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

[Stephan Gerhold]

On Mon, Aug 24, 2020 at 04:57:44PM +0530, Viresh Kumar wrote:
> 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@xxxxxxxxxxx>
> > ---
> > Related discussion: https://lore.kernel.org/linux-arm-msm/20200426123140.GA190483@xxxxxxxxxxx/
> > 
> > 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) :)
> 

So I'm really not an expert for power domains, but here is my
understanding:

We attach the power domains in dev_pm_opp_attach_genpd(opp_dev, names),
where opp_dev is the device the OPP table belongs to.

To do that, the genpd core creates a set of virtual devices. These
virtual devices are not related to opp_dev in any way. Therefore, the
power domains stay off until we run pm_runtime_get(virt_dev) for each of
the virtual devices. (Which is what is implemented in this patch...)

If I understand correctly, what you would like to do is to have a single
pm_runtime_get(opp_dev) call also enable all the virtual devices?

As far as I understand, this can be done by adding "device links"
between opp_dev and the virtual devices, e.g.

	device_link_add(opp_dev, virt_dev, DL_FLAG_PM_RUNTIME);

for each of the virtual devices.

But the problem with that approach is that it assumes that someone
actually calls pm_runtime_get(opp_dev), i.e. we assume that opp_dev is
managed by runtime PM. As far as I know, this isn't the case for the CPU
OPP table for example.

Maybe Ulf can correct me if I'm wrong :)

Thanks!
Stephan