Re: [PATCH v2 00/12] coresight: Add CPU cluster funnel/replicator/tmc support

[Yuanfang Zhang]

Hi Sudeep & Suzuki,

Gentle reminder on this patch. Any feedback would be appreciated.

Thanks,
Yuanfang.

On 5/25/2026 9:17 PM, Maulik Shah (mkshah) wrote:
> 
> 
> On 12/19/2025 3:51 PM, Sudeep Holla wrote:
>> On Fri, Dec 19, 2025 at 10:13:14AM +0800, yuanfang zhang wrote:
>>>
>>>
>>> On 12/18/2025 7:33 PM, Sudeep Holla wrote:
>>>> On Thu, Dec 18, 2025 at 12:09:40AM -0800, Yuanfang Zhang wrote:
>>>>> This patch series adds support for CoreSight components local to CPU clusters,
>>>>> including funnel, replicator, and TMC, which reside within CPU cluster power
>>>>> domains. These components require special handling due to power domain
>>>>> constraints.
>>>>>
>>>>
>>>> Could you clarify why PSCI-based power domains associated with clusters in
>>>> domain-idle-states cannot address these requirements, given that PSCI CPU-idle
>>>> OSI mode was originally intended to support them? My understanding of this
>>>> patch series is that OSI mode is unable to do so, which, if accurate, appears
>>>> to be a flaw that should be corrected.
>>>
>>> It is due to the particular characteristics of the CPU cluster power
>>> domain.Runtime PM for CPU devices works little different, it is mostly used
>>> to manage hierarchicalCPU topology (PSCI OSI mode) to talk with genpd
>>> framework to manage the last CPU handling in cluster.
>>
>> That is indeed the intended design. Could you clarify which specific
>> characteristics differentiate it here?
> 
> Sorry for coming very late on this.
> 
> This series is intended to handle coresight components which resides within CPU cluster.
> For the cases where cluster is in deepest idle low power mode or all CPUs belonging to cluster
> are hotplugged off, access to coresight components can not be done.
> 
> The implementation tried to address in two parts,
> 1. Using cluster power-domain to know which coresight component belongs to which cluster/CPUs
> 2. Schedule the task on intended cluster's CPU to make sure the CPU (and cluster) power is
> ON while coresight component of the cluster is being accessed (using smp_call_function_single()).
> 
> The use of power-domains in (1) will limit this to PSCI OS-Initiated mode,
> to have this support on PSCI Platform-Coordinated mode too, probably instead of power-domains,
> cpu-maps (which also defines the clusters) from device tree is a better choice which will give
> the information on which CPU belongs to which cluster.
> 
> (2) ensured that scheduling happened on intended CPU and while the access is in progress, CPU (and
> cluster) will not enter power down in between.
> 
>>
>>> It doesn’t really send IPI to wakeup CPU device (It don’t have
>>> .power_on/.power_off) callback implemented which gets invoked from
>>> .runtime_resume callback. This behavior is aligned with the upstream Kernel.
>>>
>>
>> I am quite lost here. Why is it necessary to wake up the CPU? If I understand
>> correctly, all of this complexity is meant to ensure that the cluster power
>> domain is enabled before any of the funnel registers are accessed. Is that
>> correct?
> 
> Yes, This is to ensure that CPU (and cluster) power is ON while coresight components
> for same cluster are being accessed.
> 
>>
>> If so, and if the cluster domains are already defined as the power domains for
>> these funnel devices, then they should be requested to power on automatically
>> before any register access occurs. Is that not the case?
> 
> Cluster power-domains will be only available for PSCI OS-initiated mode but also
> will not help for cases where all CPUs in cluster are hotplugged off as hotplugs are
> platform coordinated.
> 
> After discussion with our HW team to automatically request power on for coresight
> component GPR [1] can be used but they seems not working as intended on the existing
> SoCs and will be available on next generation SoC.
> 
> [1] https://developer.arm.com/documentation/ddi0480/d/Functional-Overview/Granular-Power-Requestor
> 
>>
>> What am I missing in this reasoning?
>>
>> The only explanation I can see is that the firmware does not properly honor
>> power-domain requests coming directly from the OS. I believe that may be the
>> case, but I would be glad to be proven wrong.
>>
> 
> please see below comment for more details, This seems not a firmware issue.
> 
>>>>
>>>>> Unlike system-level CoreSight devices, these components share the CPU cluster's
>>>>> power domain. When the cluster enters low-power mode (LPM), their registers
>>>>> become inaccessible. Notably, `pm_runtime_get` alone cannot bring the cluster
>>>>> out of LPM, making standard register access unreliable.
>>>>>
>>>>
>>>> Are these devices the only ones on the system that are uniquely bound to
>>>> cluster-level power domains? If not, what additional devices share this
>>>> dependency so that we can understand how they are managed in comparison?
>>>>
>>>
>>> Yes, devices like ETM and TRBE also share this power domain and access constraint.
>>> Their drivers naturally handle enablement/disablement on the specific CPU they
>>> belong to (e.g., via hotplug callbacks or existing smp_call_function paths).
>>
>> I understand many things are possible to implement, but the key question
>> remains: why doesn’t the existing OSI mechanism - added specifically to cover
>> cases like this solve the problem today?
>>
>> Especially on platforms with OSI enabled, what concrete limitation forces us
>> into this additional “wake-up” path instead of relying on OSI to manage the
>> dependency/power sequencing?
> 
> + Ulf in loop.
> 
> Current platforms with OSI enabled, Linux PSCI do not implement the power_on/power_off
> requests, as far as i know, runtime PM was never meant to implement this part and
> pm_runtime_get_sync() (from drivers/cpuidle/cpuidle-psci.c) call is only used to convey
> to cluster power domains about a child CPU/ sub domain being on after it has already
> been landed in Linux.
> 
> The standalone invoke of pm_runtime_get_sync() from another CPU do not really turn on/get
> the CPU (and cluster), as the CPUs either use CPUidle / CPU hotplug paths to enter/exit
> low power mode.
> 
> To put it other way,
> For a hot-plugged off CPU, invoking a pm_runtime_get_sync() won't get the CPU (and make
> its cluster power domain) ON. In order to turn on the CPU, one has to still request
> the online of the CPU, say via sysfs command echo 1 > /sys/devices/system/cpu/cpuX/online
> which would invoke PSCI CPU_ON function and the power domain for CPU gets marked as ON
> only after CPU already landed in Linux via psci_idle_cpuhp_up() invoking pm_runtime_get_sync().
> 
> I used specific hotplug example but same applies to idle powered down CPU (or Cluster) too.
> 
>>
>>>>> To address this, the series introduces:
>>>>> - Identifying cluster-bound devices via a new `qcom,cpu-bound-components`
>>>>>   device tree property.
>>>>
>>>> Really, no please.
>>>>
>>>
>>> Our objective is to determine which CoreSight components are physically locate
>>> within the CPU cluster power domain.
>>>
>>> Would it be acceptable to derive this relationship from the existing
>>> power-domains binding?
>>
>> In my opinion, this is not merely a possibility but an explicit expectation.
>>
>>> For example, if a Funnel or Replicator node is linked to a power-domains
>>> entry that specifies a cpumask, the driver could recognize this shared
>>> dependency and automatically apply the appropriate cluster-aware behavior.
>>>
>>
>> Sure, but for the driver to use that information, we need clear explanation
>> for all the questions above. In short, why it is not working with the existing
>> PSCI domain idle support.
>>
> 
> Explained above.
> 
> Thanks,
> Maulik