[RFC/PATCH 0/9] IOMMU probe deferral support

Laura Abbott labbott at redhat.com
Thu May 28 10:36:46 PDT 2015

On 05/14/2015 04:00 PM, Laurent Pinchart wrote:
> Hello,
> This patch series attempts to implement support for deferring probe of both
> IOMMU drivers and bus master drivers.
> The relationship between bus masters and IOMMUs creates a strong ordering
> during initialization of devices. As in the general case IOMMUs are hidden
> behind the DMA mapping API, IOMMU support relies on the automatic setup of DMA
> operations without any direct intervention of bus master drivers.
> DMA operations are set up when platform devices are added to the system. This
> requires IOMMUs to be available at that time. On systems where ordering of
> device add and probe can't be guaranteed (such as, but not limited to,
> DT-based systems) this caused incorrect DMA operation setup. This has been
> addressed by a patches series [1] that introduced a DT-based early
> registration mechanism for IOMMUs.
> However, that mechanism fails to address all issues. Various dependencies
> exist between IOMMU devices and other devices, in particular on clocks and on
> power domains (as mentioned by Marek in [2]). While there are mechanisms to
> handle some of them without probe deferral (for instance by using the
> OF_DECLARE macros to register clock drivers), generalizing those mechanisms
> would essentially recreate a probe ordering mechanism similar to link order
> probe ordering and couldn't really scale.
> Additionally, IOMMUs could also be present hot-pluggable devices and depend on
> resources that are thus hot-plugged. OF_DECLARE wouldn't help in that case.
> For all those reasons probe deferral for IOMMUs has been considered as desired
> if it can be implemented cleanly. For more in-depth information see [3].
> This RFC series is a first attempt at implementing IOMMU probe deferral
> support cleanly.
> The core idea is to move setup of DMA operations from device add time to
> device probe time, implemented in patch 6/9. It could be possible to move
> setup of other DMA parameters (namely masks and offset) to probe time as well,
> but that change would be more intrusive and has a higher risk of introducing
> regressions. For that reason I've decided to keep DMA masks and offset setup
> at device add time and thus split DMA configuration in masks and operations
> (patch 5/9). This can be revisited if we decide that the DMA mapping API
> shouldn't require masks and offset to be set before probe time.
> Patch 8/9 then defers probe of bus master drivers when required IOMMUs are not
> available yet. This requires knowing when a failed IOMMU lookup should be
> considered as permanent or temporary. I've reused the OF_DECLARE_IOMMU for
> this purpose, considering that the presence of a driver compatible with the
> IOMMU DT node indicates that the failure is temporary and probing of the bus
> master device should be deferred.
> Note that only IOMMU drivers using the recent .of_xlate() mechanism for
> DT-based IOMMU reference can cause probe deferral of bus master devices. The
> .add_device() mechanism isn't supported in this case.
> As an example I've converted the ipmmu-vmsa driver to the new API in patch 9/9.
> At this point many enhancements are possible, but I'd like to receive feedback
> on the proposed approach before basing more patches on this series. One
> particular point I would like to address (or see being addressed) in the
> future is the use of struct iommu_ops with of_iommu_get_ops() and
> of_iommu_set_ops(). I believe we should introduct a struct iommu and register
> IOMMU instances instead of IOMMU operations. That should bring us one step
> closer to removing bus_set_iommu().
> [1] http://www.spinics.net/lists/arm-kernel/msg382787.html
> [2] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-February/323238.html
> [3] https://lkml.org/lkml/2015/2/16/345
> Laurent Pinchart (9):
>    arm: dma-mapping: Don't override dma_ops in arch_setup_dma_ops()
>    arm: dma-mapping: Support IOMMU mappings spanning the full 32 bits
>      range
>    of: dma: Move range size workaround to of_dma_get_range()
>    of: dma: Make of_dma_deconfigure() public
>    of: dma: Split of_configure_dma() into mask and ops configuration
>    drivers: platform: Configure dma operations at probe time
>    iommu: of: Document the of_iommu_configure() function
>    iommu: of: Handle IOMMU lookup failure with deferred probing or error
>    iommu/ipmmu-vmsa: Use DT-based instantiation
>   arch/arm/include/asm/dma-iommu.h |   2 +-
>   arch/arm/mm/dma-mapping.c        |  21 +++--
>   drivers/base/platform.c          |   9 ++
>   drivers/iommu/ipmmu-vmsa.c       | 189 +++++++++++++--------------------------
>   drivers/iommu/of_iommu.c         |  29 +++++-
>   drivers/of/address.c             |  20 ++++-
>   drivers/of/device.c              |  77 ++++++++++------
>   drivers/of/of_pci.c              |   3 +-
>   drivers/of/platform.c            |  16 ++--
>   include/linux/of_device.h        |  14 ++-
>   10 files changed, 195 insertions(+), 185 deletions(-)

I no longer have hardware to test this on but the entire approach looks
reasonable to me.

Reviewed-by: Laura Abbott <labbott at redhat.com>

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