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Date:   Mon, 6 Aug 2018 12:21:11 -0700
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Chanwoo Choi <cw00.choi@samsung.com>
Cc:     MyungJoo Ham <myungjoo.ham@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>, linux-pm@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        Brian Norris <briannorris@chromium.org>,
        Douglas Anderson <dianders@chromium.org>,
        Enric Balletbo i Serra <enric.balletbo@collabora.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Lee Jones <lee.jones@linaro.org>,
        Benson Leung <bleung@chromium.org>,
        Olof Johansson <olof@lixom.net>
Subject: Re: [PATCH v5 05/12] PM / devfreq: Add support for policy notifiers
Message-ID: <20180806192111.GB160295@google.com>
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Hi Chanwoo,

On Fri, Aug 03, 2018 at 09:14:46AM +0900, Chanwoo Choi wrote:
> Hi Matthias,
> 
> On 2018년 08월 03일 08:48, Matthias Kaehlcke wrote:
> > On Thu, Aug 02, 2018 at 04:13:43PM -0700, Matthias Kaehlcke wrote:
> >> Hi Chanwoo,
> >>
> >> On Thu, Aug 02, 2018 at 10:58:59AM +0900, Chanwoo Choi wrote:
> >>> Hi Matthias,
> >>>
> >>> On 2018년 08월 02일 02:08, Matthias Kaehlcke wrote:
> >>>> Hi Chanwoo,
> >>>>
> >>>> On Wed, Aug 01, 2018 at 10:22:16AM +0900, Chanwoo Choi wrote:
> >>>>> On 2018년 08월 01일 04:39, Matthias Kaehlcke wrote:
> >>>>>> On Mon, Jul 16, 2018 at 10:50:50AM -0700, Matthias Kaehlcke wrote:
> >>>>>>> On Thu, Jul 12, 2018 at 05:44:33PM +0900, Chanwoo Choi wrote:
> >>>>>>>> Hi Matthias,
> >>>>>>>>
> >>>>>>>> On 2018년 07월 07일 02:53, Matthias Kaehlcke wrote:
> >>>>>>>>> Hi Chanwoo,
> >>>>>>>>>
> >>>>>>>>> On Wed, Jul 04, 2018 at 03:41:46PM +0900, Chanwoo Choi wrote:
> >>>>>>>>>
> >>>>>>>>>> Firstly,
> >>>>>>>>>> I'm not sure why devfreq needs the devfreq_verify_within_limits() function.
> >>>>>>>>>>
> >>>>>>>>>> devfreq already used the OPP interface as default. It means that
> >>>>>>>>>> the outside of 'drivers/devfreq' can disable/enable the frequency
> >>>>>>>>>> such as drivers/thermal/devfreq_cooling.c. Also, when some device
> >>>>>>>>>> drivers disable/enable the specific frequency, the devfreq core
> >>>>>>>>>> consider them.
> >>>>>>>>>>
> >>>>>>>>>> So, devfreq doesn't need to devfreq_verify_within_limits() because
> >>>>>>>>>> already support some interface to change the minimum/maximum frequency
> >>>>>>>>>> of devfreq device. 
> >>>>>>>>>>
> >>>>>>>>>> In case of cpufreq subsystem, cpufreq only provides 'cpufreq_verify_with_limits()'
> >>>>>>>>>> to change the minimum/maximum frequency of cpu. some device driver cannot
> >>>>>>>>>> change the minimum/maximum frequency through OPP interface.
> >>>>>>>>>>
> >>>>>>>>>> But, in case of devfreq subsystem, as I explained already, devfreq support
> >>>>>>>>>> the OPP interface as default way. devfreq subsystem doesn't need to add
> >>>>>>>>>> other way to change the minimum/maximum frequency.
> >>>>>>>>>
> >>>>>>>>> Using the OPP interface exclusively works as long as a
> >>>>>>>>> enabling/disabling of OPPs is limited to a single driver
> >>>>>>>>> (drivers/thermal/devfreq_cooling.c). When multiple drivers are
> >>>>>>>>> involved you need a way to resolve conflicts, that's the purpose of
> >>>>>>>>> devfreq_verify_within_limits(). Please let me know if there are
> >>>>>>>>> existing mechanisms for conflict resolution that I overlooked.
> >>>>>>>>>
> >>>>>>>>> Possibly drivers/thermal/devfreq_cooling.c could be migrated to use
> >>>>>>>>> devfreq_verify_within_limits() instead of the OPP interface if
> >>>>>>>>> desired, however this seems beyond the scope of this series.
> >>>>>>>>
> >>>>>>>> Actually, if we uses this approach, it doesn't support the multiple drivers too.
> >>>>>>>> If non throttler drivers uses devfreq_verify_within_limits(), the conflict
> >>>>>>>> happen.
> >>>>>>>
> >>>>>>> As long as drivers limit the max freq there is no conflict, the lowest
> >>>>>>> max freq wins. I expect this to be the usual case, apparently it
> >>>>>>> worked for cpufreq for 10+ years.
> >>>>>>>
> >>>>>>> However it is correct that there would be a conflict if a driver
> >>>>>>> requests a min freq that is higher than the max freq requested by
> >>>>>>> another. In this case devfreq_verify_within_limits() resolves the
> >>>>>>> conflict by raising p->max to the min freq. Not sure if this is
> >>>>>>> something that would ever occur in practice though.
> >>>>>>>
> >>>>>>> If we are really concerned about this case it would also be an option
> >>>>>>> to limit the adjustment to the max frequency.
> >>>>>>>
> >>>>>>>> To resolve the conflict for multiple device driver, maybe OPP interface
> >>>>>>>> have to support 'usage_count' such as clk_enable/disable().
> >>>>>>>
> >>>>>>> This would require supporting negative usage count values, since a OPP
> >>>>>>> should not be enabled if e.g. thermal enables it but the throttler
> >>>>>>> disabled it or viceversa.
> >>>>>>>
> >>>>>>> Theoretically there could also be conflicts, like one driver disabling
> >>>>>>> the higher OPPs and another the lower ones, with the outcome of all
> >>>>>>> OPPs being disabled, which would be a more drastic conflict resolution
> >>>>>>> than that of devfreq_verify_within_limits().
> >>>>>>>
> >>>>>>> Viresh, what do you think about an OPP usage count?
> >>>>>>
> >>>>>> Ping, can we try to reach a conclusion on this or at least keep the
> >>>>>> discussion going?
> >>>>>>
> >>>>>> Not that it matters, but my preferred solution continues to be
> >>>>>> devfreq_verify_within_limits(). It solves conflicts in some way (which
> >>>>>> could be adjusted if needed) and has proven to work in practice for
> >>>>>> 10+ years in a very similar sub-system.
> >>>>>
> >>>>> It is not true. Current cpufreq subsystem doesn't support external OPP
> >>>>> control to enable/disable the OPP entry. If some device driver
> >>>>> controls the OPP entry of cpufreq driver with opp_disable/enable(),
> >>>>> the operation is not working. Because cpufreq considers the limit
> >>>>> through 'cpufreq_verify_with_limits()' only.
> >>>>
> >>>> Ok, we can probably agree that using cpufreq_verify_with_limits()
> >>>> exclusively seems to have worked well for cpufreq, and that in their
> >>>> overall purpose cpufreq and devfreq are similar subsystems.
> >>>>
> >>>> The current throttler series with devfreq_verify_within_limits() takes
> >>>> the enabled OPPs into account, the lowest and highest OPP are used as
> >>>> a starting point for the frequency adjustment and (in theory) the
> >>>> frequency range should only be narrowed by
> >>>> devfreq_verify_within_limits().
> >>>>
> >>>>> As I already commented[1], there is different between cpufreq and devfreq.
> >>>>> [1] https://lkml.org/lkml/2018/7/4/80
> >>>>>
> >>>>> Already, subsystem already used OPP interface in order to control
> >>>>> specific OPP entry. I don't want to provide two outside method
> >>>>> to control the frequency of devfreq driver. It might make the confusion.
> >>>>
> >>>> I understand your point, it would indeed be preferable to have a
> >>>> single method. However I'm not convinced that the OPP interface is
> >>>> a suitable solution, as I exposed earlier in this thread (quoted
> >>>> below).
> >>>>
> >>>> I would like you to at least consider the possibility of changing
> >>>> drivers/thermal/devfreq_cooling.c to devfreq_verify_within_limits().
> >>>> Besides that it's not what is currently used, do you see any technical
> >>>> concerns that would make devfreq_verify_within_limits() an unsuitable
> >>>> or inferior solution?
> >>>
> >>> As we already discussed, devfreq_verify_within_limits() doesn't support
> >>> the multiple outside controllers (e.g., devfreq-cooling.c).
> >>
> >> That's incorrect, its purpose is precisely that.
> >>
> >> Are you suggesting that cpufreq with its use of
> >> cpufreq_verify_within_limits() (the inspiration for
> >> devfreq_verify_within_limits()) is broken? It is used by cpu_cooling.c
> >> and other drivers when receiving a CPUFREQ_ADJUST event, essentially
> >> what I am proposing with DEVFREQ_ADJUST.
> >>
> >> Could you elaborate why this model wouldn't work for devfreq? "OPP
> >> interface is mandatory for devfreq" isn't really a technical argument,
> >> is it mandatory for any other reason than that it is the interface
> >> that is currently used?
> >>
> >>> After you are suggesting the throttler core, there are at least two
> >>> outside controllers (e.g., devfreq-cooling.c and throttler driver).
> >>> As I knew the problem about conflict, I cannot agree the temporary
> >>> method. OPP interface is mandatory for devfreq in order to control
> >>> the OPP (frequency/voltage). In this situation, we have to try to
> >>> find the method through OPP interface.
> >>
> >> What do you mean with "temporary method"?
> >>
> >> We can try to find a method through the OPP interface, but at this
> >> point I'm not convinced that it is technically necessary or even
> >> preferable.
> >>
> >> Another inconvenient of the OPP approach for both devfreq-cooling.c
> >> and the throttler is that they have to bother with disabling all OPPs
> >> above/below the max/min (they don't/shouldn't have to care), instead
> >> of just telling devfreq the max/min.
> > 
> > And a more important one: both drivers now have to keep track which
> > OPPs they enabled/disabled previously, done are the days of a simple
> > dev_pm_opp_enable/disable() in devfreq_cooling. Certainly it is
> > possible and not very complex to implement, but is it really the
> > best/a good solution?
> 
> 
> As I replied them right before, Each outside driver has their own throttling
> policy to control OPP entries. They don't care the requirement of other
> driver and cannot know the requirement of other driver. devfreq core can only
> recognize them and then only consider enabled OPP entris without disabled OPP entries.
> 
> For example1,
>        | devfreq-cooling| throttler
> ---------------------------------------
> 500Mhz | disabled       | disabled
> 400Mhz | disabled       | disabled
> 300Mhz |                | disabled
> 200Mhz |                |
> 100Mhz |                |
> => devfreq driver can use only 100/200Mhz
> 
> 
> For example2,
>        | devfreq-cooling| throttler
> ---------------------------------------
> 500Mhz | disabled       | disabled
> 400Mhz | disabled       |
> 300Mhz | disabled       |
> 200Mhz |                |
> 100Mhz |                |
> => devfreq driver can use only 100/200Mhz
> 
> 
> For example3,
>        | devfreq-cooling| throttler
> ---------------------------------------
> 500Mhz | disabled       | disabled
> 400Mhz |                |
> 300Mhz |                |
> 200Mhz |                | disabled
> 100Mhz |                | disabled
> => devfreq driver can use only 300/400Mhz

These are all cases without conflicts, my concern is about this:

>        | devfreq-cooling| throttler
> ---------------------------------------
> 500Mhz | disabled       |
> 400Mhz | disabled       |
> 300Mhz |                | disabled
> 200Mhz |                | disabled
> 100Mhz |                | disabled
> => devfreq driver can't use any frequency?

Actually my above comment wasn't about this case, but about the
added complexity in devfreq-cooling.c and the throttler:

A bit simplified partition_enable_opps() currently does this:

for_each_opp(opp) {
  if (opp->freq <= max)
     opp_enable(opp)
  else
     opp_disable(opp)
}

With the OPP usage/disable count this doesn't work any longer. Now we
need to keep track of the enabled/disabled state of the OPP, something
like:

dev_pm_opp_enable(opp) {
  if (opp->freq <= max) {
    if (opp->freq > prev_max)
      opp_enable(opp)
  } else {
    if (opp->freq < prev_max)
      opp_disable(opp)
  }
}

And duplicate the same in the throttler (and other possible
drivers). Obviously it can be done, but is there really any gain
from it?

Instead they just could do:

devfreq_verify_within_limits(policy/freq_pair, 0, max_freq)

without being concerned about implementation details of devfreq.

Thanks

Matthias