drm: virtio: reinstate drm_virtio_set_busid()

Before commit a325725633c2 ("drm: Lobotomize set_busid nonsense for !pci
drivers"), several DRM drivers for platform devices used to expose an
explicit "drm_driver.set_busid" callback, invariably backed by
drm_platform_set_busid().

Commit a325725633c2 removed drm_platform_set_busid(), along with the
referring .set_busid field initializations. This was justified because
interchangeable functionality had been implemented in drm_dev_alloc() /
drm_dev_init(), which DRM_IOCTL_SET_VERSION would rely on going forward.

However, commit a325725633c2 also removed drm_virtio_set_busid(), for
which the same consolidation was not appropriate: this .set_busid callback
had been implemented with drm_pci_set_busid(), and not
drm_platform_set_busid(). The error regressed Xorg/xserver on QEMU's
"virtio-vga" card; the drmGetBusid() function from libdrm would no longer
return stable PCI identifiers like "pci:0000:00:02.0", but rather unstable
platform ones like "virtio0".

Reinstate drm_virtio_set_busid() with judicious use of

git checkout -p a325725633c2^ -- drivers/gpu/drm/virtio

Cc: Daniel Vetter <[e-mail]>
Cc: Emil Velikov <[e-mail]>
Cc: Gerd Hoffmann <[e-mail]>
Cc: Gustavo Padovan <[e-mail]>
Cc: Hans de Goede <[e-mail]>
Cc: Joachim Frieben <[e-mail]>
Cc: [e-mail] # v4.8
Reported-by: Joachim Frieben <[e-mail]>
Fixes: a325725633c26aa66ab940f762a6b0778edf76c0
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1366842
Signed-off-by: Laszlo Ersek <[e-mail]>
Reviewed-by: Emil Velikov <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/virtio/virtgpu_drm_bus.c

···
 
 #include "virtgpu_drv.h"
 
+int drm_virtio_set_busid(struct drm_device *dev, struct drm_master *master)
+{
+	struct pci_dev *pdev = dev->pdev;
+
+	if (pdev) {
+		return drm_pci_set_busid(dev, master);
+	}
+	return 0;
+}
+
 static void virtio_pci_kick_out_firmware_fb(struct pci_dev *pci_dev)
 {
 	struct apertures_struct *ap;

drivers/gpu/drm/virtio/virtgpu_drv.c

···
 
 static struct drm_driver driver = {
 	.driver_features = DRIVER_MODESET | DRIVER_GEM | DRIVER_PRIME | DRIVER_RENDER | DRIVER_ATOMIC,
+	.set_busid = drm_virtio_set_busid,
 	.load = virtio_gpu_driver_load,
 	.unload = virtio_gpu_driver_unload,
 	.open = virtio_gpu_driver_open,

drivers/gpu/drm/virtio/virtgpu_drv.h

···
 #define DRIVER_PATCHLEVEL 1
 
 /* virtgpu_drm_bus.c */
+int drm_virtio_set_busid(struct drm_device *dev, struct drm_master *master);
 int drm_virtio_init(struct drm_driver *driver, struct virtio_device *vdev);
 
 struct virtio_gpu_object {