Re: [PATCHv7 2/7] iommu/arm-smmu: Add domain attribute for system cache

[Will Deacon]

On Fri, Oct 30, 2020 at 02:53:09PM +0530, Sai Prakash Ranjan wrote:
> Add iommu domain attribute for using system cache aka last level
> cache by client drivers like GPU to set right attributes for caching
> the hardware pagetables into the system cache.
> 
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx>
> ---
>  drivers/iommu/arm/arm-smmu/arm-smmu.c | 17 +++++++++++++++++
>  drivers/iommu/arm/arm-smmu/arm-smmu.h |  1 +
>  include/linux/iommu.h                 |  1 +
>  3 files changed, 19 insertions(+)
> 
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> index b1cf8f0abc29..070d13f80c7e 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> @@ -789,6 +789,9 @@ static int arm_smmu_init_domain_context(struct iommu_domain *domain,
>  	if (smmu_domain->non_strict)
>  		pgtbl_cfg.quirks |= IO_PGTABLE_QUIRK_NON_STRICT;
>  
> +	if (smmu_domain->sys_cache)
> +		pgtbl_cfg.quirks |= IO_PGTABLE_QUIRK_SYS_CACHE;
> +
>  	pgtbl_ops = alloc_io_pgtable_ops(fmt, &pgtbl_cfg, smmu_domain);
>  	if (!pgtbl_ops) {
>  		ret = -ENOMEM;
> @@ -1520,6 +1523,9 @@ static int arm_smmu_domain_get_attr(struct iommu_domain *domain,
>  		case DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE:
>  			*(int *)data = smmu_domain->non_strict;
>  			return 0;
> +		case DOMAIN_ATTR_SYS_CACHE:
> +			*((int *)data) = smmu_domain->sys_cache;
> +			return 0;
>  		default:
>  			return -ENODEV;
>  		}
> @@ -1551,6 +1557,17 @@ static int arm_smmu_domain_set_attr(struct iommu_domain *domain,
>  			else
>  				smmu_domain->stage = ARM_SMMU_DOMAIN_S1;
>  			break;
> +		case DOMAIN_ATTR_SYS_CACHE:
> +			if (smmu_domain->smmu) {
> +				ret = -EPERM;
> +				goto out_unlock;
> +			}
> +
> +			if (*((int *)data))
> +				smmu_domain->sys_cache = true;
> +			else
> +				smmu_domain->sys_cache = false;
> +			break;
>  		default:
>  			ret = -ENODEV;
>  		}
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.h b/drivers/iommu/arm/arm-smmu/arm-smmu.h
> index 885840f3bec8..dfc44d806671 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.h
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.h
> @@ -373,6 +373,7 @@ struct arm_smmu_domain {
>  	struct mutex			init_mutex; /* Protects smmu pointer */
>  	spinlock_t			cb_lock; /* Serialises ATS1* ops and TLB syncs */
>  	struct iommu_domain		domain;
> +	bool				sys_cache;
>  };
>  
>  struct arm_smmu_master_cfg {
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index b95a6f8db6ff..4f4bb9c6f8f6 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -118,6 +118,7 @@ enum iommu_attr {
>  	DOMAIN_ATTR_FSL_PAMUV1,
>  	DOMAIN_ATTR_NESTING,	/* two stages of translation */
>  	DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE,
> +	DOMAIN_ATTR_SYS_CACHE,

I think you're trying to make this look generic, but it's really not.
If we need to funnel io-pgtable quirks through domain attributes, then I
think we should be open about that and add something like
DOMAIN_ATTR_IO_PGTABLE_CFG which could take a struct of page-table
configuration data for the domain (this could just be quirks initially,
but maybe it's worth extending to take ias, oas and page size)

Will