[PATCH kernel] powerpc/iommu: Report the correct most efficient DMA mask for PCI devices

[Michael Ellerman]

On Thu, 30 Sep 2021 13:44:54 +1000, Alexey Kardashevskiy wrote:
> According to dma-api.rst, the dma_get_required_mask() helper should return
> "the mask that the platform requires to operate efficiently". Which in
> the case of PPC64 means the bypass mask and not a mask from an IOMMU table
> which is shorter and slower to use due to map/unmap operations (especially
> expensive on "pseries").
> 
> However the existing implementation ignores the possibility of bypassing
> and returns the IOMMU table mask on the pseries platform which makes some
> drivers (mpt3sas is one example) choose 32bit DMA even though bypass is
> supported. The powernv platform sort of handles it by having a bigger
> default window with a mask >=40 but it only works as drivers choose
> 63/64bit if the required mask is >32 which is rather pointless.
> 
> [...]

Applied to powerpc/fixes.

[1/1] powerpc/iommu: Report the correct most efficient DMA mask for PCI devices
      https://git.kernel.org/powerpc/c/23c216b335d1fbd716076e8263b54a714ea3cf0e

cheers