On Mon, Jul 29, 2019 at 11:57:19AM +0200, Geert Uytterhoeven wrote:
Hi Christoph,
On Thu, Jul 25, 2019 at 8:35 AM Christoph Hellwig <hch@xxxxxx> wrote:
Most dma_map_ops instances are IOMMUs that work perfectly fine in 32-bits
of IOVA space, and the generic direct mapping code already provides its
own routines that is intelligent based on the amount of memory actually
present. Wire up the dma-direct routine for the ARM direct mapping code
as well, and otherwise default to the constant 32-bit mask. This way
we only need to override it for the occasional odd IOMMU that requires
64-bit IOVA support, or IOMMU drivers that are more efficient if they
can fall back to the direct mapping.
As I know you like diving into cans of worms ;-)
Does 64-bit IOVA support actually work in general? Or only on 64-bit
platforms, due to dma_addr_t to unsigned long truncation on 32-bit?
Most IOMMUs use 32-bit IOVAs, and thus we default to the 32-bit mask
because it is common and failsafe vs the normal linux assumptions.
However the ia64 SGI SN2 platform, and the powerpc IBM ebus
implementations seem to require a 64-bit mask already, so we keep that
behavior as is.
