linux-kernel - [RFC PATCH] arm64: dts: ls1028a: mark ARM SMMU as DMA coherent

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Date:   Thu,  8 Dec 2022 17:15:14 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     devicetree@...r.kernel.org, iommu@...ts.linux.dev
Cc:     Laurentiu Tudor <laurentiu.tudor@....com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Shawn Guo <shawnguo@...nel.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: [RFC PATCH] arm64: dts: ls1028a: mark ARM SMMU as DMA coherent

Since commit df198b37e72c ("iommu/arm-smmu: Report
IOMMU_CAP_CACHE_COHERENCY better"), the SMMU driver will say that a
device has the IOMMU_CAP_CACHE_COHERENCY capability if the
ARM_SMMU_FEAT_COHERENT_WALK bit was set in smmu->features.

This breaks vfio-pci, as can be seen below:

$ echo 0000:00:00.0 > /sys/bus/pci/drivers/fsl_enetc/unbind
$ echo vfio-pci > /sys/bus/pci/devices/0000\:00\:00.0/driver_override
$ echo 0000:00:00.0 > /sys/bus/pci/drivers/vfio-pci/bind
[   25.261941] vfio-pci 0000:00:00.0: arm_smmu_capable: smmu features 0xe9e
[   25.268877] vfio-pci 0000:00:00.0: vfio_group_find_or_alloc: device_iommu_capable() returned false
[   25.279271] vfio-pci 0000:00:00.0: vfio_pci_core_register_device: failed to register group dev: -EINVAL
[   25.301377] vfio-pci: probe of 0000:00:00.0 failed with error -22

The ARM_SMMU_FEAT_COHERENT_WALK feature is set in
arm_smmu_device_dt_probe() if the OF node of the SMMU device was set as
dma-coherent. In the case of LS1028A, it wasn't.

Fix that.

Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
---
The LS1028A is not the only SoC affected by this, it seems.
fsl-ls1088a.dtsi seems to be in the same situation where vfio-pci worked
before. There are also other SoCs which don't have dma-coherent in the
iommu node. There's also something I don't quite like about this patch
technically introducing a regression which requires a device tree update.

Can something different be done about that, or are LS1028A/LS1088A
simply to blame because of breaching the dt-bindings contract, and in
that case, I'll have to suck it up, put a Fixes tag here, write another
patch for LS1088A, and resend?

 arch/arm64/boot/dts/freescale/fsl-ls1028a.dtsi | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/boot/dts/freescale/fsl-ls1028a.dtsi b/arch/arm64/boot/dts/freescale/fsl-ls1028a.dtsi
index 383829ec7be7..bcce189c7a0a 100644
--- a/arch/arm64/boot/dts/freescale/fsl-ls1028a.dtsi
+++ b/arch/arm64/boot/dts/freescale/fsl-ls1028a.dtsi
@@ -715,6 +715,7 @@ smmu: iommu@...0000 {
 			#global-interrupts = <8>;
 			#iommu-cells = <1>;
 			stream-match-mask = <0x7c00>;
+			dma-coherent;
 			/* global secure fault */
 			interrupts = <GIC_SPI 13 IRQ_TYPE_LEVEL_HIGH>,
 			/* combined secure interrupt */
-- 
2.34.1