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 -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
