Re: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711

[Ard Biesheuvel]

On Fri, 9 Oct 2020 at 09:11, Christoph Hellwig <hch@xxxxxx> wrote:
>
> On Thu, Oct 08, 2020 at 12:05:25PM +0200, Nicolas Saenz Julienne wrote:
> > Sadly I just realised that the series is incomplete, we have RPi4 users that
> > want to boot unsing ACPI, and this series would break things for them. I'll
> > have a word with them to see what we can do for their use-case.
>
> Stupid question:  why do these users insist on a totally unsuitable
> interface?  And why would we as Linux developers care to support such
> a aims?
>

The point is really whether we want to revert changes in Linux that
made both DT and ACPI boot work without quirks on RPi4. Having to
check the RPi4 compatible string or OEM id in core init code is awful,
regardless of whether you boot via ACPI or via DT.

The problem with this hardware is that it uses a DMA mask which is
narrower than 32, and the arm64 kernel is simply not set up to deal
with that at all. On DT, we have DMA ranges properties and the likes
to describe such limitations, on ACPI we have _DMA methods as well as
DMA range attributes in the IORT, both of which are now handled
correctly. So all the information is there, we just have to figure out
how to consume it early on.

Interestingly, this limitation always existed in the SoC, but it
wasn't until they started shipping it with more than 1 GB of DRAM that
it became a problem. This means issues like this could resurface in
the future with existing SoCs when they get shipped with more memory,
and so I would prefer fixing this in a generic way.

Also, I assume papering over the issue like this does not fix the
kdump issue fundamentally, it just works around it, and so we might
run into this again in the future.