Re: [PATCH] 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:
- [Medium] Incorrect DTC logical ID assignment cross-wires DTC domains.
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commit 621fea5b4bb35e497f17f2f20187c68b18f49751
Author: Aviv Bakal <avivb@amazon.com>

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

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

> diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> index f5305c8fdca4..368fe1a86bfb 100644
> --- a/drivers/perf/arm-cmn.c
> +++ b/drivers/perf/arm-cmn.c

[ ... ]

> @@ -2443,6 +2468,10 @@ 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;

Will this cross-wire the DTC domains?

In arm_cmn_init_dtcs(), the cmn->dns array is sorted by logid, and the
cmn->dtc array is populated sequentially based on this sorted order.

During PMU event programming, the driver uses the domain ID (dn->dtc) as an
index into the cmn->dtc array.

If the physical layout of the DTC XPs causes their xp->logid values (typically
y * mesh_x + x) to sort differently than their computed domain IDs, wouldn't
the cmn->dtc array be misaligned?

For example, a Domain 1 DTC at (5,0) has a smaller logid (5) than a Domain 0
DTC at (0,6) (60), meaning Domain 1 would incorrectly occupy cmn->dtc[0]. This
cross-wiring could cause PMU events to program the wrong physical DTC hardware
and map IRQs to the wrong domains, resulting in incorrect performance
statistics and unhandled interrupts.

Would it be better to assign the computed domain ID to the logical ID here
using dn->logid = xp->dtc?

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

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Sashiko AI review · https://sashiko.dev/#/patchset/20260503155118.16191-1-avivb@amazon.com?part=1