linux-kernel - Re: [PATCH v2 RESEND] PM / devfreq: add PM QoS support

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Message-Id: <201203042330.34083.rjw@sisk.pl>
Date:	Sun, 4 Mar 2012 23:30:33 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Turquette, Mike" <mturquette@...com>
Cc:	MyungJoo Ham <myungjoo.ham@...sung.com>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Kevin Hilman <khilman@...com>,
	mark gross <markgross@...gnar.org>, myungjoo.ham@...il.com,
	Jean Pihet <jean.pihet@...oldbits.com>
Subject: Re: [PATCH v2 RESEND] PM / devfreq: add PM QoS support

On Wednesday, February 29, 2012, Turquette, Mike wrote:
> On Wed, Feb 29, 2012 at 1:43 AM, MyungJoo Ham <myungjoo.ham@...sung.com> wrote:
> > +       /* Check the sanity of qos_list/qos_type */
> > +       if (profile->qos_type || profile->qos_list) {
> > +               switch (profile->qos_type) {
> > +               case PM_QOS_CPU_DMA_LATENCY:
> > +               case PM_QOS_NETWORK_LATENCY:
> > +                       devfreq->qos_use_max = false;
> > +                       break;
> > +               case PM_QOS_NETWORK_THROUGHPUT:
> > +                       devfreq->qos_use_max = true;
> > +                       break;
> 
> Hello MyungJoo!
> 
> I see that you re-using the same old PM QoS handles in this
> implementation.  Do you feel this is the right way to do it?  Your
> example of using DMA for multimedia devices (given in the changelog)
> has nothing to do with network throughput, yet that constraint-type is
> used here.
> 
> I wonder if a better solution than overloading these classifications exist.
> 
> Just to toss around ideas, what about having per-device PM QoS
> throughput constraints which are generalized (e.g., not tied to a
> concept such as "network").  I've Cc'd Jean Pihet (yet again) who has
> some good experience making PM QoS-type interfaces work on a
> per-device basis.
> 
> I wonder, ultimately, if instead of feeding QoS constraints into
> devfreq if a better design might be to have devfreq feed input into a
> greater QoS framework.  E.g:
> 
> A scalable bus used by many devices might have two different device
> drivers that want to call pm_qos_device_tput(...), and also the
> devfreq driver for that bus also calls pm_qos_device_tput(...).  So
> essentially there are three points in the code where inputs can be
> driven into one common per-device QoS layer for the generic concept of
> "device throughput".  This way devfreq support is not a prerequisite
> for scaling a device in a generic way, but a nice framework for
> devices which can monitor their own activity level, built on top of a
> per-device pm qos layer.
> 
> Thoughts?

I agree with the general idea, definitely would prefer it to what is
currently being proposed.

Thanks,
Rafael
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