Subject: Re: [PATCH v4 2/2] perf/arm-cmn: Add workarounds for CMN-S3 on Graviton5

From: Ilkka Koskinen <[email-masked]>
Date: Mon, 1 Jun 2026 22:55:30 -0700 (PDT)

On Sun, 31 May 2026, Aviv Bakal wrote:

> Graviton5 uses a customised CMN-S3 implementation where certain
> discovery registers report zeroed fields. Add the following workarounds:
>
> - Introduce a dedicated ACPI HID to identify the Graviton5 CMN variant.
> - Derive the DTC domain from the XP node ID, since the unit info
>   register reports it as zero.
> - Set the DTC logical ID from the XP's logical ID, since the node info
>   register's logical ID field is also zeroed.
>
> Signed-off-by: Aviv Bakal <[email-masked]>

Thanks for the patch. It seems surprisingly clean approach. And thanks to
Robin for answering to Sashiko bot!

The patch looks good to me,

 	Reviewed-by: Ilkka Koskinen <[email-masked]>

Cheers, Ilkka

> ---
> drivers/perf/arm-cmn.c | 32 +++++++++++++++++++++++++++++++-
> 1 file changed, 31 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> index f1978a53d1c1..3fb71d9a57eb 100644
> --- a/drivers/perf/arm-cmn.c
> +++ b/drivers/perf/arm-cmn.c
> @@ -31,7 +31,8 @@
> #define CMN_CHILD_NODE_ADDR		GENMASK(29, 0)
> #define CMN_CHILD_NODE_EXTERNAL		BIT(31)
>
> -#define CMN_MAX_DIMENSION		12
> +/* Some implementations use a mesh larger than the architectural max of 12 */
> +#define CMN_MAX_DIMENSION		14
> #define CMN_MAX_XPS			(CMN_MAX_DIMENSION * CMN_MAX_DIMENSION)
> #define CMN_MAX_DTMS			(CMN_MAX_XPS + (CMN_MAX_DIMENSION - 1) * 4)
>
> @@ -214,6 +215,8 @@ enum cmn_part {
> 	PART_CMN700 = 0x43c,
> 	PART_CI700 = 0x43a,
> 	PART_CMN_S3 = 0x43e,
> +	/* Synthetic part number, overridden to PART_CMN_S3 during discovery */
> +	PART_GRAVITON5 = 0xa5,
> };
>
> /* CMN-600 r0px shouldn't exist in silicon, thankfully */
> @@ -2250,6 +2253,18 @@ static unsigned int arm_cmn_dtc_domain(struct arm_cmn *cmn, void __iomem *xp_reg
> 	return FIELD_GET(CMN_DTM_UNIT_INFO_DTC_DOMAIN, readl_relaxed(xp_region + offset));
> }
>
> +static unsigned int arm_cmn_graviton5_dtc_domain(u16 xp_id)
> +{
> +	unsigned int x = (xp_id >> 7) & 0xf;
> +	unsigned int y = (xp_id >> 3) & 0xf;
> +
> +	/*
> +	 * The unit info register reads as zero; derive the DTC domain from
> +	 * the XP's mesh coordinates over the 10x14 mesh.
> +	 */
> +	return (x / 5) + (y / 7) * 2;
> +}
> +
> static void arm_cmn_init_node_info(struct arm_cmn *cmn, u32 offset, struct arm_cmn_node *node)
> {
> 	int level;
> @@ -2295,6 +2310,7 @@ static int arm_cmn_discover(struct arm_cmn *cmn, unsigned int rgn_offset)
> 	u64 reg;
> 	int i, j;
> 	size_t sz;
> +	bool graviton5_workaround = false;
>
> 	arm_cmn_init_node_info(cmn, rgn_offset, &cfg);
> 	if (cfg.type != CMN_TYPE_CFG)
> @@ -2305,6 +2321,13 @@ static int arm_cmn_discover(struct arm_cmn *cmn, unsigned int rgn_offset)
> 	reg = readq_relaxed(cfg_region + CMN_CFGM_PERIPH_ID_01);
> 	part = FIELD_GET(CMN_CFGM_PID0_PART_0, reg);
> 	part |= FIELD_GET(CMN_CFGM_PID1_PART_1, reg) << 8;
> +
> +	/* Graviton5 has a customised CMN-S3 which needs some fixups */
> +	if (cmn->part == PART_GRAVITON5) {
> +		cmn->part = PART_CMN_S3;
> +		graviton5_workaround = true;
> +	}
> +
> 	/* 600AE is close enough that it's not really worth more complexity */
> 	if (part == PART_CMN600AE)
> 		part = PART_CMN600;
> @@ -2394,6 +2417,8 @@ static int arm_cmn_discover(struct arm_cmn *cmn, unsigned int rgn_offset)
>
> 		if (cmn->part == PART_CMN600)
> 			xp->dtc = -1;
> +		else if (graviton5_workaround)
> +			xp->dtc = arm_cmn_graviton5_dtc_domain(xp->id);
> 		else
> 			xp->dtc = arm_cmn_dtc_domain(cmn, xp_region);
>
> @@ -2472,6 +2497,10 @@ 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;
> +				}
> 				cmn->num_dtcs++;
> 				dn++;
> 				break;
> @@ -2687,6 +2716,7 @@ static const struct acpi_device_id arm_cmn_acpi_match[] = {
> 	{ "ARMHC650" },
> 	{ "ARMHC700" },
> 	{ "ARMHC003" },
> +	{ "AMZN0070", PART_GRAVITON5 },
> 	{}
> };
> MODULE_DEVICE_TABLE(acpi, arm_cmn_acpi_match);
> -- 
> 2.47.3
>
>