Messages in this thread | | | Subject | Re: [PATCH v4 6/7] arm64: mm: Set ZONE_DMA size based on early IORT scan | From | Hanjun Guo <> | Date | Wed, 28 Oct 2020 19:08:53 +0800 |
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On 2020/10/21 20:34, Nicolas Saenz Julienne wrote: > From: Ard Biesheuvel <ardb@kernel.org> > > 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@arm.com> > Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> > Cc: Nicolas Saenz Julienne <nsaenzjulienne@suse.de> > Cc: Rob Herring <robh+dt@kernel.org> > Cc: Christoph Hellwig <hch@lst.de> > Cc: Robin Murphy <robin.murphy@arm.com> > Cc: Hanjun Guo <guohanjun@huawei.com> > Cc: Sudeep Holla <sudeep.holla@arm.com> > Cc: Anshuman Khandual <anshuman.khandual@arm.com> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > [nsaenz: Rebased, removed documentation change and add declaration in acpi_iort.h] > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de> > > --- > > 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@huawei.com>
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