On Tue, Jul 24, 2018 at 12:16 AM, Robin Murphy <robin.mur...@arm.com> wrote:
> 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 (e.g. in arch/arm/ and other OF users) to catch all
> possible incarnations of this problem, but at least any that I'm not
> fixing here have always been broken. It is also noteworthy that
> of_dma_get_range() has never worked properly for the way PCI host
> bridges are passed into of_dma_configure() - I'll be working on
> further patches to sort that out once this part is done.
Thanks a lot for working on this, this has bugged me for many years,
and I've discussed possible solutions with lots of people over time.
I /think/ all your patches are good, but I'm currently travelling and don't
have a chance to review the resulting overall implementation.
Could you summarize what happens in the following corner cases of
DT dma-ranges after your changes (with a driver not setting a mask,
setting a 64-bit mask and setting a 24-bit mask, respectively)?
a) a device with no dma-ranges property anywhere in its parents
b) a device with with a 64-bit dma-ranges translation in its parent
but none in its grandparent
c) a device with no dma-ranges in its parent but a 64-bit mask
in its grandparent
d) a device with a 24-bit mask in its parent.
Thanks,
Arnd
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