Re: [PATCH] iommu: arm-smmu: Fix Tegra workaround for PAGE_SIZE mappings

[Will Deacon]

On Wed, Jul 17, 2024 at 07:21:45AM +0000, Ashish Mhetre wrote:
> PAGE_SIZE can be 16KB for Tegra which is not supported by MMU-500 on
> both Tegra194 and Tegra234. So, use 4KB mappings when PAGE_SIZE is 16KB.
> 
> Signed-off-by: Ashish Mhetre <amhetre@xxxxxxxxxx>
> ---
>  drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c b/drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c
> index 4b2994b6126d..bb621a94f6fe 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c
> @@ -273,11 +273,12 @@ static int nvidia_smmu_init_context(struct arm_smmu_domain *smmu_domain,
>  	 * release of PMD entry and avoid translations seeing stale PMD entry in
>  	 * walk cache.
>  	 * Fix this by limiting the page mappings to PAGE_SIZE on Tegra194 and
> -	 * Tegra234.
> +	 * Tegra234. Use 4K page mappings if PAGE_SIZE is 16K as MMU500 doesn't
> +	 * support it.
>  	 */
>  	if (of_device_is_compatible(np, "nvidia,tegra234-smmu") ||
>  	    of_device_is_compatible(np, "nvidia,tegra194-smmu")) {
> -		smmu->pgsize_bitmap = PAGE_SIZE;
> +		smmu->pgsize_bitmap = (PAGE_SIZE == SZ_16K) ? SZ_4K : PAGE_SIZE;

Perhaps it would be more robust to do a read-modify-write of the
pgsize_bitmap, preserving all the page-sizes that are <= PAGE_SIZE (i.e.
and it with something like GENMASK(PAGE_SHIFT, 0)?

Will