Re: [PATCH v3 2/2] perf/arm-cmn: Add workarounds for CMN-S3 on Graviton5

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Setting the DTC node's logical ID to the parent XP's logical ID breaks the DTC array indexing, leading to incorrect PMU event programming and IRQ routing.
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commit 20d5a81cc2d5eb87a70576532c9a783cb6e011c3
Author: Aviv Bakal <avivb@amazon.com>

perf/arm-cmn: Add workarounds for CMN-S3 on Graviton5

This commit introduces workarounds for a custom CMN-S3 implementation on
Graviton5 where certain discovery registers report zeroed fields. It adds
a dedicated ACPI HID and derives the DTC domain and logical ID from the
XP node.

> diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> --- a/drivers/perf/arm-cmn.c
> +++ b/drivers/perf/arm-cmn.c

[ ... ]

> @@ -2419,6 +2443,10 @@ static int arm_cmn_discover(struct arm_cmn *cmn, unsigned int rgn_offset)
>  
>  			switch (dn->type) {
>  			case CMN_TYPE_DTC:
> +				if (graviton5_workaround) {
> +					/* Node info logical ID is zeroed; use the XP's */
> +					dn->logid = xp->logid;

[Severity: High]
Will setting the DTC's logical ID to the parent XP's logical ID break the
DTC array indexing?

For standard CMN hardware, a DTC's logid is its Domain ID. Nodes are sorted
by type and logid, which means DTCs naturally sort by Domain ID.

By using the XP's logid instead, the DTCs will be sorted by their physical
XP order rather than their Domain ID. The cmn->dtc array and ACPI IRQs are
populated sequentially based on this sorted order.

Later, when programming PMU events, the driver indexes into the cmn->dtc
array using the Domain ID:

    d = max_t(int, dn->dtc, 0);

If the physical XP sort order doesn't match the Domain ID order, will this
cause PMU events to be programmed into the wrong hardware blocks and
misroute IRQs? 

Could this be assigned the Domain ID directly using dn->logid = xp->dtc?

> +				}
>  				cmn->num_dtcs++;
>  				dn++;
>  				break;

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Sashiko AI review · https://sashiko.dev/#/patchset/20260524153848.16334-1-avivb@amazon.com?part=2