Date: Mon, 27 Mar 2017 18:50:11 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Juri Lelli <juri.lelli@arm.com>
Cc: mingo@redhat.com, rjw@rjwysocki.net, viresh.kumar@linaro.org,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        tglx@linutronix.de, vincent.guittot@linaro.org, rostedt@goodmis.org,
        luca.abeni@santannapisa.it, claudio@evidence.eu.com,
        tommaso.cucinotta@santannapisa.it, bristot@redhat.com,
        mathieu.poirier@linaro.org, tkjos@android.com, joelaf@google.com,
        andresoportus@google.com, morten.rasmussen@arm.com,
        dietmar.eggemann@arm.com, patrick.bellasi@arm.com,
        Ingo Molnar <mingo@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Mark Brown <broonie@kernel.org>
Subject: Re: [RFD PATCH 3/5] sched/cpufreq_schedutil: make worker kthread be
 SCHED_DEADLINE
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On Fri, Mar 24, 2017 at 02:08:58PM +0000, Juri Lelli wrote:
> Worker kthread needs to be able to change frequency for all other
> threads.
> 
> Make it special, just under STOP class.

*yuck* ;-)

So imagine our I2C/SPI bus is 'busy' and its mutex taken, then this
'soecial' task will need to boost it. Now add BWI to your thinking and
shudder.


On IRC broonie mentioned that:

 - most PMIC operations are fire and forget (no need to wait for a
   response).
 - PMIC 'packets' are 'small'.
 - SPI has the possibility to push stuff on the queue.

Taken together this seems to suggest we can rework cpufreq drivers to
function in-context, either directly push the packet on the bus if
available, or queue it and let whoever owns it sort it without blocking.

It might be possible to rework/augment I2C to also support pushing stuff
on a queue.


So if we can make all that work, we can do away with this horrible
horrible kthread. Which is, IMO, a much better solution.

Thoughts?