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

[Lorenzo Pieralisi <lorenzo.pieralisi-5wv7dgnIgG8@public.gmane.org>]

On Mon, Oct 17, 2016 at 03:19:46PM +0100, Robin Murphy wrote:
> 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-5wv7dgnIgG8@public.gmane.org>
> >> Signed-off-by: Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
> >> ---
> >>  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.

No worries, it was to understand if there was a reason to keep
the code different and after another look I agree with what
you are saying (by checking if ops are present you could eg
avoid calling pci_request_acs() every probe but that's a detail).

Thanks for fixing it !
Lorenzo

> > Anyway:
> > 
> > Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi-5wv7dgnIgG8@public.gmane.org>
> 
> Thanks!
> 
> Robin.
>