Re: [QUESTION] ARM DSU PMU: 32-bit counter assumption and handling of 64-bit event counters

[Will Deacon]

[+Robin and Suzuki]

On Fri, Feb 06, 2026 at 11:51:38AM +0800, feng.zhou wrote:
> Hi Will and Mark,
> 
> I have a question regarding the ARM DSU PMU driver implementation in
> drivers/perf/arm_dsu_pmu.c, specifically about the counter width
> assumptions.
> 
> Current Implementation:
> ======================
> The driver currently assumes all event counters are 32-bit wide, with
> only the cycle counter being 64-bit (lines 69-71):
> 
> /* All event counters are 32bit, with a 64bit Cycle counter */
> #define DSU_PMU_COUNTER_WIDTH(idx)	\
> 	(((idx) == DSU_PMU_IDX_CYCLE_COUNTER) ? 64 : 32)
> 
> This assumption is used throughout the code for handling counter
> overflow and calculating deltas (e.g., line 344 in dsu_pmu_event_update).
> 
> Issue Observed:
> ===============
> I've encountered overflow issues when using perf tools on newer ARM CPUs
> where the event counters appear to be 64-bit rather than 32-bit. The
> current implementation causes incorrect event counting and potential data
> loss due to premature overflow handling based on the 32-bit mask.

I suspect either the hardware or the architecture have evolved since the
driver was initially written, possibly in a non-backwards-compatible way
which is why you see problems.

> Questions:
> ==========
> 1. Is the 32-bit event counter assumption based on the ARMv8 PMU
>    specification or DSU-specific hardware documentation?
> 
> 2. Are there known ARM CPU implementations where DSU event counters
>    exceed 32 bits? If so, how should the driver handle this variation?
> 
> 3. Would it be appropriate to detect the actual counter width at runtime
>    (similar to how num_counters is probed in dsu_pmu_probe_pmu), or
>    should this be handled through device tree / ACPI properties?
> 
> 4. Should there be a mechanism to query the hardware for the actual
>    counter width to ensure compatibility with future implementations?
> 
> Thank you for your time and any insights you can provide.

Suzuki and Robin should be able to help with these...

Will