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

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Message-ID: <20251218104027.GA1790402@e132581.arm.com>
Date: Thu, 18 Dec 2025 10:40:27 +0000
From: Leo Yan <leo.yan@....com>
To: Yuanfang Zhang <yuanfang.zhang@....qualcomm.com>
Cc: Suzuki K Poulose <suzuki.poulose@....com>,
	Mike Leach <mike.leach@...aro.org>,
	James Clark <james.clark@...aro.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Mathieu Poirier <mathieu.poirier@...aro.org>,
	Leo Yan <leo.yan@...ux.dev>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konradybcio@...nel.org>, kernel@....qualcomm.com,
	coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, maulik.shah@....qualcomm.com,
	Jie Gan <jie.gan@....qualcomm.com>
Subject: Re: [PATCH v2 00/12] coresight: Add CPU cluster
 funnel/replicator/tmc support

Hi,

On Thu, Dec 18, 2025 at 12:09:40AM -0800, Coresight ML wrote:

[...]

> - Utilizing `smp_call_function_single()` to ensure register accesses
>   (initialization, enablement, sysfs reads) are always executed on a
>   powered CPU within the target cluster.

This is concerned as Mike suggested earlier.

Let me convert to a common question: how does the Linux kernel manage
a power domain shared by multiple hardware modules?

A general solution is to bind a power domain (let's say PD1) to both
module A (MOD_A) and module B (MOD_B).  Each time before accessing MOD_A
or MOD_B, PD1 must be powered on first via the pm_runtime APIs, with
its refcount increased accordingly.

My understanding is the problem in your case is that the driver fails to
create a relationship between the funnel/replicator modules and the
cluster power domain.  Instead, you are trying to use the CPUs in the
same cluster as a delegate for power operations - when you want to
access MOD_B, your wake up MOD_A which sharing the same power domain,
only to turn on the PD_A in order to access MOD_B.

Have you discussed with the firmware and hardware engineers whether it
is feasible to provide explicit power and clock control interfaces for
the funnel and replicator modules?  I can imagine the cluster power
domain's design might differ from other device power domains, but
should not the hardware provide a sane design that allows software to
control power for the access logic within it?

General speaking, using smp_call_function_single() makes sense if only
when accessing logics within the CPU boundary.

P.s., currently you can use "taskset" as a temporary solution without
any code change, something like:

  taskset -c 0 echo 1 > /sys/bus/coresight/devices/etm0/enable_source

Thanks,
Leo