Re: [Linaro-mm-sig] [PATCH v8 06/11] dma-buf: provide phys_vec to scatter-gather mapping routine

["Christian König" <christian.koenig@amd.com>]

On 11/11/25 10:57, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@nvidia.com>
> 
> Add dma_buf_map() and dma_buf_unmap() helpers to convert an array of
> MMIO physical address ranges into scatter-gather tables with proper
> DMA mapping.
> 
> These common functions are a starting point and support any PCI
> drivers creating mappings from their BAR's MMIO addresses. VFIO is one
> case, as shortly will be RDMA. We can review existing DRM drivers to
> refactor them separately. We hope this will evolve into routines to
> help common DRM that include mixed CPU and MMIO mappings.
> 
> Compared to the dma_map_resource() abuse this implementation handles
> the complicated PCI P2P scenarios properly, especially when an IOMMU
> is enabled:
> 
>  - Direct bus address mapping without IOVA allocation for
>    PCI_P2PDMA_MAP_BUS_ADDR, using pci_p2pdma_bus_addr_map(). This
>    happens if the IOMMU is enabled but the PCIe switch ACS flags allow
>    transactions to avoid the host bridge.
> 
>    Further, this handles the slightly obscure, case of MMIO with a
>    phys_addr_t that is different from the physical BAR programming
>    (bus offset). The phys_addr_t is converted to a dma_addr_t and
>    accommodates this effect. This enables certain real systems to
>    work, especially on ARM platforms.
> 
>  - Mapping through host bridge with IOVA allocation and DMA_ATTR_MMIO
>    attribute for MMIO memory regions (PCI_P2PDMA_MAP_THRU_HOST_BRIDGE).
>    This happens when the IOMMU is enabled and the ACS flags are forcing
>    all traffic to the IOMMU - ie for virtualization systems.
> 
>  - Cases where P2P is not supported through the host bridge/CPU. The
>    P2P subsystem is the proper place to detect this and block it.
> 
> Helper functions fill_sg_entry() and calc_sg_nents() handle the
> scatter-gather table construction, splitting large regions into
> UINT_MAX-sized chunks to fit within sg->length field limits.
> 
> Since the physical address based DMA API forbids use of the CPU list
> of the scatterlist this will produce a mangled scatterlist that has
> a fully zero-length and NULL'd CPU list. The list is 0 length,
> all the struct page pointers are NULL and zero sized. This is stronger
> and more robust than the existing mangle_sg_table() technique. It is
> a future project to migrate DMABUF as a subsystem away from using
> scatterlist for this data structure.
> 
> Tested-by: Alex Mastro <amastro@fb.com>
> Tested-by: Nicolin Chen <nicolinc@nvidia.com>
> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> ---
>  drivers/dma-buf/dma-buf.c | 235 ++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/dma-buf.h   |  18 ++++
>  2 files changed, 253 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 2bcf9ceca997..cb55dff1dad5 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1254,6 +1254,241 @@ void dma_buf_unmap_attachment_unlocked(struct dma_buf_attachment *attach,
>  }
>  EXPORT_SYMBOL_NS_GPL(dma_buf_unmap_attachment_unlocked, "DMA_BUF");
>  
> +static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length,
> +					 dma_addr_t addr)
> +{
> +	unsigned int len, nents;
> +	int i;
> +
> +	nents = DIV_ROUND_UP(length, UINT_MAX);
> +	for (i = 0; i < nents; i++) {
> +		len = min_t(size_t, length, UINT_MAX);
> +		length -= len;
> +		/*
> +		 * DMABUF abuses scatterlist to create a scatterlist
> +		 * that does not have any CPU list, only the DMA list.
> +		 * Always set the page related values to NULL to ensure
> +		 * importers can't use it. The phys_addr based DMA API
> +		 * does not require the CPU list for mapping or unmapping.
> +		 */
> +		sg_set_page(sgl, NULL, 0, 0);
> +		sg_dma_address(sgl) = addr + i * UINT_MAX;
> +		sg_dma_len(sgl) = len;
> +		sgl = sg_next(sgl);
> +	}
> +
> +	return sgl;
> +}
> +
> +static unsigned int calc_sg_nents(struct dma_iova_state *state,
> +				  struct dma_buf_phys_vec *phys_vec,
> +				  size_t nr_ranges, size_t size)
> +{
> +	unsigned int nents = 0;
> +	size_t i;
> +
> +	if (!state || !dma_use_iova(state)) {
> +		for (i = 0; i < nr_ranges; i++)
> +			nents += DIV_ROUND_UP(phys_vec[i].len, UINT_MAX);
> +	} else {
> +		/*
> +		 * In IOVA case, there is only one SG entry which spans
> +		 * for whole IOVA address space, but we need to make sure
> +		 * that it fits sg->length, maybe we need more.
> +		 */
> +		nents = DIV_ROUND_UP(size, UINT_MAX);
> +	}
> +
> +	return nents;
> +}
> +
> +/**
> + * struct dma_buf_dma - holds DMA mapping information
> + * @sgt:    Scatter-gather table
> + * @state:  DMA IOVA state relevant in IOMMU-based DMA
> + * @size:   Total size of DMA transfer
> + */
> +struct dma_buf_dma {
> +	struct sg_table sgt;
> +	struct dma_iova_state *state;
> +	size_t size;
> +};
> +
> +/**
> + * dma_buf_map - Returns the scatterlist table of the attachment from arrays
> + * of physical vectors. This funciton is intended for MMIO memory only.
> + * @attach:	[in]	attachment whose scatterlist is to be returned
> + * @provider:	[in]	p2pdma provider
> + * @phys_vec:	[in]	array of physical vectors
> + * @nr_ranges:	[in]	number of entries in phys_vec array
> + * @size:	[in]	total size of phys_vec
> + * @dir:	[in]	direction of DMA transfer
> + *
> + * Returns sg_table containing the scatterlist to be returned; returns ERR_PTR
> + * on error. May return -EINTR if it is interrupted by a signal.
> + *
> + * On success, the DMA addresses and lengths in the returned scatterlist are
> + * PAGE_SIZE aligned.
> + *
> + * A mapping must be unmapped by using dma_buf_unmap().
> + */
> +struct sg_table *dma_buf_map(struct dma_buf_attachment *attach,

That is clearly not a good name for this function. We already have overloaded the term *mapping* with something completely different.

> +			     struct p2pdma_provider *provider,
> +			     struct dma_buf_phys_vec *phys_vec,
> +			     size_t nr_ranges, size_t size,
> +			     enum dma_data_direction dir)
> +{
> +	unsigned int nents, mapped_len = 0;
> +	struct dma_buf_dma *dma;
> +	struct scatterlist *sgl;
> +	dma_addr_t addr;
> +	size_t i;
> +	int ret;
> +
> +	dma_resv_assert_held(attach->dmabuf->resv);
> +
> +	if (WARN_ON(!attach || !attach->dmabuf || !provider))
> +		/* This function is supposed to work on MMIO memory only */
> +		return ERR_PTR(-EINVAL);
> +
> +	dma = kzalloc(sizeof(*dma), GFP_KERNEL);
> +	if (!dma)
> +		return ERR_PTR(-ENOMEM);
> +
> +	switch (pci_p2pdma_map_type(provider, attach->dev)) {
> +	case PCI_P2PDMA_MAP_BUS_ADDR:
> +		/*
> +		 * There is no need in IOVA at all for this flow.
> +		 */
> +		break;
> +	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> +		dma->state = kzalloc(sizeof(*dma->state), GFP_KERNEL);
> +		if (!dma->state) {
> +			ret = -ENOMEM;
> +			goto err_free_dma;
> +		}
> +
> +		dma_iova_try_alloc(attach->dev, dma->state, 0, size);

Oh, that is a clear no-go for the core DMA-buf code.

It's intentionally up to the exporter how to create the DMA addresses the importer can work with.

We could add something like a dma_buf_sg_helper.c or similar and put it in there.

Regards,
Christian.


> +		break;
> +	default:
> +		ret = -EINVAL;
> +		goto err_free_dma;
> +	}
> +
> +	nents = calc_sg_nents(dma->state, phys_vec, nr_ranges, size);
> +	ret = sg_alloc_table(&dma->sgt, nents, GFP_KERNEL | __GFP_ZERO);
> +	if (ret)
> +		goto err_free_state;
> +
> +	sgl = dma->sgt.sgl;
> +
> +	for (i = 0; i < nr_ranges; i++) {
> +		if (!dma->state) {
> +			addr = pci_p2pdma_bus_addr_map(provider,
> +						       phys_vec[i].paddr);
> +		} else if (dma_use_iova(dma->state)) {
> +			ret = dma_iova_link(attach->dev, dma->state,
> +					    phys_vec[i].paddr, 0,
> +					    phys_vec[i].len, dir,
> +					    DMA_ATTR_MMIO);
> +			if (ret)
> +				goto err_unmap_dma;
> +
> +			mapped_len += phys_vec[i].len;
> +		} else {
> +			addr = dma_map_phys(attach->dev, phys_vec[i].paddr,
> +					    phys_vec[i].len, dir,
> +					    DMA_ATTR_MMIO);
> +			ret = dma_mapping_error(attach->dev, addr);
> +			if (ret)
> +				goto err_unmap_dma;
> +		}
> +
> +		if (!dma->state || !dma_use_iova(dma->state))
> +			sgl = fill_sg_entry(sgl, phys_vec[i].len, addr);
> +	}
> +
> +	if (dma->state && dma_use_iova(dma->state)) {
> +		WARN_ON_ONCE(mapped_len != size);
> +		ret = dma_iova_sync(attach->dev, dma->state, 0, mapped_len);
> +		if (ret)
> +			goto err_unmap_dma;
> +
> +		sgl = fill_sg_entry(sgl, mapped_len, dma->state->addr);
> +	}
> +
> +	dma->size = size;
> +
> +	/*
> +	 * No CPU list included — set orig_nents = 0 so others can detect
> +	 * this via SG table (use nents only).
> +	 */
> +	dma->sgt.orig_nents = 0;
> +
> +
> +	/*
> +	 * SGL must be NULL to indicate that SGL is the last one
> +	 * and we allocated correct number of entries in sg_alloc_table()
> +	 */
> +	WARN_ON_ONCE(sgl);
> +	return &dma->sgt;
> +
> +err_unmap_dma:
> +	if (!i || !dma->state) {
> +		; /* Do nothing */
> +	} else if (dma_use_iova(dma->state)) {
> +		dma_iova_destroy(attach->dev, dma->state, mapped_len, dir,
> +				 DMA_ATTR_MMIO);
> +	} else {
> +		for_each_sgtable_dma_sg(&dma->sgt, sgl, i)
> +			dma_unmap_phys(attach->dev, sg_dma_address(sgl),
> +				       sg_dma_len(sgl), dir, DMA_ATTR_MMIO);
> +	}
> +	sg_free_table(&dma->sgt);
> +err_free_state:
> +	kfree(dma->state);
> +err_free_dma:
> +	kfree(dma);
> +	return ERR_PTR(ret);
> +}
> +EXPORT_SYMBOL_NS_GPL(dma_buf_map, "DMA_BUF");
> +
> +/**
> + * dma_buf_unmap - unmaps the buffer
> + * @attach:	[in]	attachment to unmap buffer from
> + * @sgt:	[in]	scatterlist info of the buffer to unmap
> + * @direction:	[in]	direction of DMA transfer
> + *
> + * This unmaps a DMA mapping for @attached obtained by dma_buf_map().
> + */
> +void dma_buf_unmap(struct dma_buf_attachment *attach, struct sg_table *sgt,
> +		   enum dma_data_direction dir)
> +{
> +	struct dma_buf_dma *dma = container_of(sgt, struct dma_buf_dma, sgt);
> +	int i;
> +
> +	dma_resv_assert_held(attach->dmabuf->resv);
> +
> +	if (!dma->state) {
> +		; /* Do nothing */
> +	} else if (dma_use_iova(dma->state)) {
> +		dma_iova_destroy(attach->dev, dma->state, dma->size, dir,
> +				 DMA_ATTR_MMIO);
> +	} else {
> +		struct scatterlist *sgl;
> +
> +		for_each_sgtable_dma_sg(sgt, sgl, i)
> +			dma_unmap_phys(attach->dev, sg_dma_address(sgl),
> +				       sg_dma_len(sgl), dir, DMA_ATTR_MMIO);
> +	}
> +
> +	sg_free_table(sgt);
> +	kfree(dma->state);
> +	kfree(dma);
> +
> +}
> +EXPORT_SYMBOL_NS_GPL(dma_buf_unmap, "DMA_BUF");
> +
>  /**
>   * dma_buf_move_notify - notify attachments that DMA-buf is moving
>   *
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index d58e329ac0e7..545ba27a5040 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -22,6 +22,7 @@
>  #include <linux/fs.h>
>  #include <linux/dma-fence.h>
>  #include <linux/wait.h>
> +#include <linux/pci-p2pdma.h>
>  
>  struct device;
>  struct dma_buf;
> @@ -530,6 +531,16 @@ struct dma_buf_export_info {
>  	void *priv;
>  };
>  
> +/**
> + * struct dma_buf_phys_vec - describe continuous chunk of memory
> + * @paddr:   physical address of that chunk
> + * @len:     Length of this chunk
> + */
> +struct dma_buf_phys_vec {
> +	phys_addr_t paddr;
> +	size_t len;
> +};
> +
>  /**
>   * DEFINE_DMA_BUF_EXPORT_INFO - helper macro for exporters
>   * @name: export-info name
> @@ -609,4 +620,11 @@ int dma_buf_vmap_unlocked(struct dma_buf *dmabuf, struct iosys_map *map);
>  void dma_buf_vunmap_unlocked(struct dma_buf *dmabuf, struct iosys_map *map);
>  struct dma_buf *dma_buf_iter_begin(void);
>  struct dma_buf *dma_buf_iter_next(struct dma_buf *dmbuf);
> +struct sg_table *dma_buf_map(struct dma_buf_attachment *attach,
> +			     struct p2pdma_provider *provider,
> +			     struct dma_buf_phys_vec *phys_vec,
> +			     size_t nr_ranges, size_t size,
> +			     enum dma_data_direction dir);
> +void dma_buf_unmap(struct dma_buf_attachment *attach, struct sg_table *sgt,
> +		   enum dma_data_direction dir);
>  #endif /* __DMA_BUF_H__ */
>