[PATCH] iommu: Fix bypass of IOMMU readiness check for multi-IOMMU devices

[Tudor Ambarus]

Commit da33e87bd2bf ("iommu: Handle yet another race around
registration") introduced a readiness check in `iommu_fwspec_init()` to
prevent client drivers from configuring their IOMMUs before
`bus_iommu_probe()` has completed.

To optimize the replay path, the readiness check was conditionally
gated behind `!dev->iommu`:
    if (!dev->iommu && !READ_ONCE(iommu->ready))
        return -EPROBE_DEFER;

However, this assumption breaks down for devices that map to multiple
IOMMU instances. During the initialization loop over multiple IOMMUs in
`of_iommu_configure_device()`, the first IOMMU successfully allocates
`dev->iommu`. When `iommu_fwspec_init()` is called for the second
IOMMU, `!dev->iommu` evaluates to false, short-circuiting the logic and
entirely bypassing the `iommu->ready` check.

If the second IOMMU is still executing its `bus_iommu_probe()`
concurrently, this allows the client driver to proceed prematurely,
resulting in a late IOMMU probe warning:
    dev: late IOMMU probe at driver bind, something fishy here!
    WARNING: drivers/iommu/iommu.c:645 at __iommu_probe_device

Fix this by making the `iommu->ready` check unconditional, ensuring
that a device will defer its probe until *all* of its required IOMMUs
are fully registered and ready.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: da33e87bd2bf ("iommu: Handle yet another race around registration")
Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Signed-off-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
---
The warning was observed using an Android 6.19 tree, using downstream
drivers (exynos-decon and samsung-sysmmu-v9).
---
 drivers/iommu/iommu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 78756c3f3c40..e61927b4d41f 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -3042,7 +3042,7 @@ int iommu_fwspec_init(struct device *dev, struct fwnode_handle *iommu_fwnode)
 
 	if (!iommu)
 		return driver_deferred_probe_check_state(dev);
-	if (!dev->iommu && !READ_ONCE(iommu->ready))
+	if (!READ_ONCE(iommu->ready))
 		return -EPROBE_DEFER;
 
 	if (fwspec)

---
base-commit: ca3bbc9287400c1274d87ee57a16e3126ba2969a
change-id: 20260320-iommu-ready-check-4976863957c2

Best regards,
-- 
Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>