Re: [PATCH v4 0/5] Add support for Broadcom BCM2712 IOMMU driver (Raspberry Pi 5)

[Jim Quinlan]

s

On Wed, Sep 2, 2026 at 1:58 PM Daniel Drake <dan@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> This series adds a driver for the Broadcom BCM2712 IOMMU found on
> Raspberry Pi 5, and hooks up the display controller IOMMU for efficient
> management of graphics memory. This is adapted from the downstream driver
> from Raspberry Pi (original author Nick Hollinghurst), with main changes:
>  - In page mapping mode, operate the translation aperture from address 0,
>    with no bypass window. More like a IOMMU and less like a GART.
>  - Implement the page table management using generic_pt
>  - Support 4MB hugepage mapping in top level page table
>  - Implement brcm,iommu-cache as standards-compliant DT property
>  - Drop the dma-iova-offset hack, used to work around some issue seen with
>    dma-ranges. This will need to be investigated separately and solved
>    properly. (It's not needed for display controller iommu support included
>    here.)

Hi Daniel,
I wish you had kept the dma-iova offset hack but I understand why you
did not.  I've built and successfully tested your driver on the
Broadcom sister chip of the RpI 5.  I am mostly interested in the
iommu attached to the PCIe controller.  I tested this by running
multiple tests where 10MiB is sent by a Wifi chip.  Data integrity was
validated of course.

I did not use the driver code as is.  I tried merely decrementing the
MMMU_PT_PA_BASE value for the 4GB iova-offset required for our
configuration, but that did not work for some reason.  So I just
hacked in some iova conversions and got it working.

At any rate, LGTM and thanks for submitting this.

Regards,
Jim Quinlan
Broadcom


>  - Misc simplifications/standardisations/cleanups
>
> The IOMMU works strictly with 4KB pages. This means that unfortunately
> when the kernel is compiled with PAGE_SIZE=16KB (the Raspberry Pi 5
> kernel default), 12KB is wasted in each page that is allocated for page
> tables. I plan to address this in followup work.
>
> It has been tested on Raspberry Pi 5 using a 3D-accelerated graphical
> environment which causes plenty of IOMMU maps & unmaps.
>
> Thanks for the review comments and discussion. Further feedback and
> testing welcome!
>
> Changes in v4:
> - Add Jim Quinlan and Florian Fainelli as bindings maintainers
> - Replace MMU register access macros with static inline functions
> - Clear illegal address register and flush caches on blocking domain attach
> - Add __must_hold annotations in TLB helpers
> - Use BIT() macro for MMUC_CONTROL definitions
> - Link to v3: https://lore.kernel.org/r/20260825-bcm2712-iommu-submit-v3-0-7f143e7289b6@xxxxxxxxxxxxxxx
>
> Changes in v3:
> - Switch from a aperture and bypass window GART-like approach
>   to IOMMU-like 0-based translation
> - Add 3 SW bits which seem unused (per the chipset docs); drop patch
>   for NO_SW_BIT handling. This reduces the claimed address space, so I had
>   to tweak a iommu_pt test accordingly.
> - Disallow domain sharing across multiple hardware IOMMU instances
> - Clear and flush local MMU TLB during domain attach
> - Handle unattached domains safely during TLB sync
> - Set cache platform driver data late during probe to prevent race condition
> - Improved handling of cache flush timeouts
> - Use format string for device name in iommu_device_sysfs_add()
> - Use cleanup.h helpers for scope management
> - Expand DT binding descriptions for IOMMU and IOMMUC
> - Add iommus property to HVS display node in DT
> - Drop iommuc back-compat with current RPi DT
> - Link to v2: https://lore.kernel.org/r/20260727-bcm2712-iommu-submit-v2-0-0247b5c03de8@xxxxxxxxxxxxxxx
>
> Changes in v2:
> - Use iommu_fwspec in xlate & probe_device
> - Add blocked_domain support
> - Simplify page size configuration between iommu driver and format
> - Introduce PT_FEAT_NO_SW_BIT flag to allow formats to explicitly opt-in to
>   a defensive flushing algorithm when no sw bit is available
> - Implement 4MB pages at Level 1; this is more efficient and means the
>   "allow full-table contiguous leaves in unit tests" commit has been
>   dropped because it is no longer needed.
> - Hardcode the aperture in an unused part of the address space, removing
>   it from DT (it's a software-specific decision, not an attribute of the
>   hardware), adding more explanatory comments
> - Use pt_full_va_prefix to have the PT format operate within the aperture
>   space, enabling use of IOMMU_PT_DOMAIN_OPS
> - Add more dt-bindings explanation to brcm,iommu-cache and drop docs
>   around the cache property used in RPi firmware (the driver still supports
>   this however, for fw compat reasons)
> - Correct iommu device address in bcm2712.dtsi
> - Link to v1: https://lore.kernel.org/r/20260712-bcm2712-iommu-submit-v1-0-80e10cdde2ea@xxxxxxxxxxxxxxx
>
> ---
> Daniel Drake (5):
>       iommu_pt: Fix test_pgsize_boundary() failure on narrow-OA formats
>       dt-bindings: iommu: Add Broadcom BCM2712 IOMMU
>       iommu/generic_pt: Add Broadcom BCM2712 page table format
>       iommu: Add Broadcom BCM2712 IOMMU driver
>       arm64: dts: broadcom: bcm2712: Add GPU IOMMU and IOMMU cache nodes
>
>  .../bindings/iommu/brcm,bcm2712-iommu.yaml         |  54 ++
>  .../bindings/iommu/brcm,bcm2712-iommuc.yaml        |  40 ++
>  arch/arm64/boot/dts/broadcom/bcm2712.dtsi          |  15 +
>  drivers/iommu/Kconfig                              |  15 +
>  drivers/iommu/Makefile                             |   1 +
>  drivers/iommu/bcm2712-iommu-cache.c                |  84 +++
>  drivers/iommu/bcm2712-iommu-cache.h                |   9 +
>  drivers/iommu/bcm2712-iommu.c                      | 577 +++++++++++++++++++++
>  drivers/iommu/generic_pt/.kunitconfig              |   1 +
>  drivers/iommu/generic_pt/Kconfig                   |  10 +
>  drivers/iommu/generic_pt/fmt/Makefile              |   2 +
>  drivers/iommu/generic_pt/fmt/bcm2712.h             | 288 ++++++++++
>  drivers/iommu/generic_pt/fmt/defs_bcm2712.h        |  18 +
>  drivers/iommu/generic_pt/fmt/iommu_bcm2712.c       |   6 +
>  drivers/iommu/generic_pt/kunit_iommu_pt.h          |   2 +-
>  include/linux/generic_pt/common.h                  |   6 +
>  include/linux/generic_pt/iommu.h                   |  12 +
>  17 files changed, 1139 insertions(+), 1 deletion(-)
> ---
> base-commit: 786262be6048deab760f68c8acc2c85607165894
> change-id: 20260712-bcm2712-iommu-submit-2e09899e65c4
>
> Best regards,
> --
> Daniel Drake <dan@xxxxxxxxxxxxxxx>
>

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