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

Christian König
2025-11-19T14:11:14Z

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.

>
>>
>>> + 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.
>
> I didn't fully understand the email either. The importer needs to
> configure DMA and it supports only MMIO addresses. Exporter controls it
> by asking for peer2peer.

I miss interpreted the call to pci_p2pdma_map_type() here in that now the DMA-buf code decides if transactions go over the root complex or not.

But the exporter can call pci_p2pdma_map_type() even before calling this function, so that looks fine to me.

Regards,
Christian.

>
> Thanks