Re: [PATCH] iommu/arm-smmu-v3: Skip prefetch cmd for MMU-600, MMU-700

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

Hi Daniel,

On 2024-05-13 10:54 pm, Daniel Mentz wrote:
> Arm CoreLink MMU-600 and MMU-700 both silently ignore the
> CMD_PREFETCH_CONFIG command. Let's not waste cycles adding this command to
> the command queue if it is ignored anyway.

It is true that our implementations don't do anything with prefetch 
commands so far, however this is a valid architectural behaviour, and 
the intent of the quirk is really for buggy implementations where the 
commands erroneously fault.

TBH if saving cycles in arm_smmu_attach_dev() matters to you, then 
you're probably not going to like what's now queued in next that makes 
it absurdly overcomplicated with loads of indirect function calls...

Thanks,
Robin.

> Signed-off-by: Daniel Mentz <danielmentz@google.com>
> ---
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> 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 aa62f0ecd053..98f1c4b44ef3 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -3684,6 +3684,8 @@ static void arm_smmu_device_iidr_probe(struct arm_smmu_device *smmu)
>   	case IIDR_IMPLEMENTER_ARM:
>   		switch (productid) {
>   		case IIDR_PRODUCTID_ARM_MMU_600:
> +			/* MMU-600 silently ignores CMD_PREFETCH_CONFIG */
> +			smmu->options |= ARM_SMMU_OPT_SKIP_PREFETCH;
>   			/* Arm erratum 1076982 */
>   			if (variant == 0 && revision <= 2)
>   				smmu->features &= ~ARM_SMMU_FEAT_SEV;
> @@ -3692,6 +3694,8 @@ 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:
> +			/* MMU-700 silently ignores CMD_PREFETCH_CONFIG */
> +			smmu->options |= ARM_SMMU_OPT_SKIP_PREFETCH;
>   			/* Arm erratum 2812531 */
>   			smmu->features &= ~ARM_SMMU_FEAT_BTM;
>   			smmu->options |= ARM_SMMU_OPT_CMDQ_FORCE_SYNC;