[PATCH] drivers/of_iommu: ignore SMMU DT nodes with status 'disabled'

Ard Biesheuvel ard.biesheuvel at linaro.org
Fri Apr 28 09:14:49 EDT 2017

On 28 April 2017 at 14:11, Will Deacon <will.deacon at arm.com> wrote:
> Hi Ard,
> [+ devicetree@]
> On Fri, Apr 14, 2017 at 01:43:15PM +0100, Ard Biesheuvel wrote:
>> DT nodes may have a status property, and if they do, such nodes should
>> only be considered present if the status property is set to 'okay'.
>> Currently, we call the init function of IOMMUs described by the device
>> tree without taking this into account, which may result in the output
>> below on systems where some SMMUs may be legally disabled.
>>  Failed to initialise IOMMU /smb/smmu at e0200000
>>  Failed to initialise IOMMU /smb/smmu at e0c00000
>>  arm-smmu e0a00000.smmu: probing hardware configuration...
>>  arm-smmu e0a00000.smmu: SMMUv1 with:
>>  arm-smmu e0a00000.smmu:  stage 2 translation
>>  arm-smmu e0a00000.smmu:  coherent table walk
>>  arm-smmu e0a00000.smmu:  stream matching with 32 register groups, mask 0x7fff
>>  arm-smmu e0a00000.smmu:  8 context banks (8 stage-2 only)
>>  arm-smmu e0a00000.smmu:  Supported page sizes: 0x60211000
>>  arm-smmu e0a00000.smmu:  Stage-2: 40-bit IPA -> 40-bit PA
>>  Failed to initialise IOMMU /smb/smmu at e0600000
>>  Failed to initialise IOMMU /smb/smmu at e0800000
>> Since this is not an error condition, only call the init function if
>> the device is enabled, which also inhibits the spurious error messages.
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>  drivers/iommu/of_iommu.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
>> index 2683e9fc0dcf..2dd1206e6c0d 100644
>> --- a/drivers/iommu/of_iommu.c
>> +++ b/drivers/iommu/of_iommu.c
>> @@ -183,7 +183,7 @@ static int __init of_iommu_init(void)
>>       for_each_matching_node_and_match(np, matches, &match) {
>>               const of_iommu_init_fn init_fn = match->data;
>> -             if (init_fn(np))
>> +             if (of_device_is_available(np) && init_fn(np))
>>                       pr_err("Failed to initialise IOMMU %s\n",
>>                               of_node_full_name(np));
>>       }
> Is there a definition of what status = "disabled" is supposed to mean for an
> IOMMU? For example, that could mean that the firmware has pre-programmed the
> SMMU with particular translations or memory attributes (a bit like the
> CCA=1, CPM=1, DACS=0 case in ACPI IORT), or even disabled DMA traffic
> altogether.
> So I think we'd need an update to the generic IOMMU binding text to say
> exactly what the semantics are supposed to be here.

I agree that it might make sense to describe the behavior of the IOMMU
when it is left in the state we found it in. But that is not the same
as status=disabled.

The DTS subtree contains loads and loads of boilerplate
configurations, where only some pieces are enabled in the final image
by setting status=okay. So a node that has status 'disabled' should be
treated as 'not present', not as 'present but can be ignored under
assumptions such and such'

In other words, I think we are talking about two different issues here.

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