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

[Leon Romanovsky <leon@kernel.org>]

On Wed, Nov 19, 2025 at 03:53:30PM +0100, Christian König wrote:
> 
> 
> On 11/19/25 15:50, Leon Romanovsky wrote:
> > On Wed, Nov 19, 2025 at 03:11:01PM +0100, Christian König wrote:
> >> On 11/19/25 14:42, Leon Romanovsky wrote:
> >>> On Wed, Nov 19, 2025 at 02:16:57PM +0100, Christian König wrote:
> >>>>
> >>>>
> >>>> 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.
> >>>
> >>> This function performs DMA mapping, so what name do you suggest instead of dma_buf_map()?
> >>
> >> Something like dma_buf_phys_vec_to_sg_table(). I'm not good at naming either.
> > 
> > Can I call it simply dma_buf_mapping() as I plan to put that function in dma_buf_mapping.c
> > file per-your request.
> 
> No, just completely drop the term "mapping" here. This is about phys_vector to sg_table conversion and nothing else.

We have both map and unmap, so dma_buf_*_to_*() can be applicable to dma_buf_map() only.
And it is not simple conversion, most of the logic is actually handles mapping:

  137         for (i = 0; i < nr_ranges; i++) {
  138                 if (!dma->state) {
  139                         addr = pci_p2pdma_bus_addr_map(provider,
  140                                                        phys_vec[i].paddr);
  141                 } else if (dma_use_iova(dma->state)) {
  142                         ret = dma_iova_link(attach->dev, dma->state,
  143                                             phys_vec[i].paddr, 0,
  144                                             phys_vec[i].len, dir,
  145                                             DMA_ATTR_MMIO);
  146                         if (ret)
  147                                 goto err_unmap_dma;
  148
  149                         mapped_len += phys_vec[i].len;
  150                 } else {
  151                         addr = dma_map_phys(attach->dev, phys_vec[i].paddr,
  152                                             phys_vec[i].len, dir,
  153                                             DMA_ATTR_MMIO);
  154                         ret = dma_mapping_error(attach->dev, addr);
  155                         if (ret)
  156                                 goto err_unmap_dma;
  157                 }
  158
  159                 if (!dma->state || !dma_use_iova(dma->state))
  160                         sgl = fill_sg_entry(sgl, phys_vec[i].len, addr);
  161         }
  162
  163         if (dma->state && dma_use_iova(dma->state)) {
  164                 WARN_ON_ONCE(mapped_len != size);
  165                 ret = dma_iova_sync(attach->dev, dma->state, 0, mapped_len);
  166                 if (ret)
  167                         goto err_unmap_dma;
  168
  169                 sgl = fill_sg_entry(sgl, mapped_len, dma->state->addr);
  170         }

SG table conversion is only two lines (160 and 169) which are here
because of DMABUF dependency on SG.

What about dma_buf_phys_vec_mapping()/dma_buf_phys_vec_unmapping()?

> 
> That we create an IOVA mapping when the access needs to go through the root complex is an implementation detail.
> 
> > 
> > Regarding SG, the long term plan is to remove SG table completely, so at
> > least external users of DMABUF shouldn't be exposed to internal implementation
> > details (SG table).
> 
> Hui? Well I suggested to remove the sg_table, but that doesn't mean that implementations shouldn't be aware of that.

VFIO which is first user of this interface. It doesn't care how
internally DMABUF handles array of phys_vecs. Today, it is sg_table,
tomorrow it will be something else.

Thanks

> 
> Regards,
> Christian.
> 
> > 
> > Thanks
>