Re: [PATCH v4 6/7] arm64: mm: Set ZONE_DMA size based on early IORT scan

From: Hanjun Guo
Date: Wed Oct 28 2020 - 17:46:37 EST


On 2020/10/21 20:34, 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.

Cc: Jeremy Linton <jeremy.linton@xxxxxxx>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
Cc: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
Cc: Rob Herring <robh+dt@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: Hanjun Guo <guohanjun@xxxxxxxxxx>
Cc: Sudeep Holla <sudeep.holla@xxxxxxx>
Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
[nsaenz: Rebased, removed documentation change and add declaration in acpi_iort.h]
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>

---

Changes since v3:
- Use min_not_zero()
- Check ACPI revision
- Remove unnecessary #ifdef in zone_sizes_init()

arch/arm64/mm/init.c | 3 ++-
drivers/acpi/arm64/iort.c | 52 +++++++++++++++++++++++++++++++++++++++
include/linux/acpi_iort.h | 4 +++

Acked-by: Hanjun Guo <guohanjun@xxxxxxxxxx>