Subject: Re: [PATCH v4 00/16] dma-mapping: migrate to physical address-based API

From: Jason Gunthorpe <[email-masked]>
Date: Fri, 29 Aug 2025 10:16:25 -0300

On Tue, Aug 19, 2025 at 08:36:44PM +0300, Leon Romanovsky wrote:

> This series does the core code and modern flows. A followup series
> will give the same treatment to the legacy dma_ops implementation.

I took a quick check over this to see that it is sane.  I think using
phys is an improvement for most of the dma_ops implemenations.

  arch/sparc/kernel/pci_sun4v.c
  arch/sparc/kernel/iommu.c
    Uses __pa to get phys from the page, never touches page

  arch/alpha/kernel/pci_iommu.c
  arch/sparc/mm/io-unit.c
  drivers/parisc/ccio-dma.c
  drivers/parisc/sba_iommu.c
    Does page_addres() and later does __pa on it. Doesn't touch struct page

  arch/x86/kernel/amd_gart_64.c
  drivers/xen/swiotlb-xen.c
  arch/mips/jazz/jazzdma.c
    Immediately does page_to_phys(), never touches struct page

  drivers/vdpa/vdpa_user/vduse_dev.c
    Does page_to_phys() to call iommu_map()

  drivers/xen/grant-dma-ops.c
    Does page_to_pfn() and nothing else

  arch/powerpc/platforms/ps3/system-bus.c
   This is a maze but I think it wants only phys and the virt is only
   used for debug prints.

The above all never touch a KVA and just want a phys_addr_t.

The below are touching the KVA somehow:

  arch/sparc/mm/iommu.c
  arch/arm/mm/dma-mapping.c
    Uses page_address to cache flush, would be happy with phys_to_virt()
    and a PhysHighMem()

  arch/powerpc/kernel/dma-iommu.c
  arch/powerpc/platforms/pseries/vio.c
   Uses iommu_map_page() which wants phys_to_virt(), doesn't touch
   struct page

  arch/powerpc/platforms/pseries/ibmebus.c
    Returns phys_to_virt() as dma_addr_t.

The two PPC ones are weird, I didn't figure out how that was working..

It would be easy to make map_phys patches for about half of these, in
the first grouping. Doing so would also grant those arches
map_resource capability.

Overall I didn't think there was any reduction in maintainability in
these places. Most are improvements eliminating code, and some are
just switching to phys_to_virt() from page_address(), which we could
further guard with DMA_ATTR_MMIO and a check for highmem.

Jason