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

[Lorenzo Pieralisi]

On Fri, Oct 09, 2020 at 11:13:59AM +0200, Ard Biesheuvel wrote:
> On Fri, 9 Oct 2020 at 10:36, Nicolas Saenz Julienne
> <nsaenzjulienne@xxxxxxx> wrote:
> >
> > 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.
> >
> 
> 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)

Is this information that we can infer from IORT nodes and make it
generic (ie make a DMA limit out of all IORT nodes allowed ranges) ?

We can move this check to IORT code and call it from arm64 if it
can be made to work.

Thanks,
Lorenzo