[PATCH 1/2] iommu/arm-smmu: Don't inadvertently reject multiple SMMUv3s

[Robin Murphy]

On 19/10/16 13:49, Will Deacon wrote:
> On Mon, Oct 17, 2016 at 12:06:20PM +0100, Robin Murphy wrote:
>> We now delay installing our per-bus iommu_ops until we know an SMMU has
>> successfully probed, as they don't serve much purpose beforehand, and
>> doing so also avoids fights between multiple IOMMU drivers in a single
>> kernel. However, the upshot of passing the return value of bus_set_iommu()
>> back from our probe function is that if there happens to be more than
>> one SMMUv3 device in a system, the second and subsequent probes will
>> wind up returning -EBUSY to the driver core and getting torn down again.
>>
>> There are essentially 3 cases in which bus_set_iommu() returns nonzero:
>> 1. The bus already has iommu_ops installed
>> 2. One of the add_device callbacks from the initial notifier failed
>> 3. Allocating or installing the notifier itself failed
>>
>> The first two are down to devices other than the SMMU in question, so
>> shouldn't abort an otherwise-successful SMMU probe, whilst the third is
>> indicative of the kind of catastrophic system failure which isn't going
>> to get much further anyway. Consequently, there is little harm in
>> ignoring the return value either way.
>>
>> CC: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
>> Signed-off-by: Robin Murphy <robin.murphy at arm.com>
>> ---
>>  drivers/iommu/arm-smmu-v3.c | 11 ++++-------
>>  1 file changed, 4 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>> index 15c01c3cd540..74fbef384deb 100644
>> --- a/drivers/iommu/arm-smmu-v3.c
>> +++ b/drivers/iommu/arm-smmu-v3.c
>> @@ -2637,16 +2637,13 @@ static int arm_smmu_device_dt_probe(struct platform_device *pdev)
>>  	of_iommu_set_ops(dev->of_node, &arm_smmu_ops);
>>  #ifdef CONFIG_PCI
>>  	pci_request_acs();
>> -	ret = bus_set_iommu(&pci_bus_type, &arm_smmu_ops);
>> -	if (ret)
>> -		return ret;
>> +	bus_set_iommu(&pci_bus_type, &arm_smmu_ops);
>>  #endif
>>  #ifdef CONFIG_ARM_AMBA
>> -	ret = bus_set_iommu(&amba_bustype, &arm_smmu_ops);
>> -	if (ret)
>> -		return ret;
>> +	bus_set_iommu(&amba_bustype, &arm_smmu_ops);
>>  #endif
>> -	return bus_set_iommu(&platform_bus_type, &arm_smmu_ops);
>> +	bus_set_iommu(&platform_bus_type, &arm_smmu_ops);
>> +	return 0;
> 
> In which case, we should probably add an iommu_present check, like we
> have for the v2 driver.

As touched upon in the commit message, the first thing bus_set_iommu()
does is perform that same check and return immediately if any ops are
already set. Do you really want redundant checks added, or shall I spin
that cleanup patch removing them from v2 that I proposed to Lorenzo?

Robin.

> 
> Will
>