Re: [PATCH v3 7/8] arm64: mm: Set ZONE_DMA size based on early IORT scan

Hanjun Guo
2020-10-15T14:26:33Z

On 2020/10/15 3:12, 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.

Sorry, I'm still a little bit confused. With this patch, if we have
a device which set the right _DMA method (DMA size >= 32), but with the
wrong DMA size in IORT, we still have the ZONE_DMA created which
is actually not needed?

>
> 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, warnings and add
> declaration in acpi_iort.h]
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
> ---
> arch/arm64/mm/init.c | 6 +++++
> drivers/acpi/arm64/iort.c | 51 +++++++++++++++++++++++++++++++++++++++
> include/linux/acpi_iort.h | 4 +++
> 3 files changed, 61 insertions(+)
>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 97b0d2768349..f321761eedb2 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -29,6 +29,7 @@
> #include <linux/kexec.h>
> #include <linux/crash_dump.h>
> #include <linux/hugetlb.h>
> +#include <linux/acpi_iort.h>
>
> #include <asm/boot.h>
> #include <asm/fixmap.h>
> @@ -196,6 +197,11 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
> #ifdef CONFIG_ZONE_DMA
> zone_dma_bits = min(zone_dma_bits,
> (unsigned int)ilog2(of_dma_get_max_cpu_address(NULL)));
> +
> + if (IS_ENABLED(CONFIG_ACPI))
> + 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
> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
> index 9929ff50c0c0..8f530bf3c03b 100644
> --- a/drivers/acpi/arm64/iort.c
> +++ b/drivers/acpi/arm64/iort.c
> @@ -1718,3 +1718,54 @@ 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)
> + limit = min(limit, rc->memory_address_limit);

There is no "Memory address size limit" field in revision 0 table, so as
Lorenzo reminded, please add a revision check here.

Thanks
Hanjun

> + break;
> + }
> + node = ACPI_ADD_PTR(struct acpi_iort_node, node, node->length);
> + }
> + acpi_put_table(&iort->header);
> + return limit;
> +}
> +#endif
> diff --git a/include/linux/acpi_iort.h b/include/linux/acpi_iort.h
> index 20a32120bb88..7d2e184f0d4d 100644
> --- a/include/linux/acpi_iort.h
> +++ b/include/linux/acpi_iort.h
> @@ -38,6 +38,7 @@ void iort_dma_setup(struct device *dev, u64 *dma_addr, u64 *size);
> const struct iommu_ops *iort_iommu_configure_id(struct device *dev,
> const u32 *id_in);
> int iort_iommu_msi_get_resv_regions(struct device *dev, struct list_head *head);
> +unsigned int acpi_iort_get_zone_dma_size(void);
> #else
> static inline void acpi_iort_init(void) { }
> static inline u32 iort_msi_map_id(struct device *dev, u32 id)
> @@ -55,6 +56,9 @@ static inline const struct iommu_ops *iort_iommu_configure_id(
> static inline
> int iort_iommu_msi_get_resv_regions(struct device *dev, struct list_head *head)
> { return 0; }
> +
> +static inline unsigned int acpi_iort_get_zone_dma_size(void)
> +{ return 32; }
> #endif
>
> #endif /* __ACPI_IORT_H__ */