[PATCH V4 6/8] xen/grant-dma-iommu: Introduce stub IOMMU driver

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

In order to reuse generic IOMMU device tree bindings by Xen grant
DMA-mapping layer we need to add this stub driver from a fw_devlink
perspective (grant-dma-ops cannot be converted into the proper
IOMMU driver).

Otherwise, just reusing IOMMU bindings (without having a corresponding
driver) leads to the deferred probe timeout afterwards, because
the IOMMU device never becomes available.

This stub driver does nothing except registering empty iommu_ops,
the upper layer "of_iommu" will treat this as NO_IOMMU condition
and won't return -EPROBE_DEFER.

As this driver is quite different from the most hardware IOMMU
implementations and only needed in Xen guests, place it in drivers/xen
directory. The subsequent commit will make use of it.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
According to the discussion at:
https://lore.kernel.org/xen-devel/c0f78aab-e723-fe00-a310-9fe52ec75e48@xxxxxxxxx/

Change V2 -> V3:
   - new patch

Changes V3 -> V4:
   - add Stefano's R-b
---
 drivers/xen/Kconfig           |  4 +++
 drivers/xen/Makefile          |  1 +
 drivers/xen/grant-dma-iommu.c | 78 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 83 insertions(+)
 create mode 100644 drivers/xen/grant-dma-iommu.c

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index a7bd8ce..35d20d9 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -335,6 +335,10 @@ config XEN_UNPOPULATED_ALLOC
          having to balloon out RAM regions in order to obtain physical memory
          space to create such mappings.
 
+config XEN_GRANT_DMA_IOMMU
+       bool
+       select IOMMU_API
+
 config XEN_GRANT_DMA_OPS
        bool
        select DMA_OPS
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 1a23cb0..c0503f1 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -40,3 +40,4 @@ xen-privcmd-y                         := privcmd.o 
privcmd-buf.o
 obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF)    += xen-front-pgdir-shbuf.o
 obj-$(CONFIG_XEN_UNPOPULATED_ALLOC)    += unpopulated-alloc.o
 obj-$(CONFIG_XEN_GRANT_DMA_OPS)                += grant-dma-ops.o
+obj-$(CONFIG_XEN_GRANT_DMA_IOMMU)      += grant-dma-iommu.o
diff --git a/drivers/xen/grant-dma-iommu.c b/drivers/xen/grant-dma-iommu.c
new file mode 100644
index 00000000..16b8bc0
--- /dev/null
+++ b/drivers/xen/grant-dma-iommu.c
@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Stub IOMMU driver which does nothing.
+ * The main purpose of it being present is to reuse generic IOMMU device tree
+ * bindings by Xen grant DMA-mapping layer.
+ *
+ * Copyright (C) 2022 EPAM Systems Inc.
+ */
+
+#include <linux/iommu.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+
+struct grant_dma_iommu_device {
+       struct device *dev;
+       struct iommu_device iommu;
+};
+
+/* Nothing is really needed here */
+static const struct iommu_ops grant_dma_iommu_ops;
+
+static const struct of_device_id grant_dma_iommu_of_match[] = {
+       { .compatible = "xen,grant-dma" },
+       { },
+};
+
+static int grant_dma_iommu_probe(struct platform_device *pdev)
+{
+       struct grant_dma_iommu_device *mmu;
+       int ret;
+
+       mmu = devm_kzalloc(&pdev->dev, sizeof(*mmu), GFP_KERNEL);
+       if (!mmu)
+               return -ENOMEM;
+
+       mmu->dev = &pdev->dev;
+
+       ret = iommu_device_register(&mmu->iommu, &grant_dma_iommu_ops, 
&pdev->dev);
+       if (ret)
+               return ret;
+
+       platform_set_drvdata(pdev, mmu);
+
+       return 0;
+}
+
+static int grant_dma_iommu_remove(struct platform_device *pdev)
+{
+       struct grant_dma_iommu_device *mmu = platform_get_drvdata(pdev);
+
+       platform_set_drvdata(pdev, NULL);
+       iommu_device_unregister(&mmu->iommu);
+
+       return 0;
+}
+
+static struct platform_driver grant_dma_iommu_driver = {
+       .driver = {
+               .name = "grant-dma-iommu",
+               .of_match_table = grant_dma_iommu_of_match,
+       },
+       .probe = grant_dma_iommu_probe,
+       .remove = grant_dma_iommu_remove,
+};
+
+static int __init grant_dma_iommu_init(void)
+{
+       struct device_node *iommu_np;
+
+       iommu_np = of_find_matching_node(NULL, grant_dma_iommu_of_match);
+       if (!iommu_np)
+               return 0;
+
+       of_node_put(iommu_np);
+
+       return platform_driver_register(&grant_dma_iommu_driver);
+}
+subsys_initcall(grant_dma_iommu_init);
-- 
2.7.4