Re: [PATCH v3 7/8] arm64: mm: Set ZONE_DMA size based on early IORT scan

[Catalin Marinas]

On Thu, Oct 15, 2020 at 10:26:18PM +0800, Hanjun Guo wrote:
> On 2020/10/15 3:12, Nicolas Saenz Julienne wrote:
> > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > 
> > We recently introduced a 1 GB sized ZONE_DMA to cater for platforms
> > incorporating masters that can address less than 32 bits of DMA, in
> > particular the Raspberry Pi 4, which has 4 or 8 GB of DRAM, but has
> > peripherals that can only address up to 1 GB (and its PCIe host
> > bridge can only access the bottom 3 GB)
> > 
> > Instructing the DMA layer about these limitations is straight-forward,
> > even though we had to fix some issues regarding memory limits set in
> > the IORT for named components, and regarding the handling of ACPI _DMA
> > methods. However, the DMA layer also needs to be able to allocate
> > memory that is guaranteed to meet those DMA constraints, for bounce
> > buffering as well as allocating the backing for consistent mappings.
> > 
> > This is why the 1 GB ZONE_DMA was introduced recently. Unfortunately,
> > it turns out the having a 1 GB ZONE_DMA as well as a ZONE_DMA32 causes
> > problems with kdump, and potentially in other places where allocations
> > cannot cross zone boundaries. Therefore, we should avoid having two
> > separate DMA zones when possible.
> > 
> > So let's do an early scan of the IORT, and only create the ZONE_DMA
> > if we encounter any devices that need it. This puts the burden on
> > the firmware to describe such limitations in the IORT, which may be
> > redundant (and less precise) if _DMA methods are also being provided.
> > However, it should be noted that this situation is highly unusual for
> > arm64 ACPI machines. Also, the DMA subsystem still gives precedence to
> > the _DMA method if implemented, and so we will not lose the ability to
> > perform streaming DMA outside the ZONE_DMA if the _DMA method permits
> > it.
> 
> Sorry, I'm still a little bit confused. With this patch, if we have
> a device which set the right _DMA method (DMA size >= 32), but with the
> wrong DMA size in IORT, we still have the ZONE_DMA created which
> is actually not needed?

With the current kernel, we get a ZONE_DMA already with an arbitrary
size of 1GB that matches what RPi4 needs. We are trying to eliminate
such unnecessary ZONE_DMA based on some heuristics (well, something that
looks "better" than a OEM ID based quirk). Now, if we learn that IORT
for platforms in the field is that broken as to describe few bits-wide
DMA masks, we may have to go back to the OEM ID quirk.

-- 
Catalin