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

[Robin Murphy]

Hi Lorenzo,

On 17/10/16 14:21, Lorenzo Pieralisi 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;
> 
> Nit: I do not see why you would not take the same approach as
> the ARM SMMUv1/v2, namely checking if ops are already set and
> skip the call if that's the case.

Well, I'd say it really goes the other way around - since the very first
thing bus_set_iommu() does is check if ops are present, and return if
so, and the v2 driver already doesn't care about that return value,
there's not really any need for it to duplicate the check either. I
didn't change it at the time to avoid cluttering the gigantic rework any
further, but I could spin a cleanup patch if you like.

> Anyway:
> 
> Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>

Thanks!

Robin.