[PATCH] xen/grant-dma-iommu: Implement a dummy probe_device() callback

[Oleksandr Tyshchenko <olekstysh@gmail.com>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Update stub IOMMU driver (which main purpose is to reuse generic
IOMMU device-tree bindings by Xen grant DMA-mapping layer on Arm)
according to the recent changes done in the following
commit 57365a04c921 ("iommu: Move bus setup to IOMMU device registration").

With probe_device() callback being called during IOMMU device registration,
the uninitialized callback just leads to the "kernel NULL pointer
dereference" issue during boot. Fix that by adding a dummy callback.

Looks like the release_device() callback is not mandatory to be
implemented as IOMMU framework makes sure that callback is initialized
before dereferencing.

Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
---
 drivers/xen/grant-dma-iommu.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/grant-dma-iommu.c b/drivers/xen/grant-dma-iommu.c
index 16b8bc0c0b33..6a9fe02c6bfc 100644
--- a/drivers/xen/grant-dma-iommu.c
+++ b/drivers/xen/grant-dma-iommu.c
@@ -16,8 +16,15 @@ struct grant_dma_iommu_device {
 	struct iommu_device iommu;
 };
 
-/* Nothing is really needed here */
-static const struct iommu_ops grant_dma_iommu_ops;
+static struct iommu_device *grant_dma_iommu_probe_device(struct device *dev)
+{
+	return ERR_PTR(-ENODEV);
+}
+
+/* Nothing is really needed here except a dummy probe_device callback */
+static const struct iommu_ops grant_dma_iommu_ops = {
+	.probe_device = grant_dma_iommu_probe_device,
+};
 
 static const struct of_device_id grant_dma_iommu_of_match[] = {
 	{ .compatible = "xen,grant-dma" },
-- 
2.34.1