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SubjectRe: [RFC 1/4] virtio: Define virtio_direct_dma_ops structure
> +/*
> + * Virtio direct mapping DMA API operations structure
> + *
> + * This defines DMA API structure for all virtio devices which would not
> + * either bring in their own DMA OPS from architecture or they would not
> + * like to use architecture specific IOMMU based DMA OPS because QEMU
> + * expects GPA instead of an IOVA in absence of VIRTIO_F_IOMMU_PLATFORM.
> + */
> +dma_addr_t virtio_direct_map_page(struct device *dev, struct page *page,
> + unsigned long offset, size_t size,
> + enum dma_data_direction dir,
> + unsigned long attrs)

All these functions should probably be marked static.

> +void virtio_direct_unmap_page(struct device *hwdev, dma_addr_t dev_addr,
> + size_t size, enum dma_data_direction dir,
> + unsigned long attrs)
> +{
> +}

No need to implement no-op callbacks in struct dma_map_ops.

> +
> +int virtio_direct_mapping_error(struct device *hwdev, dma_addr_t dma_addr)
> +{
> + return 0;
> +}

Including this one.

> +void *virtio_direct_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
> + gfp_t gfp, unsigned long attrs)
> +{
> + void *queue = alloc_pages_exact(PAGE_ALIGN(size), gfp);
> +
> + if (queue) {
> + phys_addr_t phys_addr = virt_to_phys(queue);
> + *dma_handle = (dma_addr_t)phys_addr;
> +
> + if (WARN_ON_ONCE(*dma_handle != phys_addr)) {
> + free_pages_exact(queue, PAGE_ALIGN(size));
> + return NULL;
> + }
> + }
> + return queue;

queue is a very odd name in a generic memory allocator.

> +void virtio_direct_free(struct device *dev, size_t size, void *vaddr,
> + dma_addr_t dma_addr, unsigned long attrs)
> +{
> + free_pages_exact(vaddr, PAGE_ALIGN(size));
> +}
> +
> +const struct dma_map_ops virtio_direct_dma_ops = {
> + .alloc = virtio_direct_alloc,
> + .free = virtio_direct_free,
> + .map_page = virtio_direct_map_page,
> + .unmap_page = virtio_direct_unmap_page,
> + .mapping_error = virtio_direct_mapping_error,
> +};

This is missing a dma_map_sg implementation. In general this is
mandatory for dma_ops. So either you implement it or explain in
a common why you think you can skip it.

> +EXPORT_SYMBOL(virtio_direct_dma_ops);

EXPORT_SYMBOL_GPL like all virtio symbols, please.

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