Hi Viresh, I'm glad it helped. Please find below my reply. On 10/9/20 6:39 AM, Viresh Kumar wrote: > On 08-10-20, 17:00, Nicola Mazzucato wrote: >> On 10/8/20 4:03 PM, Ionela Voinescu wrote: >>> Hi Viresh, >>> >>> On Thursday 08 Oct 2020 at 16:32:41 (+0530), Viresh Kumar wrote: >>>> On 07-10-20, 13:58, Nicola Mazzucato wrote: >>>>> Hi Viresh, >>>>> >>>>> performance controls is what is exposed by the firmware through a protocol that >>>>> is not capable of describing hardware (say SCMI). For example, the firmware can >>>>> tell that the platform has N controls, but it can't say to which hardware they >>>>> are "wired" to. This is done in dt, where, for example, we map these controls >>>>> to cpus, gpus, etc. >>>>> >>>>> Let's focus on cpus. >>>>> >>>>> Normally we would have N of performance controls (what comes from f/w) >>>>> that that correspond to hardware clock/dvfs domains. >>>>> >>>>> However, some firmware implementations might benefit from having finer >>>>> grained information about the performance requirements (e.g. >>>>> per-CPU) and therefore choose to present M performance controls to the >>>>> OS. DT would be adjusted accordingly to "wire" these controls to cpus >>>>> or set of cpus. >>>>> In this scenario, the f/w will make aggregation decisions based on the >>>>> requests it receives on these M controls. >>>>> >>>>> Here we would have M cpufreq policies which do not necessarily reflect the >>>>> underlying clock domains, thus some s/w components will underperform >>>>> (EAS and thermal, for example). >>>>> >>>>> A real example would be a platform in which the firmware describes the system >>>>> having M per-cpu control, and the cpufreq subsystem will have M policies while >>>>> in fact these cpus are "performance-dependent" each other (e.g. are in the same >>>>> clock domain). >>>> >>>> If the CPUs are in the same clock domain, they must be part of the >>>> same cpufreq policy. >>> >>> But cpufreq does not currently support HW_ALL (I'm using the ACPI >>> coordination type to describe the generic scenario of using hardware >>> aggregation and coordination when establishing the clock rate of CPUs). >>> >>> Adding support for HW_ALL* will involve either bypassing some >>> assumptions around cpufreq policies or making core cpufreq changes. >>> >>> In the way I see it, support for HW_ALL involves either: >>> >>> - (a) Creating per-cpu policies in order to allow each of the CPUs to >>> send their own frequency request to the hardware which will do >>> aggregation and clock rate decision at the level of the clock >>> domain. The PSD domains (ACPI) and the new DT binding will tell >>> which CPUs are actually in the same clock domain for whomever is >>> interested, despite those CPUs not being in the same policy. >>> This requires the extra mask that Nicola introduced. >>> >>> - (b) Making deep changes to cpufreq (core/governors/drivers) to allow: >>> - Governors to stop aggregating (usually max) the information >>> for each of the CPUs in the policy and convey to the core >>> information for each CPU. >>> - Cpufreq core to be able to receive and pass this information >>> down to the drivers. >>> - Drivers to be able to have some per cpu structures to hold >>> frequency control (let's say SCP fast channel addresses) for >>> each of the CPUs in the policy. Or have these structures in the >>> cpufreq core/policy, to avoid code duplication in drivers. >>> >>> Therefore (a) is the least invasive but we'll be bypassing the rule >>> above. But to make that rule stick we'll have to make invasive cpufreq >>> changes (b). >> >> Regarding the 'rule' above of one cpufreq policy per clock domain, I would like >> to share my understanding on it. Perhaps it's a good opportunity to shed some light. >> >> Looking back in the history of CPUFreq, related_cpus was originally designed >> to hold the map of cpus within the same clock. Later on, the meaning of this >> cpumask changed [1]. >> This led to the introduction of a new cpumask 'freqdomain_cpus' >> within acpi-cpufreq to keep the knowledge of hardware clock domains for >> sysfs consumers since related_cpus was not suitable anymore for this. >> Further on, this cpumask was assigned to online+offline cpus within the same clk >> domain when sw coordination is in use [2]. >> >> My interpretation is that there is no guarantee that related_cpus holds the >> 'real' hardware clock implementation. As a consequence, it is not true anymore >> that cpus that are in the same clock domain will be part of the same >> policy. >> >> This guided me to think it would be better to have a cpumask which always holds >> the real hw clock domains in the policy. >> >>> >>> This is my current understanding and I'm leaning towards (a). What do >>> you think? >>> >>> *in not so many words, this is what these patches are trying to propose, >>> while also making sure it's supported for both ACPI and DT. >>> >>> BTW, thank you for your effort in making sense of this! >>> >>> Regards, >>> Ionela. >>> >> >> This could be a platform where per-cpu and perf-dependencies will be used: >> >> CPU: 0 1 2 3 4 5 6 7 >> Type: A A A A B B B B >> Cluster: [ ] >> perf-controls: [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] >> perf-dependency: [ ] [ ] >> HW clock: [ ] [ ] >> >> The firmware will present 8 controls to the OS and each control is mapped to a >> cpu device via the standard dt. This is done so we can achieve hw coordination. >> What is required in these systems is to present to OS the information of which >> cpus belong to which clock domain. In other words, when hw coordinates we don't >> have any way at present in dt to understand how these cpus are dependent >> each other, from performance perspective (as opposed to ACPI where we have >> _PSD). Hence my proposal for the new cpu-perf-dependencies. >> This is regardless whether we decide to go for either a policy per-cpu or a >> policy per-domain. >> >> Hope it helps. > > Oh yes, I get it now. Finally. Thanks for helping me out :) > > So if I can say all this stuff in simple terms, this is what it will > be like: > > - We don't want software aggregation of frequencies and so we need to > have per-cpu policies even when they share their clock lines. > > - But we still need a way for other frameworks to know which CPUs > share the clock lines (that's what the perf-dependency is all about, > right ?). > > - We can't get it from SCMI, but need a DT based solution. > > - Currently for the cpufreq-case we relied for this on the way OPP > tables for the CPUs were described. i.e. the opp-table is marked as > "shared" and multiple CPUs point to it. > > - I wonder if we can keep using that instead of creating new bindings > for exact same stuff ? Though the difference here would be that the > OPP may not have any other entries. I thought about it and looked for other platforms' DT to see if can reuse existing opp information. Unfortunately I don't think it is optimal. The reason being that, because cpus have the same opp table it does not necessarily mean that they share a clock wire. It just tells us that they have the same capabilities (literally just tells us they have the same V/f op points). Unless I am missing something? When comparing with ACPI/_PSD it becomes more intuitive that there is no equivalent way to reveal "perf-dependencies" in DT. Thank you for time on this. Regards Nicola >
