Re: [PATCH] iommu/arm-smmu-v3: Restrict MMU-700 errata 2268618 and 2812531 to affected

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

On 05/02/2026 9:33 am, Zhiyuan Dai wrote:
> According to SDEN-1786925, Arm errata 2268618 and 2812531 are present in
> r0p0, r0p1, and r1p0, and fixed in r1p1.

This is a bit fiddly, since these workarounds also overlap some other 
errata that IIRC were *not* fixed in r1p1 - it's been on my to-do list 
to figure this out for a while now, sorry I haven't had a chance to get 
to it.

> Introduce this patch to avoid performance degradation caused by
> restricting features on unaffected revisions.

I'm curious, can you clarify the performance impact? At the time I was 
pretty sure that the ARM_SMMU_OPT_CMDQ_FORCE_SYNC was just for safety 
and never likely to be hit in practice, since with range invalidations 
any unmap should never end up with anywhere near 16 TLBIs in a single 
batch anyway. Is that no longer the case?

> Signed-off-by: Zhiyuan Dai <daizhiyuan@phytium.com.cn>
> ---
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 12 +++++++-----
>   1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index d16d35c78c06..7eadb27eb9be 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -4272,11 +4272,13 @@ static void arm_smmu_device_iidr_probe(struct arm_smmu_device *smmu)
>   				smmu->features &= ~ARM_SMMU_FEAT_NESTING;
>   			break;
>   		case IIDR_PRODUCTID_ARM_MMU_700:
> -			/* Arm erratum 2812531 */
> -			smmu->features &= ~ARM_SMMU_FEAT_BTM;
> -			smmu->options |= ARM_SMMU_OPT_CMDQ_FORCE_SYNC;
> -			/* Arm errata 2268618, 2812531 */
> -			smmu->features &= ~ARM_SMMU_FEAT_NESTING;
> +			if (variant <= 1 && revision < 0) {

revision is unsigned, it cannot be < 0. This just breaks the workarounds 
entirely.

Thanks,
Robin.

> +				/* Arm erratum 2812531 */
> +				smmu->features &= ~ARM_SMMU_FEAT_BTM;
> +				smmu->options |= ARM_SMMU_OPT_CMDQ_FORCE_SYNC;
> +				/* Arm errata 2268618, 2812531 */
> +				smmu->features &= ~ARM_SMMU_FEAT_NESTING;
> +			}
>   			break;
>   		}
>   		break;