Re: [PATCH 1/1] iommu/arm-smmu: Add support to use Last level cache

[Will Deacon]

Hi Vivek,

On Fri, Jun 15, 2018 at 04:23:29PM +0530, Vivek Gautam wrote:
> Qualcomm SoCs have an additional level of cache called as
> System cache or Last level cache[1]. This cache sits right
> before the DDR, and is tightly coupled with the memory
> controller.
> The cache is available to all the clients present in the
> SoC system. The clients request their slices from this system
> cache, make it active, and can then start using it. For these
> clients with smmu, to start using the system cache for
> dma buffers and related page tables [2], few of the memory
> attributes need to be set accordingly.
> This change makes the related memory Outer-Shareable, and
> updates the MAIR with necessary protection.
> 
> The MAIR attribute requirements are:
>     Inner Cacheablity = 0
>     Outer Cacheablity = 1, Write-Back Write Allocate
>     Outer Shareablity = 1

Hmm, so is this cache coherent with the CPU or not? Why don't normal
non-cacheable mappings allocated in the LLC by default?

> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> index f7a96bcf94a6..8058e7205034 100644
> --- a/drivers/iommu/arm-smmu.c
> +++ b/drivers/iommu/arm-smmu.c
> @@ -249,6 +249,7 @@ 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				has_sys_cache;
>  };
>  
>  struct arm_smmu_option_prop {
> @@ -862,6 +863,8 @@ static int arm_smmu_init_domain_context(struct iommu_domain *domain,
>  
>  	if (smmu->features & ARM_SMMU_FEAT_COHERENT_WALK)
>  		pgtbl_cfg.quirks = IO_PGTABLE_QUIRK_NO_DMA;
> +	if (smmu_domain->has_sys_cache)
> +		pgtbl_cfg.quirks |= IO_PGTABLE_QUIRK_SYS_CACHE;
>  
>  	smmu_domain->smmu = smmu;
>  	pgtbl_ops = alloc_io_pgtable_ops(fmt, &pgtbl_cfg, smmu_domain);
> @@ -1477,6 +1480,9 @@ static int arm_smmu_domain_get_attr(struct iommu_domain *domain,
>  	case DOMAIN_ATTR_NESTING:
>  		*(int *)data = (smmu_domain->stage == ARM_SMMU_DOMAIN_NESTED);
>  		return 0;
> +	case DOMAIN_ATTR_USE_SYS_CACHE:
> +		*((int *)data) = smmu_domain->has_sys_cache;
> +		return 0;

I really don't like exposing this to clients directly like this,
particularly as there aren't any in-tree users. I would prefer that we
provide a way for the io-pgtable code to have its MAIR values overridden
so that all non-coherent DMA ends up using the system cache.

Will