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

[Hanjun Guo]

Hi Ard,

On 2020/10/16 14:54, Ard Biesheuvel wrote:
> On Fri, 16 Oct 2020 at 08:51, Hanjun Guo <guohanjun@xxxxxxxxxx> wrote:
> > On 2020/10/16 2:03, Catalin Marinas wrote:
> > > 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.
> >
> > Some platforms using 0 as the memory size limit, for example D05 [0] and
> > D06 [1], I think we need to go back to the OEM ID quirk.
> >
> > For D05/D06, there are multi interrupt controllers named as mbigen,
> > mbigen is using the named component to describe the mappings with
> > the ITS controller, and mbigen is using 0 as the memory size limit.
> >
> > Also since the memory size limit for PCI RC was introduced by later
> > IORT revision, so firmware people may think it's fine to set that
> > as 0 because the system works without it.
>
> Hello Hanjun,
>
> The patch only takes the address limit field into account if its value > 0.

Sorry I missed the if (*->memory_address_limit) check, thanks
for the reminding.

> Also, before commit 7fb89e1d44cb6aec ("ACPI/IORT: take _DMA methods
> into account for named components"), the _DMA method was not taken
> into account for named components at all, and only the IORT limit was
> used, so I do not anticipate any problems with that.

Then this patch is fine to me.

Thanks
Hanjun