drm/hyperv: Fix device removal on Gen1 VMs

The Hyper-V DRM driver tries to free MMIO region on removing
the device regardless of VM type, while Gen1 VMs don't use MMIO
and hence causing the kernel to crash on a NULL pointer dereference.

Fix this by making deallocating MMIO only on Gen2 machines and implement
removal for Gen1

Fixes: 76c56a5a ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
Reviewed-by: Deepak Rawat <drawat.floss@gmail.com>
Signed-off-by: Deepak Rawat <drawat.floss@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211119112900.300537-1-mgamal@redhat.com

drivers/gpu/drm/hyperv/hyperv_drm_drv.c

@@ -225,12 +225,29 @@ static int hyperv_vmbus_remove(struct hv_device *hdev)
 {
 	struct drm_device *dev = hv_get_drvdata(hdev);
 	struct hyperv_drm_device *hv = to_hv(dev);
+	struct pci_dev *pdev;
 
 	drm_dev_unplug(dev);
 	drm_atomic_helper_shutdown(dev);
 	vmbus_close(hdev->channel);
 	hv_set_drvdata(hdev, NULL);
+
+	/*
+	 * Free allocated MMIO memory only on Gen2 VMs.
+	 * On Gen1 VMs, release the PCI device
+	 */
+	if (efi_enabled(EFI_BOOT)) {
 		vmbus_free_mmio(hv->mem->start, hv->fb_size);
+	} else {
+		pdev = pci_get_device(PCI_VENDOR_ID_MICROSOFT,
+				      PCI_DEVICE_ID_HYPERV_VIDEO, NULL);
+		if (!pdev) {
+			drm_err(dev, "Unable to find PCI Hyper-V video\n");
+			return -ENODEV;
+		}
+		pci_release_region(pdev, 0);
+		pci_dev_put(pdev);
+	}
 
 	return 0;
 }