Re: [PATCH V3 6/8] xen/grant-dma-iommu: Introduce stub IOMMU driver

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 31 May 2022, Oleksandr Tyshchenko wrote:
> 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
> ---
>  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
> 
> 
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