linux-kernel - Re: [PATCH v2] opp: Power on (virtual) power domains managed by the OPP core

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Date:   Tue, 1 Sep 2020 11:30:09 +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: [PATCH v2] opp: Power on (virtual) power domains managed by the
 OPP core

On 31-08-20, 17:49, Stephan Gerhold wrote:
> I appreciate it, thank you! But actually after our discussion regarding
> the "manage multiple power domains which might not always need to be on"
> use case I would like to explore that a bit further before we decide on
> a final solution that complicates changes later.
> 
> The reason I mention this is that after our discussion I have been
> (again) staring at the vendor implementation of CPU frequency scaling of
> the platform I'm working on (qcom msm8916). Actually there seems to be yet
> another power domain that is scaled only for some CPU frequencies within
> the clock driver. (The vendor driver implies this is a requirement of
> the PLL clock that is used for higher CPU frequencies, but not sure...)
> 
> I don't fully understand how to implement this in mainline yet. I have
> started some research on these "voltage requirements" for clocks, and
> based on previous discussions [1] and patches [2] it seems like I'm
> *not* supposed to add this to the clock driver, but rather as another
> required-opp to the CPU OPP table.
> 
> If that is the case, we would pretty much have a situation like you
> described, a power domain that should only be scaled for some of the
> OPPs (the higher CPU frequencies).
> 
> But first I need to do some more research, and probably discuss how to
> handle that power domain separately first. I think it would be easier if
> we postpone this patch till then. I don't think anyone else needs this
> patch at the moment.

Heh, okay, I can keep it out of my tree then :)

> [1]: https://lore.kernel.org/linux-clk/9439bd29e3ccd5424a8e9b464c8c7bd9@codeaurora.org/
> [2]: https://lore.kernel.org/linux-pm/20190320094918.20234-1-rnayak@codeaurora.org/
> 
> > Signed-off-by: Stephan Gerhold <stephan@...hold.net>
> > [ Viresh: Rearranged the code to remove extra loop and minor cleanup ]
> 
> FWIW, the reason I put this into an extra loop is that the
> dev_pm_domain_attach_by_name() might return -EPROBE_DEFER (or some other
> error) for some of the power domains. If you create the device links
> before attaching all domains you might unnecessarily turn on+off some of
> them.

Hmm, but that shouldn't have any significant side affects, right ? And
shouldn't result in some other sort of error. It makes the code look
more sensible/readable and so I would prefer to keep a single loop if
it doesn't make something not-work.

> Having it in a separate loop avoids that, because we only start powering
> on the power domains when we know that all the power domains are
> available.

-- 
viresh