linux-kernel - Re: [PATCH v1 09/17] docs: core-api: document the IOVA-based API

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Message-ID: <19cf7d58-4a28-4ce8-9524-8c99fdc79062@infradead.org>
Date: Wed, 30 Oct 2024 18:41:21 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Leon Romanovsky <leon@...nel.org>, Jens Axboe <axboe@...nel.dk>,
 Jason Gunthorpe <jgg@...pe.ca>, Robin Murphy <robin.murphy@....com>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>
Cc: Keith Busch <kbusch@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
 Logan Gunthorpe <logang@...tatee.com>, Yishai Hadas <yishaih@...dia.com>,
 Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
 Kevin Tian <kevin.tian@...el.com>,
 Alex Williamson <alex.williamson@...hat.com>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 Jérôme Glisse <jglisse@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-block@...r.kernel.org, linux-rdma@...r.kernel.org,
 iommu@...ts.linux.dev, linux-nvme@...ts.infradead.org,
 linux-pci@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 09/17] docs: core-api: document the IOVA-based API

(nits)

On 10/30/24 8:12 AM, Leon Romanovsky wrote:
> From: Christoph Hellwig <hch@....de>
> 
> Add an explanation of the newly added IOVA-based mapping API.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> ---
>  Documentation/core-api/dma-api.rst | 70 ++++++++++++++++++++++++++++++
>  1 file changed, 70 insertions(+)
> 
> diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst
> index 8e3cce3d0a23..6095696a65a7 100644
> --- a/Documentation/core-api/dma-api.rst
> +++ b/Documentation/core-api/dma-api.rst
> @@ -530,6 +530,76 @@ routines, e.g.:::
>  		....
>  	}
>  
> +Part Ie - IOVA-based DMA mappings
> +---------------------------------
> +
> +These APIs allow a very efficient mapping when using an IOMMU.  They are an
> +optional path that requires extra code and are only recommended for drivers
> +where DMA mapping performance, or the space usage for storing the DMA addresses
> +matter.  All the consideration from the previous section apply here as well.

                    considerations

> +
> +::
> +
> +    bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state,
> +		phys_addr_t phys, size_t size);
> +
> +Is used to try to allocate IOVA space for mapping operation.  If it returns
> +false this API can't be used for the given device and the normal streaming
> +DMA mapping API should be used.  The ``struct dma_iova_state`` is allocated
> +by the driver and must be kept around until unmap time.
> +
> +::
> +
> +    static inline bool dma_use_iova(struct dma_iova_state *state)
> +
> +Can be used by the driver to check if the IOVA-based API is used after a
> +call to dma_iova_try_alloc.  This can be useful in the unmap path.
> +
> +::
> +
> +    int dma_iova_link(struct device *dev, struct dma_iova_state *state,
> +		phys_addr_t phys, size_t offset, size_t size,
> +		enum dma_data_direction dir, unsigned long attrs);
> +
> +Is used to link ranges to the IOVA previously allocated.  The start of all
> +but the first call to dma_iova_link for a given state must be aligned
> +to the DMA merge boundary returned by ``dma_get_merge_boundary())``, and
> +the size of all but the last range must be aligned to the DMA merge boundary
> +as well.
> +
> +::
> +
> +    int dma_iova_sync(struct device *dev, struct dma_iova_state *state,
> +		size_t offset, size_t size);
> +
> +Must be called to sync the IOMMU page tables for IOVA-range mapped by one or
> +more calls to ``dma_iova_link()``.
> +
> +For drivers that use a one-shot mapping, all ranges can be unmapped and the
> +IOVA freed by calling:
> +
> +::
> +
> +   void dma_iova_destroy(struct device *dev, struct dma_iova_state *state,
> +		enum dma_data_direction dir, unsigned long attrs);
> +
> +Alternatively drivers can dynamically manage the IOVA space by unmapping
> +and mapping individual regions.  In that case
> +
> +::
> +
> +    void dma_iova_unlink(struct device *dev, struct dma_iova_state *state,
> +		size_t offset, size_t size, enum dma_data_direction dir,
> +		unsigned long attrs);
> +
> +is used to unmap a range previous mapped, and

                            previously

> +
> +::
> +
> +   void dma_iova_free(struct device *dev, struct dma_iova_state *state);
> +
> +is used to free the IOVA space.  All regions must have been unmapped using
> +``dma_iova_unlink()`` before calling ``dma_iova_free()``.
>  
>  Part II - Non-coherent DMA allocations
>  --------------------------------------

-- 
~Randy