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

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Date:   Sat, 31 Dec 2022 21:34:55 +0800
From:   Shawn Guo <shawnguo@...nel.org>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     devicetree@...r.kernel.org, iommu@...ts.linux.dev,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org, Li Yang <leoyang.li@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-kernel@...r.kernel.org, Michael Walle <michael@...le.cc>
Subject: Re: [PATCH v2 1/2] arm64: dts: ls1028a: declare cache-coherent page
 table walk feature for IOMMU

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@arm.com/
> Suggested-by: Robin Murphy <robin.murphy@....com>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Applied both, thanks!