Hi Matthias, On 2018년 08월 07일 09:23, Matthias Kaehlcke wrote: > Hi Chanwoo, > > On Tue, Aug 07, 2018 at 07:31:16AM +0900, Chanwoo Choi wrote: >> Hi Matthias, >> >> On 2018년 08월 07일 04:21, Matthias Kaehlcke wrote: >>> 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? >> >> There are no any enabled frequency. Because device driver >> (devfreq-cooling, throttler) disable all frequencies. >> >> Outside drivers(devfreq-cooling, throttler) can enable/disable >> specific OPP entries. As I already commented, each outside driver >> doesn't consider the policy of other device driver about OPP entries. > > And wouldn't it be preferable to have an interface that tries to avoid > this situation in the first place and has a clear policy for conflict > resolution? > >> OPP interface is independent on devfreq and just control OPP entries. >> After that, devfreq just consider the only enabled OPP entries. >> >>> >>> 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) I have a new question about using devfreq_verify_within_limits(). For example, Driver A has following opp-table - 500Mhz - 400Mhz - 300Mhz - 200Mhz - 100Mhz Basically, driver A has following init value: - policy->min is 100 - policy->max is 500 Driver B, devfreq_verify_within_limits(200, 300) policy->min is 200 policy->max is 300 Driver C, devfreq_verify_within_limits(300, 400) policy->min is 300 policy->max is 300 Driver D, devfreq_verify_within_limits(400, 500) policy->min is 400 policy->max is 400 In result, it looks like the requirement of Driver B are disappeared. is it the intention of devfreq_verify_within_limits()? >>> >>> without being concerned about implementation details of devfreq. >>> >> >> I don't think so. > > What are you referring to, the change that I claim that will be needed > in partition_enable_opps() when OPPs have usage/disable counts? If so, > how do you avoid that the function doesn't enable/disable an OPP that > was already enabled/disabled in the previous iteration? Just about changes of "dev_pm_opp_enable(opp)". > >> dev_pm_opp_enable()/dev_pm_opp_disable() have to consider only one >> OPP entry without any other OPP entry. > > I agree with this :) > >> dev_pm_opp_enable()/dev_pm_opp_disable() can never know the other >> OPP entries. After some driver(devfreq-cooling.c and throttler) >> enable or disable specific OPP entries, the remaining OPP entry >> with enabled state will be considered on devfreq driver. > > Having multiple drivers (or even a single one) enable and disable > OPPs independently and at the time of their choosing sounds like a > recipe for race conditions. > > What happens if e.g. the devfreq core calls > dev_pm_opp_find_freq_ceil/floor() and right after returning another > driver disables the OPP? devfreq uses the disabled OPP. Probably not a\ devfreq doesn't use the disabled OPP. For example, 1. devfreq-cooling.c disable/enable some OPP and OPP send notification about OPP changes. 2. devfreq receives the notification (devfreq_notifier_call() is executed) 3. devfreq_notifier_call() try to find scaling_min_freq/scaling_max_freq 4. devfreq_notifier_call() executes update_devfreq() in order to apply the OPP changes. 5. devfreq can consider only enabled frequencies right after dev_pm_opp_disable/enable() > big deal if disabling the OPP is only a semantic question, but I > imagine there can be worse scenarios. Currently the only user of > dev_pm_opp_disable() besides devfreq_cooling.c is imx6q-cpufreq.c, and > it is well behaved and only disables OPPs during probe(). imx6q-cpufreq.c used the dev_pm_opp_disable() before calling dev_pm_opp_init_cpufreq_table(). After registered cpufreq_register_driver(), imx6q-cpufreq.c doesn't use the dev_pm_opp_disable/enable(). It means that dev_pm_opp_disable() of imx6q-cpufreq.c doesn't affect the frequency choice of cpufreq on the runtime after registered cpufreq driver. On the other hand, devfreq_cooling.c use dev_pm_opp_disable/enable() on the runtime after registering devfreq driver. It affect the frequency choice of devfreq on the runtime. > > I keep missing a clear answer to the question in which sense > manipulating the OPPs in devfreq_cooling.c is superior over narrowing > down the frequency during DEVFREQ_ADJUST, which would avoid potential > races and allow to resolve conflicts. Does it allow for some You mentioned the race conditions eariler. Actually, I don't know the potential races. > functionality that couldn't be achieved otherwise, does it make the > code significantly less complex, is some integration with the OPP > subsystem needed that I'm overlooking, is it more efficient, ...? > > I'm not just insisting because I'm stubborn. I'd be happy to use any > interface that fits the bill, or to adjust one to fit the bill, but as > of now I mainly see drawbacks on the OPPs side and haven't seen > convincing arguments that it is really needed in devreq_cooling.c or a > better solution. During we discussed, we knew that OPP doesn't provide all operation perfectly. As of now, OPP is standard framework to control the pair of frequency/voltage. devfreq need to use OPP to the pair of frequency/voltage. Even if OPP doesn't provide all of our requirements, I think that devfreq should use OPP interface after updating OPP framework, instead of adding other functionality to control frequency in outside driver(devfreq-cooling.c, throttler). Finally, I asked to Viresh (OPP maintainer). If dev_pm_opp_enable() and dev_pm_opp_disable() don't support the usecase on multiple device drivers, opp interface could not be used in order to support both devfreq-cooling.c and throttler. So, We better to wait the opinion from OPP maintainer. -- Best Regards, Chanwoo Choi Samsung Electronics -- 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
