Re: [PATCH v2 1/2] arm64: dts: ls1028a: declare cache-coherent page table walk feature for IOMMU

[Shawn Guo]

On Thu, Dec 15, 2022 at 03:56:35PM +0200, Vladimir Oltean wrote:
> The SMMUv2 driver for MMU-500 reads the ARM_SMMU_GR0_ID0 register at
> probe time and tries to determine based on the CTTW (Coherent
> Translation Table Walk) bit whether this feature is supported.
> 
> Unfortunately, it looks like the SMMU integration in the NXP LS1028A has
> wrongly tied the cfg_cttw signal to 0, even though the SoC documentation
> specifies that "The SMMU supports cache coherency for page table walks
> and DVM transactions for page table cache maintenance operations."
> 
> Device tree provides the option of overriding the ID register via the
> dma-coherent property since commit bae2c2d421cd ("iommu/arm-smmu: Sort
> out coherency"), and that's what we do here.
> 
> Telling struct io_pgtable_cfg that the SMMU page table walks are
> coherent with the CPU caches brings performance benefits, because it
> avoids certain operations such as __arm_lpae_sync_pte() for PTE updates.
> 
> Link: https://lore.kernel.org/linux-iommu/3f3112e4-65ff-105d-8cd7-60495ec9054a@xxxxxxx/
> Suggested-by: Robin Murphy <robin.murphy@xxxxxxx>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>

Applied both, thanks!