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

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 9 Oct 2020 at 10:36, Nicolas Saenz Julienne
<nsaenzjulienne@suse.de> wrote:
>
> On Fri, 2020-10-09 at 09:37 +0200, Ard Biesheuvel wrote:
> > On Fri, 9 Oct 2020 at 09:11, Christoph Hellwig <hch@lst.de> 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.
>

Yeah that was unfortunate.

> > 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.
>

Yes, it will be harder, especially for the _DMA methods.

> > 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.
>

The problem is that, while we are providing a full description of the
SoC's capabilities, we short circuit this by inserting knowledge into
the code (that is shared between all DT architectures) that
"brcm,bcm2711" is special, and needs a DMA zone override.

I think for ACPI boot, we might be able to work around this by cold
plugging the memory above 1 GB, but I have to double check whether it
won't get pulled into ZONE_DMA32 anyway (unless anyone can answer that
for me here from the top of their head)