[PATCH/RFC 0/4] Probe deferral for IOMMU DT integration

Laurent Pinchart laurent.pinchart at ideasonboard.com
Mon Feb 16 08:14:45 PST 2015 

Hi Laura and all,

On Thursday 05 February 2015 16:31:58 Laura Abbott wrote:
> Hi,
> 
> On the heels of Exynos integrating SMMU in to the DT for probing,
> this series attempts to add support to make SMMU drivers work with
> deferred probing. This has been referenced before[1] but this is
> some actual code to use as a starting point for discussion.
> 
> The requirement for this is based on a previous patch to add clock
> support to the ARM SMMU driver[2]. Once we have clock support, it's
> possible that the driver itself may need to be defered which breaks
> a bunch of assumptions about how SMMU probing is supposed to work.
> The concept here is to have the driver call of_dma_configure which
> may return -EPROBE_DEFER. of_dma_configure could possibly be moved
> up to probe level. The existing method of initialization still needs
> to be done as well which might mean we have the worst of both worlds.
> 
> Open questions:
> - This still doesn't really address Arnd's concerns[3] about disabling
> IOMMUs

Arnd, Will and I have discussed IOMMU probe deferral last week. Here's a 
summary of the discussion, before the details blur away.

The discussion covered both higher level concepts and lower level details, in 
various directions. I'll try to make the summary clear by filling the gaps 
between pieces where needed, so some of the information below might not be a 
direct result of the discussions. Arnd and Will, please feel free to correct 
me.

The first question we've discussed was whether probe deferral for IOMMUs is 
really needed. Various dependencies between IOMMU devices and other devices 
exist, in particular on clocks (as you have mentioned above) and on power 
domains (as mentioned by Marek in http://lists.infradead.org/pipermail/linux-arm-kernel/2015-February/323238.html). While there are mechanism to handle 
some of them with 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 we have concluded that probe deferral for IOMMUs is 
desired if it can be implemented cleanly.

The second question we've discussed was how to implement probe deferral 
cleanly :-)

The current implementation configures DMA at device creation time and sets the 
following properties:

- dma_map_ops (IOMMU, SWIOTLB, linear mapping)
- initial DMA mask
- DMA offset
- DMA coherency

Additionally the IOMMU group (when an IOMMU is present) will also be 
configured at the same time (patches are under review).

The base idea is to defer probing of bus master devices when their IOMMU isn't 
present. Three cases need to be handled.

1. No IOMMU is declared by the firmware (through DT, ACPI, ...) for the bus 
master device. The bus master device probe doesn't need to be deferred due to 
the IOMMU. dma_map_ops must be set to SWIOTLB or linear mapping (the later 
should likely be implemented through SWIOTLB).

2. An IOMMU is declared for the bus master device and the IOMMU has been 
successfully probed and registered. The bus master device probe doesn't need 
to be deferred due to the IOMMU. dma_map_ops must be set to IOMMU ops.

3. An IOMMU is declared for the bus master device but the IOMMU isn't 
registered. This can be caused by different reasons:

- a. No driver is loaded for this IOMMU (for instance because DT describes the 
IOMMU connection, but the IOMMU driver hasn't been developed yet, or an older 
kernel is used). If the IOMMU is optional the bus master device probe should 
succeed, and dma_map_ops should be set to linear. If the IOMMU is mandatory 
the bus master device probe should fail.

Note that, as we require IOMMU drivers to be compiled in, we don't need to 
handle the case of loadable IOMMU drivers that haven't been loaded yet.

- b. A driver is loaded for this IOMMU but the IOMMU hasn't been probed yet, 
or its probe has been deferred. The bus master device probe should be 
deferred.

- c. A driver is loaded for this IOMMU but the IOMMU probe has failed 
permanently. It's not clear at the moment whether we should try to recover 
from this automatically using the same mechanism as case 3.a, or if we can 
considered this as an abnormal failure and disable the bus master device.

Assuming that we should try to recover from such an error, we can't predict 
this case when the device is instantiated (even if IOMMUs are registered 
before bus master devices are added, for instance using the OF_DECLARE 
mechanism that Will has implemented). We thus need to setup the dma_map_ops 
and IOMMU group at bus master device probe time.

Furthermore, we can't distinguish cases 3.a and 3.b at bus master probe time 
without early registration of IOMMU drivers. Case 3.a would instead be 
considered as 3.b, leading to infinite probe deferral of bus master devices.

For those reasons we have concluded that IOMMU probe deferral needs to be 
implemented with a combination of several mechanisms. The following steps 
should happen at bus master device probe time.

1. The IOMMU device referenced by the firmware with the bus master device is 
looked up. On DT-based systems, this will be the IOMMU DT node referenced by 
the iommus property. If no IOMMU device is associated, dma_map_ops will be set 
to linear mapping or SWIOTLB and device probe will continue.

2. An IOMMU device is referenced for the bus master device.

The corresponding IOMMU instance is looked up. This requires a new IOMMU 
registration system. IOMMU drivers will create IOMMU instances at probe time 
and register them with the IOMMU core.

If an IOMMU instance is found for the referenced IOMMU device, the IOMMU 
instance's of_xlate function will be called to setup the IOMMU.

If the of_xlate call succeeds dma_map_ops will be set to IOMMU ops and device 
probe will continue. If the call fails we can either fail the bus master 
device probe, or fall back to non-IOMMU dma_map_ops (to be discussed).

3. The IOMMU device referenced for the bus master device isn't present, due to 
the IOMMU device probe not having been performed yet, having been deferred, or 
having failed.

The IOMMU driver associated with the IOMMU device is looked up. This was 
initially thought to require an early registration mechanism for IOMMU drivers 
(using an OF_DECLARE mechanism for DT-based systems for instance), but on 
second thought it might be possible to implement this based on normal driver 
registration (to be researched).

If an IOMMU driver is found for the referenced IOMMU device, a callback 
function of the IOMMU driver is called to check whether an IOMMU instance is 
expected to be registered later (most IOMMU drivers will just return true, so 
we could skip this callback function until an IOMMU driver requires it). If an 
IOMMU instance is expected to be registered later the bus master device probe 
is deferred. Otherwise dma_map_ops will be set to linear mapping/SWIOTLB and 
device probe will continue.


The initial DMA mask and the DMA offset can still be configured at device 
instantiation time if desired.

> - Currently tested where we knew the driver was going to be deferring.
> Probably need some logic for calling of_dma_configure again.
> 
> This is based on Robin Murphy's work for dma mapping[4] and a few
> patches from Murali Kaicheri[5] for of_dma_configure.
> 
> 
> [1]
> http://lists.linuxfoundation.org/pipermail/iommu/2015-January/011764.html
> [2]
> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/279036.ht
> ml [3]
> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-December/311579.
> html [4]
> http://lists.infradead.org/pipermail/linux-arm-kernel/2015-January/315459.h
> tml [5]
> http://lists.infradead.org/pipermail/linux-arm-kernel/2015-January/319390.h
> tml
> 
> Laura Abbott (3):
>   dma-mapping: Make arch_setup_dma_ops return an error code
>   of: Return error codes from of_dma_configure
>   iommu/arm-smmu: Support deferred probing
> 
> Mitchel Humpherys (1):
>   iommu/arm-smmu: add support for specifying clocks
> 
>  arch/arm/include/asm/dma-mapping.h   |   2 +-
>  arch/arm/mm/dma-mapping.c            |   4 +-
>  arch/arm64/include/asm/dma-mapping.h |   2 +-
>  arch/arm64/mm/dma-mapping.c          |  16 +--
>  drivers/iommu/arm-smmu.c             | 186 ++++++++++++++++++++++++++++++--
>  drivers/iommu/iommu.c                |  49 ++++++++-
>  drivers/iommu/of_iommu.c             |  14 ++-
>  drivers/of/device.c                  |   9 +-
>  include/linux/dma-mapping.h          |   7 +-
>  include/linux/iommu.h                |   2 +
>  include/linux/of_device.h            |   4 +-
>  11 files changed, 268 insertions(+), 27 deletions(-)

-- 
Regards,

Laurent Pinchart

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