RFC Stop losing firmware-set DMA masks

Whilst the common firmware code invoked by dma_configure() initialises devices' DMA masks according to limitations described by the respective properties ("dma-ranges" for OF and _DMA/IORT for ACPI), the nature of the dma_set_mask() API leads to that information getting lost when well-behaved drivers probe and set a 64-bit mask, since in general there's no way to tell the difference between a firmware-described mask (which should be respected) and whatever default may have come from the bus code (which should be replaced outright). This can break DMA on systems with certain IOMMU topologies (e.g. [1]) where the IOMMU driver only knows its maximum supported address size, not how many of those address bits might actually be wired up between any of its input interfaces and the associated DMA master devices. Similarly, some PCIe root complexes only have a 32-bit native interface on their host bridge, which leads to the same DMA-address-truncation problem in systems with a larger physical memory map and RAM above 4GB (e.g. [2]).

These patches attempt to deal with this in the simplest way possible by generalising the specific quirk for 32-bit bridges into an arbitrary mask which can then also be plumbed into the firmware code. In the interest of being minimally invasive, I've only included a point fix for the IOMMU issue as seen on arm64 - there may be further tweaks needed in DMA ops to catch all possible incarnations of this problem, but this initial RFC is mostly about the impact beyond the dma-mapping subsystem itself.

Robin.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2018-May/580804.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-December/474443.html

Robin Murphy (4):
dma-mapping: Generalise dma_32bit_limit flag
ACPI/IORT: Set bus DMA mask as appropriate
of/device: Set bus DMA mask as appropriate
iommu/dma: Respect bus DMA limit for IOVAs

 arch/x86/kernel/pci-dma.c | 2 +-
 drivers/acpi/arm64/iort.c | 1 +
 drivers/iommu/dma-iommu.c | 3 +++
 drivers/of/device.c       | 1 +
 include/linux/device.h    | 6 +++---
 kernel/dma/direct.c       | 2 +-
 6 files changed, 10 insertions(+), 5 deletions(-)

2.17.1.dirty