[RFC 0/3] Unify IOMMU-based DMA-mapping code for ARM and ARM64

Marek Szyprowski m.szyprowski at samsung.com
Fri Feb 19 08:22:41 UTC 2016

Dear All,

This is an initial RFC on the unification of IOMMU-based DMA-mapping
code for ARM and ARM64 architectures.

Right now ARM architecture still use my old code for IOMMU-based
DMA-mapping glue, initially merged in commit
4ce63fcd919c32d22528e54dcd89506962933719 ("ARM: dma-mapping: add support
for IOMMU mapper"). In meantime ARM64 got a new, slightly improved
implementation provided by Robin Murphy in commit
13b8629f651164d71f4d38b821925f93ba4236c8 ("arm64: Add IOMMU dma_ops").

Both implementations are very similar thus their unification is desired
to avoid duplicating future works and simplify code, which uses this
layer on both architectures. In this patchset I've selected the new
implementation (from ARM64 architecture) as a base. This means that
ARM-specific, old interface (arm_iommu_* functions) for configuring
IOMMU domains will be no longer available and its users have to be
converted to new API.

Besides lack of old interface, the second difference is additional
requirements for IOMMU drivers. New code relies on the support for
IOMMU_DOMAIN_DMA and default IOMMU domain, which is automatically
attached by the IOMMU core.

The new code also assumes that the IOMMU-based DMA-mapping ops are
mainly configured from arch_setup_dma_ops() function, which means that
the IOMMU driver should provide needed of_xlate callbacks and initialize
IOMMU ops for device nodes. However it should be also possible to
initialize IOMMU-based DMA-mapping ops for client devices directly from
IOMMU drivers by calling common_iommu_setup_dma_ops() (some drivers used
such approach).

IOMMU drivers should be also aware of the fact that the
default domain is attached via device_attach and then device_attach
callback can be called once again with different domain without previous
detach from default domain. For more information on this issue, see the
following thread:

Currently there are 4 users of the old arm_iommu_* interface:
1. Exynos DRM driver
2. Rockchip DRM driver
3. OMAP3 ISP camera driver
4. Renesas VMSA-compatible IPMMU driver

In this patchset I've converted Exynos DRM driver for the new API (patch
1). This required some changes in the memory management model inside the
driver and removal of some hacks, which were used to setup IOMMU-based
DMA-mapping ops on the 'exynos-drm' virtual device and common IOMMU
domain for all Exynos DRM sub-devices, those changes have been posted
separately here: http://www.spinics.net/lists/dri-devel/msg100861.html 
Rockchip DRM driver requires similar conversion.

Converting OMAP3 ISP camera driver to new API requires adding support
for IOMMU groups to OMAP IOMMU driver, because the new DMA/IOMMU code
used IOMMU_DOMAIN_DMA type domains and default groups.

Renesas IPMMU driver needs also to be extended with IOMMU_DOMAIN_DMA domain
type support. It can also be prepared for IOMMU_OF_DECLARE and of_xlate
callback-based initialization to let core to automatically setup of
IOMMU-based DMA mapping implementation.

Patch 2 moves existing code from arch/arm64 to drivers/iommu and
introduces some minor changes in function names - mainly adding arch_
prefix to some dma-mapping internal functions, which stay in arch/arm64/
(functions of similar names are present in arch/arm). Patch 3 adapts ARM
architecture for the common code.

I would like to get your comments on the proposed approach. There is
still some work that need to be done to convert remaining users of the
old API and updating IOMMU drivers to the new API requirements. This
change need to be tested on the all affected ARM sub-architectures.

Right now patches were tested on only Exynos based boards: ARM 32bit:
Exynos4412 and Exynos5422 boards and ARM 64 bit Exnyos 5433 (with some
out-of-tree DTS).

To ease testing I've prepared a branch with all the patches needed
(there are all needed patches for Exynos subarch, which have been posted
as separate patchsets):
https://git.linaro.org/people/marek.szyprowski/linux-srpol.git v4.5-dma-iommu-unification

Patches are based on Linux v4.5-rc4 vanilla tree.

Best regards
Marek Szyprowski
Samsung R&D Institute Poland

Patch summary:

Marek Szyprowski (3):
  drm/exynos: rewrite IOMMU support code
  iommu: dma-iommu: move IOMMU/DMA-mapping code from ARM64 arch to drivers
  iommu: dma-iommu: use common implementation also on ARM architecture

 arch/arm/Kconfig                          |   22 +-
 arch/arm/include/asm/device.h             |    9 -
 arch/arm/include/asm/dma-iommu.h          |   37 -
 arch/arm/include/asm/dma-mapping.h        |   59 +-
 arch/arm/mm/dma-mapping.c                 | 1158 +----------------------------
 arch/arm64/include/asm/dma-mapping.h      |   39 +-
 arch/arm64/mm/dma-mapping.c               |  491 +-----------
 drivers/gpu/drm/exynos/Kconfig            |    2 +-
 drivers/gpu/drm/exynos/exynos_drm_drv.c   |    7 +-
 drivers/gpu/drm/exynos/exynos_drm_drv.h   |    2 +-
 drivers/gpu/drm/exynos/exynos_drm_iommu.c |   91 ++-
 drivers/gpu/drm/exynos/exynos_drm_iommu.h |    2 +-
 drivers/gpu/drm/rockchip/Kconfig          |    1 +
 drivers/iommu/Kconfig                     |    1 +
 drivers/iommu/Makefile                    |    2 +-
 drivers/iommu/dma-iommu-ops.c             |  471 ++++++++++++
 drivers/media/platform/Kconfig            |    1 +
 include/linux/dma-iommu.h                 |   14 +
 18 files changed, 679 insertions(+), 1730 deletions(-)
 delete mode 100644 arch/arm/include/asm/dma-iommu.h
 create mode 100644 drivers/iommu/dma-iommu-ops.c


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