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

[Ard Biesheuvel <ardb@kernel.org>]

On Tue, 13 Oct 2020 at 13:09, Lorenzo Pieralisi
<lorenzo.pieralisi@arm.com> wrote:
>
> 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@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>
> > ---
> > 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.
>

Which documentation? ACPI compliance by itself is not sufficient for a
system to be able to boot Linux/arm64, which is why we documented the
requirements for ACPI boot on Linux/arm64 in this file. I don't think
we need endorsement from ARM to decide that odd platforms like this
need to abide by some additional rules if they want to boot in ACPI
mode.


> >  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.
>

Ack.

> > +             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.
>

The address limit field was there since the beginning, and DEN0049B
defines its value as 0x0, so I don't think we need to check anything
here.


> 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
> >