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SubjectRe: arm64 iommu groups issue
On 13/02/2020 3:49 pm, John Garry wrote:
>>> The underlying issue is that, for historical reasons, OF/IORT-based
>>> IOMMU drivers have ended up with group allocation being tied to endpoint
>>> driver probing via the dma_configure() mechanism (long story short,
>>> driver probe is the only thing which can be delayed in order to wait for
>>> a specific IOMMU instance to be ready).However, in the meantime, the
>>> IOMMU API internals have evolved sufficiently that I think there's a way
>>> to really put things right - I have the spark of an idea which I'll try
>>> to sketch out ASAP...
>> OK, great.
> Hi Robin,
> I was wondering if you have had a chance to consider this problem again?

Yeah, sorry, more things came up such that it still hasn't been P yet ;)

Lorenzo and I did get as far as discussing it a bit more and writing up
a ticket, so it's formally on our internal radar now, at least.

> One simple idea could be to introduce a device link between the endpoint
> device and its parent bridge to ensure that they probe in order, as
> expected in pci_device_group():
> Subject: [PATCH] PCI: Add device link to ensure endpoint device driver
> probes after bridge
> It is required to ensure that a device driver for an endpoint will probe
> after the parent port driver, so add a device link for this.
> ---
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 512cb4312ddd..4b832ad25b20 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2383,6 +2383,7 @@ static void pci_set_msi_domain(struct pci_dev *dev)
>  void pci_device_add(struct pci_dev *dev, struct pci_bus *bus)
>  {
>      int ret;
> +    struct device *parent;
>      pci_configure_device(dev);
> @@ -2420,6 +2421,10 @@ void pci_device_add(struct pci_dev *dev, struct
> pci_bus *bus)
>      /* Set up MSI IRQ domain */
>      pci_set_msi_domain(dev);
> +    parent = dev->dev.parent;
> +    if (parent && parent->bus == &pci_bus_type)
> +        device_link_add(&dev->dev, parent, DL_FLAG_AUTOPROBE_CONSUMER);
> +
>      /* Notifier could use PCI capabilities */
>      dev->match_driver = false;
>      ret = device_add(&dev->dev);
> --
> This would work, but the problem is that if the port driver fails in
> probing - and not just for -EPROBE_DEFER - then the child device will
> never probe. This very thing happens on my dev board. However we could
> expand the device links API to cover this sort of scenario.

Yes, that's an undesirable issue, but in fact I think it's mostly
indicative that involving drivers in something which is designed to
happen at a level below drivers is still fundamentally wrong and doomed
to be fragile at best.

Another thought that crosses my mind is that when pci_device_group()
walks up to the point of ACS isolation and doesn't find an existing
group, it can still infer that everything it walked past *should* be put
in the same group it's then eventually going to return. Unfortunately I
can't see an obvious way for it to act on that knowledge, though, since
recursive iommu_probe_device() is unlikely to end well.

> As for alternatives, it looks pretty difficult to me to disassociate the
> group allocation from the dma_configure path.

Indeed it's non-trivial, but it really does need cleaning up at some point.

Having just had yet another spark, does something like the untested
super-hack below work at all? I doubt it's a viable direction to take in
itself, but it could be food for thought if it at least proves the concept.


diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index aa3ac2a03807..554cde76c766 100644
--- a/drivers/iommu/arm-smmu-v3.c
+++ b/drivers/iommu/arm-smmu-v3.c
@@ -3841,20 +3841,20 @@ static int arm_smmu_set_bus_ops(struct iommu_ops
int err;

- if (pci_bus_type.iommu_ops != ops) {
+ if (1) {
err = bus_set_iommu(&pci_bus_type, ops);
if (err)
return err;
- if (amba_bustype.iommu_ops != ops) {
+ if (1) {
err = bus_set_iommu(&amba_bustype, ops);
if (err)
goto err_reset_pci_ops;
- if (platform_bus_type.iommu_ops != ops) {
+ if (1) {
err = bus_set_iommu(&platform_bus_type, ops);
if (err)
goto err_reset_amba_ops;
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 660eea8d1d2f..b81ae2b4d4fb 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -1542,6 +1542,14 @@ static int iommu_bus_init(struct bus_type *bus,
const struct iommu_ops *ops)
return err;

+static int iommu_bus_replay(struct device *dev, void *data)
+ if (dev->iommu_group)
+ return 0;
+ return add_iommu_group(dev, data);
* bus_set_iommu - set iommu-callbacks for the bus
* @bus: bus.
@@ -1564,6 +1572,9 @@ int bus_set_iommu(struct bus_type *bus, const
struct iommu_ops *ops)
return 0;

+ if (bus->iommu_ops == ops)
+ return bus_for_each_dev(bus, NULL, NULL, iommu_bus_replay);
if (bus->iommu_ops != NULL)
return -EBUSY;
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