Re: [PATCH 05/18] dma: Provide an interface to allow allocate IOVA

[Baolu Lu]

On 2024/10/27 22:21, Leon Romanovsky wrote:
> From: Leon Romanovsky<leonro@xxxxxxxxxx>
>
> The existing .map_page() callback provides both allocating of IOVA
> and linking DMA pages. That combination works great for most of the
> callers who use it in control paths, but is less effective in fast
> paths where there may be multiple calls to map_page().
>
> These advanced callers already manage their data in some sort of
> database and can perform IOVA allocation in advance, leaving range
> linkage operation to be in fast path.
>
> Provide an interface to allocate/deallocate IOVA and next patch
> link/unlink DMA ranges to that specific IOVA.
>
> The API is exported from dma-iommu as it is the only implementation
> supported, the namespace is clearly different from iommu_* functions
> which are not allowed to be used. This code layout allows us to save
> function call per API call used in datapath as well as a lot of boilerplate
> code.
>
> Signed-off-by: Leon Romanovsky<leonro@xxxxxxxxxx>
> ---
>   drivers/iommu/dma-iommu.c   | 79 +++++++++++++++++++++++++++++++++++++
>   include/linux/dma-mapping.h | 15 +++++++
>   2 files changed, 94 insertions(+)
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index c422e36c0d66..0644152c5aad 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1745,6 +1745,85 @@ size_t iommu_dma_max_mapping_size(struct device *dev)
>   	return SIZE_MAX;
>   }
>   
> +static bool iommu_dma_iova_alloc(struct device *dev,
> +		struct dma_iova_state *state, phys_addr_t phys, size_t size)
> +{
> +	struct iommu_domain *domain = iommu_get_dma_domain(dev);
> +	struct iommu_dma_cookie *cookie = domain->iova_cookie;
> +	struct iova_domain *iovad = &cookie->iovad;
> +	size_t iova_off = iova_offset(iovad, phys);
> +	dma_addr_t addr;
> +
> +	if (WARN_ON_ONCE(!size))
> +		return false;
> +	if (WARN_ON_ONCE(size & DMA_IOVA_USE_SWIOTLB))
> +		return false;
> +
> +	addr = iommu_dma_alloc_iova(domain,
> +			iova_align(iovad, size + iova_off),
> +			dma_get_mask(dev), dev);
> +	if (!addr)
> +		return false;
> +
> +	state->addr = addr + iova_off;
> +	state->__size = size;
> +	return true;
> +}
> +
> +/**
> + * dma_iova_try_alloc - Try to allocate an IOVA space
> + * @dev: Device to allocate the IOVA space for
> + * @state: IOVA state
> + * @phys: physical address
I'm curious to know why a physical address is necessary for IOVA space
allocation. Could you please elaborate?

> + * @size: IOVA size
> + *
> + * Check if @dev supports the IOVA-based DMA API, and if yes allocate IOVA space
> + * for the given base address and size.
> + *
> + * Note: @phys is only used to calculate the IOVA alignment. Callers that always
> + * do PAGE_SIZE aligned transfers can safely pass 0 here.
> + *
> + * Returns %true if the IOVA-based DMA API can be used and IOVA space has been
> + * allocated, or %false if the regular DMA API should be used.
> + */
> +bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state,
> +		phys_addr_t phys, size_t size)
> +{
> +	memset(state, 0, sizeof(*state));
> +	if (!use_dma_iommu(dev))
> +		return false;
> +	if (static_branch_unlikely(&iommu_deferred_attach_enabled) &&
> +	    iommu_deferred_attach(dev, iommu_get_domain_for_dev(dev)))
> +		return false;
> +	return iommu_dma_iova_alloc(dev, state, phys, size);
> +}
> +EXPORT_SYMBOL_GPL(dma_iova_try_alloc);

Thanks,
baolu