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>
