Re: [PATCH v2 4/8] dma-mapping: Introduce DMA require coherency attribute

Leon Romanovsky
2026-03-12T16:46:29Z

On Thu, Mar 12, 2026 at 09:19:37AM -0300, Jason Gunthorpe wrote:
> On Wed, Mar 11, 2026 at 09:08:47PM +0200, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@nvidia.com>
> >
> > The mapping buffers which carry this attribute require DMA coherent system.
> > This means that they can't take SWIOTLB path, can perform CPU cache overlap
> > and doesn't perform cache flushing.
> >
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > ---
> > Documentation/core-api/dma-attributes.rst | 12 ++++++++++++
> > include/linux/dma-mapping.h | 7 +++++++
> > include/trace/events/dma.h | 3 ++-
> > kernel/dma/debug.c | 3 ++-
> > kernel/dma/mapping.c | 6 ++++++
> > 5 files changed, 29 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
> > index 48cfe86cc06d7..69d094f144c70 100644
> > --- a/Documentation/core-api/dma-attributes.rst
> > +++ b/Documentation/core-api/dma-attributes.rst
> > @@ -163,3 +163,15 @@ data corruption.
> >
> > All mappings that share a cache line must set this attribute to suppress DMA
> > debug warnings about overlapping mappings.
> > +
> > +DMA_ATTR_REQUIRE_COHERENT
> > +-------------------------
> > +
> > +The mapping buffers which carry this attribute require DMA coherent system. This means
> > +that they can't take SWIOTLB path, can perform CPU cache overlap and doesn't perform
> > +cache flushing.
>
> DMA mapping requests with the DMA_ATTR_REQUIRE_COHERENT fail on any
> system where SWIOTLB or cache management is required. This should only
> be used to support uAPI designs that require continuous HW DMA
> coherence with userspace processes, for example RDMA and DRM. At a
> minimum the memory being mapped must be userspace memory from
> pin_user_pages() or similar.
>
> Drivers should consider using dma_mmap_pages() instead of this
> interface when building their uAPIs, when possible.
>
> It must never be used in an in-kernel driver that only works with
> kernal memory.
>
> > @@ -164,6 +164,9 @@ dma_addr_t dma_map_phys(struct device *dev, phys_addr_t phys, size_t size,
> > if (WARN_ON_ONCE(!dev->dma_mask))
> > return DMA_MAPPING_ERROR;
> >
> > + if (!dev_is_dma_coherent(dev) && (attrs & DMA_ATTR_REQUIRE_COHERENT))
> > + return DMA_MAPPING_ERROR;
>
> This doesn't capture enough conditions.. is_swiotlb_force_bounce(),
> dma_kmalloc_needs_bounce(), dma_capable(), etc all need to be blocked
> too

These checks exist in dma_direct_map_phys() and here is the common check
between direct and IOMMU modes.

Thanks

>
> So check it inside swiotlb_map() too, and maybe shift the above
> into the existing branches:
>
> if (!dev_is_dma_coherent(dev) &&
> !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_MMIO)))
> arch_sync_dma_for_device(phys, size, dir);
>
> Jason