[PATCH] arm/mm: add option to prefer IOMMU ops for DMA on Xen

[Chuck Zmudzinski <brchuckz@aol.com>]

Enabling the new option, ARM_DMA_USE_IOMMU_XEN, fixes this error when
attaching the Exynos mixer in Linux dom0 on Xen on the Chromebook Snow
(and probably on other devices that use the Exynos mixer):

[drm] Exynos DRM: using 14400000.fimd device for DMA mapping operations
exynos-drm exynos-drm: bound 14400000.fimd (ops 0xc0d96354)
exynos-mixer 14450000.mixer: [drm:exynos_drm_register_dma] *ERROR* Device
                             14450000.mixer lacks support for IOMMU
exynos-drm exynos-drm: failed to bind 14450000.mixer (ops 0xc0d97554): -22
exynos-drm exynos-drm: adev bind failed: -22
exynos-dp: probe of 145b0000.dp-controller failed with error -22

Linux normally uses xen_swiotlb_dma_ops for DMA for all devices when
xen_swiotlb is detected even when Xen exposes an IOMMU to Linux. Enabling
the new config option allows devices such as the Exynos mixer to use the
IOMMU instead of xen_swiotlb_dma_ops for DMA and this fixes the error.

The new config option is not set by default because it is likely some
devices that use IOMMU for DMA on Xen will cause DMA errors and memory
corruption when Xen PV block and network drivers are in use on the system.

Link: https://lore.kernel.org/xen-devel/acfab1c5-eed1-4930-8c70-8681e256c820@netscape.net/

Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
---
The reported error with the Exynos mixer is not fixed by default by adding
a second patch to select the new option in the Kconfig definition for the
Exynos mixer if EXYNOS_IOMMU and SWIOTLB_XEN are enabled because it is
not certain setting the config option is suitable for all cases. So it is
necessary to explicitly select the new config option during the config
stage of the Linux kernel build to fix the reported error or similar
errors that have the same cause of lack of support for IOMMU on Xen. This
is necessary to avoid any regressions that might be caused by enabling the
new option by default for the Exynos mixer.
 arch/arm/mm/dma-mapping.c |  6 ++++++
 drivers/xen/Kconfig       | 16 ++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index 5409225b4abc..ca04fdf01be3 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -1779,6 +1779,12 @@ void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
 	if (iommu)
 		arm_setup_iommu_dma_ops(dev, dma_base, size, iommu, coherent);
 
+#ifdef CONFIG_ARM_DMA_USE_IOMMU_XEN
+	if (dev->dma_ops == &iommu_ops) {
+		dev->archdata.dma_ops_setup = true;
+		return;
+	}
+#endif
 	xen_setup_dma_ops(dev);
 	dev->archdata.dma_ops_setup = true;
 }
diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index d5989871dd5d..44e1334b6acd 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -181,6 +181,22 @@ config SWIOTLB_XEN
 	select DMA_OPS
 	select SWIOTLB
 
+config ARM_DMA_USE_IOMMU_XEN
+	bool "Prefer IOMMU DMA ops on Xen"
+	depends on SWIOTLB_XEN
+	depends on ARM_DMA_USE_IOMMU
+	help
+	  Normally on Xen, the IOMMU is used by Xen and not exposed to
+	  Linux. Some Arm systems such as Exynos have an IOMMU that
+	  Xen does not use so the IOMMU is exposed to Linux in those
+	  cases. This option enables Linux to use the IOMMU instead of
+	  using the Xen swiotlb_dma_ops for DMA on Xen.
+
+	  Say N here unless support for one or more devices that use
+	  IOMMU ops instead of Xen swiotlb ops for DMA is needed and the
+	  devices that use the IOMMU do not cause any problems on the
+	  Xen system in use.
+
 config XEN_PCI_STUB
 	bool
 
-- 
2.39.2