RPi4 can't deal with 64 bit PCI accesses

[Nicolas Saenz Julienne]

Hi everyone,
Raspberry Pi 4, a 64bit arm system on chip, contains a PCIe bus that can't
handle 64bit accesses to its MMIO address space, in other words, writeq() has
to be split into two distinct writel() operations. This isn't ideal, as it
misrepresents PCI's promise of being able to treat device memory as regular
memory, ultimately breaking a bunch of PCI device drivers[1].

I'd like to have a go at fixing this in a way that can be distributed in a
generic distro without prejudice to other users.

AFAIK there is no way to detect this limitation through generic PCIe
capabilities, so one solution would be to expose it through firmware
(devicetree in this case), and pass the limitations through 'struct device' so
as for the drivers to choose the right access method in a way that doesn't
affect performance much[2]. All in all, most of this doesn't need to be
PCI-centric as the property could be applied to any MMIO bus.

Thoughts? Opinions? Is it overkill just for a single SoC?

Regards,
Nicolas

[1] https://github.com/raspberrypi/linux/issues/4158#issuecomment-782351510
[2] Things might get even weirder as the order in which the 32bit operations
    are performed might matter (low/high vs high/low).

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