drm/i915/selftests: Push the fake iommu device from the stack to data

Since we store a pointer to the fake iommu device that is allocated on
the stack, as soon as we leave the function it goes out of scope and any
future dereference is undefined behaviour. Just in case we may need to
look at the fake iommu device after initialiation, move the allocation
from the stack into the data.

Fixes: 01b9d4e2 ("iommu/vt-d: Use dev_iommu_priv_get/set()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200916105022.28316-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/mock_gem_device.c

@@ -118,11 +118,11 @@ static struct dev_pm_domain pm_domain = {
 
 struct drm_i915_private *mock_gem_device(void)
 {
-	struct drm_i915_private *i915;
-	struct pci_dev *pdev;
 #if IS_ENABLED(CONFIG_IOMMU_API) && defined(CONFIG_INTEL_IOMMU)
-	struct dev_iommu iommu;
+	static struct dev_iommu fake_iommu = { .priv = (void *)-1 };
 #endif
+	struct drm_i915_private *i915;
+	struct pci_dev *pdev;
 	int err;
 
 	pdev = kzalloc(sizeof(*pdev), GFP_KERNEL);
@@ -141,10 +141,8 @@ struct drm_i915_private *mock_gem_device(void)
 	dma_coerce_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
 
 #if IS_ENABLED(CONFIG_IOMMU_API) && defined(CONFIG_INTEL_IOMMU)
-	/* HACK HACK HACK to disable iommu for the fake device; force identity mapping */
-	memset(&iommu, 0, sizeof(iommu));
-	iommu.priv = (void *)-1;
-	pdev->dev.iommu = &iommu;
+	/* HACK to disable iommu for the fake device; force identity mapping */
+	pdev->dev.iommu = &fake_iommu;
 #endif
 
 	pci_set_drvdata(pdev, i915);