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 11/19/25 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.

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.

Regards,
Christian.

> 
> Thanks