Re: [PATCH v2] arm64: do not set dma masks that device connection can't handle

[Nikita Yushchenko <nikita.yoush@cogentembedded.com>]

> I actually have a third variation of this problem involving a PCI root
> complex which *could* drive full-width (40-bit) addresses, but won't,
> due to the way its PCI<->AXI interface is programmed. That would require
> even more complicated dma-ranges handling to describe the windows of
> valid physical addresses which it *will* pass, so I'm not pressing the
> issue - let's just get the basic DMA mask case fixed first.

R-Car + NVMe is actually not "basic case".

It has PCI<->AXI interface involved.
PCI addresses are 64-bit and controller does handle 64-bit addresses
there. Mapping between PCI addresses and AXI addresses is defined. But
AXI is 32-bit.

SoC has iommu that probably could be used between PCIe module and RAM.
Although AFAIK nobody made that working yet.

Board I work with has 4G of RAM, in 4 banks, located at different parts
of wide address space, and only one of them is below 4G. But if iommu is
capable of translating addresses such that 4 gigabyte banks map to first
4 gigabytes of address space, then all memory will become available for
DMA from PCIe device.