[PATCH] arm64: mm: set ZONE_DMA size based on early IORT scan

[Lorenzo Pieralisi]

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 at arm.com>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Cc: Nicolas Saenz Julienne <nsaenzjulienne at suse.de>
> Cc: Rob Herring <robh+dt at kernel.org>
> Cc: Christoph Hellwig <hch at lst.de>
> Cc: Robin Murphy <robin.murphy at arm.com>
> Cc: Hanjun Guo <guohanjun at huawei.com>
> Cc: Sudeep Holla <sudeep.holla at arm.com>
> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
> This is related to the discussion in 
> 
> https://lore.kernel.org/linux-arm-kernel/20201001161740.29064-2-nsaenzjulienne@suse.de/
> 
>  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
>