linux-kernel - Re: [RFC PATCH 3/3] opp: Power on (virtual) power domains managed by the OPP core

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Date:   Mon, 24 Aug 2020 16:57:44 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Nishanth Menon <nm@...com>, Stephen Boyd <sboyd@...nel.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Niklas Cassel <nks@...wful.org>
Subject: Re: [RFC PATCH 3/3] opp: Power on (virtual) power domains managed by
 the OPP core

On 30-07-20, 10:01, Stephan Gerhold wrote:
> dev_pm_opp_attach_genpd() allows attaching an arbitrary number of
> power domains to an OPP table. In that case, the genpd core will
> create a virtual device for each of the power domains.
> 
> At the moment, the OPP core only calls
> dev_pm_genpd_set_performance_state() on these virtual devices.
> It does not attempt to power on the power domains. Therefore
> the required power domain might never get turned on.
> 
> So far, dev_pm_opp_attach_genpd() is only used in qcom-cpufreq-nvmem.c
> to attach the CPR power domain to the CPU OPP table. The CPR driver
> does not check if it was actually powered on so this did not cause
> any problems. However, other drivers (e.g. rpmpd) might ignore the
> performance state until the power domain is actually powered on.
> 
> Since these virtual devices are managed exclusively by the OPP core,
> I would say that it should also be responsible to ensure they are
> enabled. A similar approach is already used for regulators, see
> commit 8d45719caaf5 ("opp: core: add regulators enable and disable").
> 
> This commit implements similar functionality for the virtual genpd
> devices managed by the OPP core. The power domains are turned on
> the first time dev_pm_opp_set_rate() is called. They are turned off
> again when dev_pm_opp_set_rate(dev, 0) is called.
> 
> Signed-off-by: Stephan Gerhold <stephan@...hold.net>
> ---
> Related discussion: https://lore.kernel.org/linux-arm-msm/20200426123140.GA190483@gerhold.net/
> 
> There would be also other ways to implement this, e.g. device links,
> assuming that the device using the OPP table also makes use of runtime PM.
> My first thought was that it would be most consistent to handle this like
> regulators, bandwidth votes etc. RFC :)

This stuff was done ages back and I am starting to forget almost
everything now :)

Ulf, why doesn't pm_runtime_get(dev) take care of enabling multiple
power domain case ? RFP (request for patience) :)

-- 
viresh