?

It will be a BIG hack somewhere, isn't it ? How will we know when has the time
come to shoot the final sequence of set_performance_state() ? And where will we
do it? genpd_runtime_resume() ? And then we will have more problems, for example
Rajendra earlier compared this stuff to clk framework where it is possible to do
clk_set_rate() first and then only call clk_enable() and the same should be
possible with genpd as well, i.e. set performance state first and then only
enable the device/domain. And so we need this right within genpd_power_on().

I know things are repetitive here, but that's the right way of doing it IMHO.
What do you say ?

-- 
viresh