quoted

For OF platforms, this is called via of_dma_configure(), that checks
dma-ranges of node that is *parent* for host bridge. Host bridge
currently does not control this at all.

We need to think about this a bit. Is it actually the PCI host
bridge that limits the ranges here, or the bus that it is connected
to. In the latter case, the caller needs to be adapted to handle
both.

In r-car case, I'm not sure what is the source of limitation at physical
level.

pcie-rcar driver configures ranges for PCIe inbound transactions based
on dma-ranges property in it's device tree node. In the current device
tree for this platform, that only contains one range and it is in lower
memory.

NVMe driver tries i/o to kmalloc()ed area. That returns 0x5xxxxxxxx
addresses here. As a quick experiment, I tried to add second range to
pcie-rcar's dma-ranges to cover 0x5xxxxxxxx area - but that did not make
DMA to high addresses working.

My current understanding is that host bridge hardware module can't
handle inbound transactions to PCI addresses above 4G - and this
limitations comes from host bridge itself.

I've read somewhere in the lists that pcie-rcar hardware is "32-bit" -
but I don't remember where, and don't know lowlevel details. Maybe
somebody from linux-renesas can elaborate?

quoted

In current device trees no dma-ranges is defined for nodes that are
parents to pci host bridges. This will make of_dma_configure() to fall
back to 32-bit size for all devices on all current platforms.  Thus
applying this patch will immediately break 64-bit dma masks on all
hardware that supports it.

No, it won't break it, it will just fall back to swiotlb for all the
ones that are lacking the dma-ranges property. I think this is correct
behavior.

I'd say - for all ones that have parents without dma-ranges property.

As of 4.10-rc2, I see only two definitions of wide parent dma-ranges
under arch/arm64/boot/dts/ - in amd/amd-seattle-soc.dtsi and
apm/apm-storm.dtsi

Are these the only arm64 platforms that can to DMA to high addresses?
I'm not arm64 expert but I'd be surprised if that's the case.

quoted

Also related: dma-ranges property used by several pci host bridges is
*not* compatible with "legacy" dma-ranges parsed by of_get_dma_range() -
former uses additional flags word at beginning.

Can you elaborate? Do we have PCI host bridges that use wrongly formatted
dma-ranges properties?

of_dma_get_range() expects <dma_addr cpu_addr size> format.

pcie-rcar.c, pci-rcar-gen2.c, pci-xgene.c and pcie-iproc.c from
drivers/pci/host/ all parse dma-ranges using of_pci_range_parser that
uses <flags pci-addr cpu-addr size> format - i.e. something different
from what of_dma_get_range() uses.


Nikita