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

[Hanjun Guo]

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>