Re: [PATCH 2/4] perf/arm_cspmu: Improve APMT-based PMU naming

[Robin Murphy <robin.murphy@arm.com>]

On 09/07/2026 3:39 pm, Robin Murphy wrote:
> On ACPI systems, it has not actually been possible for userspace to
> reliably tell which PMU corresponds to which APMT entry for types
> other than "ACPI device" - the evidence trail only leads from the
> arbitrarily-numbered PMU device to its arbitrarily-numbered parent
> platform device that has no distinguishing features either.
> 
> While we've now improved the platform device creation to associate the
> actual APMT unique ID, we may as well also tweak the PMU devices to
> substitute the arbitrary number with a different arbitrary number that
> might be more directly meaningful based on the APMT definitions.
> 
> We don't have an equivalent for Devicetree, but in that case the
> platform devices are at least identifiable via their sysfs-visible
> of_node.
> 
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>   drivers/perf/arm_cspmu/arm_cspmu.c | 36 +++++++++++++++++-------------
>   1 file changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/perf/arm_cspmu/arm_cspmu.c b/drivers/perf/arm_cspmu/arm_cspmu.c
> index 80fb314d5135..0570be74d11f 100644
> --- a/drivers/perf/arm_cspmu/arm_cspmu.c
> +++ b/drivers/perf/arm_cspmu/arm_cspmu.c
> @@ -250,38 +250,44 @@ static const char *arm_cspmu_get_name(const struct arm_cspmu *cspmu)
>   	struct device *dev;
>   	struct acpi_apmt_node *apmt_node;
>   	u8 pmu_type;
> -	char *name;
>   	char acpi_hid_string[ACPI_ID_LEN] = { 0 };
> -	static atomic_t pmu_idx[ACPI_APMT_NODE_TYPE_COUNT] = { 0 };
> +	static atomic_t pmu_idx;
> +	u32 id;
>   
>   	dev = cspmu->dev;
>   	apmt_node = arm_cspmu_apmt_node(dev);
>   	if (!apmt_node)
>   		return devm_kasprintf(dev, GFP_KERNEL, PMUNAME "_%u",
> -				      atomic_fetch_inc(&pmu_idx[0]));
> +				      atomic_fetch_inc(&pmu_idx));
>   
>   	pmu_type = apmt_node->type;
> -
> -	if (pmu_type >= ACPI_APMT_NODE_TYPE_COUNT) {
> +	switch (pmu_type) {
> +	default:
>   		dev_err(dev, "unsupported PMU type-%u\n", pmu_type);
>   		return NULL;
> -	}
> -
> -	if (pmu_type == ACPI_APMT_NODE_TYPE_ACPI) {
> +	case ACPI_APMT_NODE_TYPE_ACPI:
>   		memcpy(acpi_hid_string,
>   			&apmt_node->inst_primary,
>   			sizeof(apmt_node->inst_primary));
> -		name = devm_kasprintf(dev, GFP_KERNEL, "%s_%s_%s_%u", PMUNAME,
> +		return devm_kasprintf(dev, GFP_KERNEL, "%s_%s_%s_%u", PMUNAME,
>   				      arm_cspmu_type_str[pmu_type],
>   				      acpi_hid_string,
>   				      apmt_node->inst_secondary);
> -	} else {
> -		name = devm_kasprintf(dev, GFP_KERNEL, "%s_%s_%d", PMUNAME,
> -				      arm_cspmu_type_str[pmu_type],
> -				      atomic_fetch_inc(&pmu_idx[pmu_type]));
> -	}
> +	case ACPI_APMT_NODE_TYPE_MC:
> +		id = apmt_node->id;
> +		break;
> +	case ACPI_APMT_NODE_TYPE_SMMU:
> +	case ACPI_APMT_NODE_TYPE_PCIE_ROOT:
> +		id = apmt_node->inst_primary;
> +		break;
> +	case ACPI_APMT_NODE_TYPE_CACHE:
> +		id = apmt_node->inst_secondary;
> +		break;
> +	};
>   
> -	return name;
> +	return devm_kasprintf(dev, GFP_KERNEL, "%s_%s_%u", PMUNAME,
> +			      arm_cspmu_type_str[pmu_type],
> +			      apmt_node->id);

Seems I did forget this when I realised the instance fields could be 
useful and added the local "id" variable. The rest of the Sashiko noise 
is mostly nonsense.

Cheers,
Robin.

>   }
>   
>   static ssize_t arm_cspmu_cpumask_show(struct device *dev,