On Mon, Aug 04, 2025 at 03:42:43PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leo...@nvidia.com>
>
> Extend base DMA page API to handle MMIO flow.
I would mention here this follows the long ago agreement that we don't
need to enable P2P in the legacy dma_ops area. Simply failing when
getting an ATTR_MMIO is OK.
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -158,6 +158,7 @@ dma_addr_t dma_map_page_attrs(struct device *dev, struct
> page *page,
> {
> const struct dma_map_ops *ops = get_dma_ops(dev);
> phys_addr_t phys = page_to_phys(page) + offset;
> + bool is_mmio = attrs & DMA_ATTR_MMIO;
> dma_addr_t addr;
>
> BUG_ON(!valid_dma_direction(dir));
> @@ -166,12 +167,23 @@ dma_addr_t dma_map_page_attrs(struct device *dev,
> struct page *page,
> return DMA_MAPPING_ERROR;
>
> if (dma_map_direct(dev, ops) ||
> - arch_dma_map_phys_direct(dev, phys + size))
> + (!is_mmio && arch_dma_map_phys_direct(dev, phys + size)))
> addr = dma_direct_map_phys(dev, phys, size, dir, attrs);
I don't know this area, maybe explain a bit in the commit message how
you see ATTR_MMIO interacts with arch_dma_map_phys_direct ?
> else if (use_dma_iommu(dev))
> addr = iommu_dma_map_phys(dev, phys, size, dir, attrs);
> - else
> + else if (is_mmio) {
> + if (!ops->map_resource)
> + return DMA_MAPPING_ERROR;
> +
> + addr = ops->map_resource(dev, phys, size, dir, attrs);
> + } else {
> + /*
> + * All platforms which implement .map_page() don't support
> + * non-struct page backed addresses.
> + */
> addr = ops->map_page(dev, page, offset, size, dir, attrs);
Comment could be clearer maybe just:
The dma_ops API contract for ops->map_page() requires kmappable memory, while
ops->map_resource() does not.
But this approach looks good to me, it prevents non-kmappable phys
from going down to the legacy dma_ops map_page where it cannot work.
>From here you could do what Marek and Christoph asked to flush the
struct page out of the ops->map_page() and replace it with
kmap_local_phys().
Jason
