drm/nv50: fix iommu errors caused by device reading from address 0

int

nv50_fb_init(struct drm_device *dev)

{

	/* This is needed to get meaningful information from 100c90

	 * on traps. No idea what these values mean exactly. */

	struct drm_nouveau_private *dev_priv = dev->dev_private;

	/* Not a clue what this is exactly.  Without pointing it at a

	 * scratch page, VRAM->GART blits with M2MF (as in DDX DFS)

	 * cause IOMMU "read from address 0" errors (rh#561267)

	 */

	nv_wr32(dev, 0x100c08, dev_priv->gart_info.sg_dummy_bus >> 8);

	/* This is needed to get meaningful information from 100c90

	 * on traps. No idea what these values mean exactly. */

	switch (dev_priv->chipset) {

	case 0x50:

		nv_wr32(dev, 0x100c90, 0x0707ff);