On Sat, Oct 10, 2020 at 11:31:53AM +0200, Ard Biesheuvel wrote: > 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> > --- > This is related to the discussion in > > https://lore.kernel.org/linux-arm-kernel/20201001161740.29064-2-nsaenzjulienne@xxxxxxx/ > > Documentation/arm64/arm-acpi.rst | 7 +++ > arch/arm64/mm/init.c | 8 +++ > drivers/acpi/arm64/iort.c | 51 ++++++++++++++++++++ > 3 files changed, 66 insertions(+) Thanks for putting it together so promptly. > diff --git a/Documentation/arm64/arm-acpi.rst b/Documentation/arm64/arm-acpi.rst > index 47ecb9930dde..947f5b5c45ef 100644 > --- a/Documentation/arm64/arm-acpi.rst > +++ b/Documentation/arm64/arm-acpi.rst > @@ -205,6 +205,13 @@ devices available. This list of tables is not meant to be all inclusive; > in some environments other tables may be needed (e.g., any of the APEI > tables from section 18) to support specific functionality. > > +It is assumed that all DMA capable devices in the system are able to > +access the lowest 4 GB of system memory. If this is not the case, an > +IORT describing those limitations is mandatory, even if an IORT is not > +otherwise necessary to describe the I/O topology, and regardless of > +whether _DMA methods are used to describe the DMA limitations more > +precisely. Once the system has booted, _DMA methods will take precedence > +over DMA addressing limits described in the IORT. If this is a boot requirement it must be in ARM's official documentation, first, not the kernel one. I understand this is an urgent (well - no comments on why bootstrapping ACPI on Raspberry PI4 is causing all this fuss, honestly) fix but that's not a reason to rush through these guidelines. I would not add this paragraph to arm-acpi.rst, yet. > ACPI Detection > -------------- > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c > index f0599ae73b8d..829fa63c3d72 100644 > --- a/arch/arm64/mm/init.c > +++ b/arch/arm64/mm/init.c > @@ -191,6 +191,14 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max) > unsigned long max_zone_pfns[MAX_NR_ZONES] = {0}; > > #ifdef CONFIG_ZONE_DMA > + if (IS_ENABLED(CONFIG_ACPI)) { > + extern unsigned int acpi_iort_get_zone_dma_size(void); Yes as Catalin asked please add a declaration in IORT headers. > + zone_dma_bits = min(zone_dma_bits, > + acpi_iort_get_zone_dma_size()); > + arm64_dma_phys_limit = max_zone_phys(zone_dma_bits); > + } > + > max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit); > #endif > #ifdef CONFIG_ZONE_DMA32 > diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c > index ec782e4a0fe4..c3db44896e49 100644 > --- a/drivers/acpi/arm64/iort.c > +++ b/drivers/acpi/arm64/iort.c > @@ -1722,3 +1722,54 @@ void __init acpi_iort_init(void) > > iort_init_platform_devices(); > } > + > +#ifdef CONFIG_ZONE_DMA > +/* > + * Check the IORT whether any devices exist whose DMA mask is < 32 bits. > + * If so, return the smallest value encountered, or 32 otherwise. > + */ > +unsigned int __init acpi_iort_get_zone_dma_size(void) > +{ > + struct acpi_table_iort *iort; > + struct acpi_iort_node *node, *end; > + acpi_status status; > + u8 limit = 32; > + int i; > + > + if (acpi_disabled) > + return limit; > + > + status = acpi_get_table(ACPI_SIG_IORT, 0, > + (struct acpi_table_header **)&iort); > + if (ACPI_FAILURE(status)) > + return limit; > + > + node = ACPI_ADD_PTR(struct acpi_iort_node, iort, iort->node_offset); > + end = ACPI_ADD_PTR(struct acpi_iort_node, iort, iort->header.length); > + > + for (i = 0; i < iort->node_count; i++) { > + if (node >= end) > + break; > + > + switch (node->type) { > + struct acpi_iort_named_component *ncomp; > + struct acpi_iort_root_complex *rc; > + > + case ACPI_IORT_NODE_NAMED_COMPONENT: > + ncomp = (struct acpi_iort_named_component *)node->node_data; > + if (ncomp->memory_address_limit) > + limit = min(limit, ncomp->memory_address_limit); > + break; > + > + case ACPI_IORT_NODE_PCI_ROOT_COMPLEX: > + rc = (struct acpi_iort_root_complex *)node->node_data; > + if (rc->memory_address_limit); You need a node->revision check here otherwise we may end up dereferencing junk. AKA ACPI versioning in all its glory. Thanks, Lorenzo > + limit = min(limit, rc->memory_address_limit); > + break; > + } > + node = ACPI_ADD_PTR(struct acpi_iort_node, node, node->length); > + } > + acpi_put_table(&iort->header); > + return limit; > +} > +#endif > -- > 2.17.1 >
