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

From: Viresh Kumar <viresh.kumar@linaro.org>
To: Stephan Gerhold <stephan@gerhold.net>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Kevin Hilman <khilman@kernel.org>,
	Ulf Hansson <ulf.hansson@linaro.org>, Nishanth Menon <nm@ti.com>,
	Stephen Boyd <sboyd@kernel.org>,
	linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Niklas Cassel <nks@flawful.org>
Subject: Re: [RFC PATCH 3/3] opp: Power on (virtual) power domains managed by the OPP core
Date: Mon, 24 Aug 2020 16:57:44 +0530	[thread overview]
Message-ID: <20200824112744.jsyaxrfbybyjpwex@vireshk-i7> (raw)
In-Reply-To: <20200730080146.25185-4-stephan@gerhold.net>

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@gerhold.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