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. -- viresh
