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

From: [email-masked] (Will Deacon)
Date: Wed, 19 Oct 2016 13:49:32 +0100

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 <[email-masked]>
> Signed-off-by: Robin Murphy <[email-masked]>
> ---
>  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.

Will