Re: [RFC PATCH 5/5] iommu/arm-smmu-v3: Invoke pm_runtime before hw access

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Mar 19, 2025 at 12:42:54AM +0000, Pranjal Shrivastava wrote:
> -static void arm_smmu_attach_dev_ste(struct iommu_domain *domain,
> +static int arm_smmu_attach_dev_ste(struct iommu_domain *domain,
>  				    struct device *dev,
>  				    struct arm_smmu_ste *ste,
>  				    unsigned int s1dss)
>  {

This can't be changed into something that fails! That would mess up
all the error handling if it ever blows up.

> @@ -3214,8 +3287,8 @@ static int arm_smmu_attach_dev_identity(struct iommu_domain *domain,
>  	struct arm_smmu_master master = dev_iommu_priv_get(dev);
>  
>  	arm_smmu_make_bypass_ste(master->smmu, &ste);
> -	arm_smmu_attach_dev_ste(domain, dev, &ste, STRTAB_STE_1_S1DSS_BYPASS);
> -	return 0;
> +	return arm_smmu_attach_dev_ste(domain, dev, &ste,
> +				       STRTAB_STE_1_S1DSS_BYPASS);
>  }
>  
>  static const struct iommu_domain_ops arm_smmu_identity_ops = {
> @@ -3233,9 +3306,8 @@ static int arm_smmu_attach_dev_blocked(struct iommu_domain *domain,
>  	struct arm_smmu_ste ste;
>  
>  	arm_smmu_make_abort_ste(&ste);
> -	arm_smmu_attach_dev_ste(domain, dev, &ste,
> -				STRTAB_STE_1_S1DSS_TERMINATE);
> -	return 0;
> +	return arm_smmu_attach_dev_ste(domain, dev, &ste,
> +				       STRTAB_STE_1_S1DSS_TERMINATE);
>  }

Because these functions are required to be non-failing.

This feels kind of wonky, if we need to update a STE but the PM is off
then we just need to update the STE memory with some synchornization
and do nothing else. We don't need to wake up the HW and issue a cache
flush if the powered off cache could never have held a copy anyhow.

Jason