Cc: coresight lists, Leo, James, Mike L

Hello !

Thanks for the report ! In the future, please use scripts/get_maintainer.pl for the clear list of people/list for reporting issues.

Response inline, below.

On 20/06/2025 08:21, Keita Morisaki wrote:

Hello folks,

I am writing to report a WARN_ON message I'm encountering in the CoreSight driver on a multi-core ARM system running a 6.12-based kernel. The warning appears consistently when disabling an Embedded Trace Extension (ETE) source after it has been active. The issue is not reproducible when CPUidle is disabled.

The problem occurs because the driver assumes the CoreSight claim register is persistent, but it could be reset by the CPUidle power management flow. The section B2.3.2 of Arm CoreSight Architecture Specification v3.0[1] indicates that the claim register must reset at “reset”. A CPU power-up from an idle state can trigger a Cold reset, which might explain this behavior.

My ftrace analysis confirms this. I traced the only two functions that modify the claim state: coresight_set_claim_tags (which sets the claim) and coresight_clear_claim_tags (which is the only part of the kernel that writes to CLAIMCLR). The trace shows the claim being set, followed by a CPUidle transition, but no subsequent call to coresight_clear_claim_tags.

Here are the steps to reproduce the issue:

modprobecoresight_etm4x

# Enable any relevant sink

echo1>/sys/bus/coresight/devices/ete0/enable_source

echo0>/sys/bus/coresight/devices/ete0/enable_source

Here is a relevant snippet from the ftrace log that illustrates the sequence:

#tracer:function_graph

#

#CPUDURATIONFUNCTIONCALLS

#|||||||

0)|coresight_claim_device_unlocked[coresight](){

0)3.750us|coresight_set_claim_tags[coresight]();//Claimissethere

0)+20.260us|}

0)|/*psci_domain_idle_enter:cpu_id=0state={Our PSCI parameter value}*/// CPUgoesidle

0)|/*psci_domain_idle_exit:cpu_id=0state={Our PSCI parameter value}*/// CPUwakesup,causingColdreset

...

0)@309346.3us|coresight_disclaim_device_unlocked[coresight]();// TriggersWARN_ON

The following WARN_ON [2] is printed because the CLAIMCLR register has already been reset at the time coresight_disclaim_device_unlocked is called, contrary to the driver's expectation.

We have the ETM driver performing the save/restore of ETM context during a CPUidle. This is only done when the ETM/ETE is described to be loosing context over PM operation. If this is not done (via DT), the driver doesn't do anything. This could be problematic. Could you try adding:

"arm,coresight-loses-context-with-cpu"

property to the ETE nodes and see if it makes a difference ?

Kind regards Suzuki

[0] https://elixir.bootlin.com/linux/v6.12/source/Documentation/devicetree/bindi...

[416.354181][C0]WARNING:CPU:0PID:0atdrivers/hwtracing/coresight/ coresight-core.c:187coresight_disclaim_device_unlocked+0x84/0x9c[coresight]

[416.535454][C0]Calltrace:

[416.538606][C0]coresight_disclaim_device_unlocked+0x84/0x9c[coresight]

[416.549359][C0]etm4_disable_hw+0x2d8/0x374[coresight_etm4x]

[416.623310][C0]do_idle+0x1d4/0x264

(Note on tracing: To get this detailed trace, I made two modifications to the kernel. First, since the trace_psci_domain_idle_enter/exit events are not available in kernel 6.12, I cherry-picked the upstream patch 7b7644831e72 [3] to add them. Second, to specifically trace the claim functions, I temporarily replaced their inline compiler hints with noinline.)

Given the evidence, it appears the driver's assumption that the claim register is persistent across CPU power states is incorrect and may need to be addressed.

Could you please provide your guidance on this?

Thank you for your time and assistance.

[1] https://developer.arm.com/documentation/ihi0029/latest/ <https:// developer.arm.com/documentation/ihi0029/latest/>_ _[2] https://elixir.bootlin.com/linux/v6.12/source/drivers/hwtracing/ coresight/coresight-core.c#L187 <https://elixir.bootlin.com/linux/v6.12/ source/drivers/hwtracing/coresight/coresight-core.c#L187>_ _[3] https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/ linux.git/commit/?id=7b7644831e7276f52a233ec685d13c965fff09d9 <https:// web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/? id=7b7644831e7276f52a233ec685d13c965fff09d9>

Best regards, Keita