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

[Nicolas Saenz Julienne]

On Fri, 2020-10-09 at 09:37 +0200, Ard Biesheuvel wrote:
> 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.

Well, and broke a big amount of devices that were otherwise fine.

> 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.

Is it worth the effort just for a single board? I don't know about ACPI but
parsing dma-ranges that early at boot time is not trivial. My intuition tells
me that it'd be even harder for ACPI, being a more complex data structure.

> 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.

Actually what I proposed here is pretty generic. Specially from arm64's
perspective. We call early_init_dt_scan(), which sets up zone_dma_bits based on
whatever it finds in DT. Both those operations are architecture independent.
arm64 arch code doesn't care about the logic involved in ascertaining
zone_dma_bits. I get that the last step isn't generic. But it's all setup so as
to make it as such whenever it's worth the effort.

> 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.

Any ideas? The way I understand it the kdump issue is just a shortcoming of
the memory zones design.

Regards,
Nicolas

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