drm: track dev_mapping in more robust and flexible way

Setting dev_mapping (pointer to the address_space structure
used for memory mappings) to the address_space of the first
opener's inode and then failing if other openers come in
through a different inode has a few restrictions that are
eliminated by this patch.

If we already have valid dev_mapping and we spot an opener
with different i_node, we force its i_mapping pointer to the
already established address_space structure (first opener's
inode). This will make all mappings from drm device hang off
the same address_space object.

Some benefits (things that now work and didn't work
before) of this patch are:

 * user space can mknod and use any number of device
   nodes and they will all work fine as long as the major
   device number is that of the drm module.
 * user space can even remove the first opener's device
   nodes and mknod the new one and the applications and
   windowing system will still work.
 * GPU drivers can safely assume that dev->dev_mapping is
   correct address_space and just blindly copy it
   into their (private) bdev.dev_mapping

For reference, some discussion that lead to this patch can
be found here:

http://lists.freedesktop.org/archives/dri-devel/2012-April/022283.htmlSigned-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fops.c

@@ -140,12 +140,12 @@ int drm_open(struct inode *inode, struct file *filp)
 	}
 	if (!retcode) {
 		mutex_lock(&dev->struct_mutex);
-		if (minor->type == DRM_MINOR_LEGACY) {
 		if (dev->dev_mapping == NULL)
-				dev->dev_mapping = inode->i_mapping;
-			else if (dev->dev_mapping != inode->i_mapping)
-				retcode = -ENODEV;
-		}
+			dev->dev_mapping = &inode->i_data;
+		/* ihold ensures nobody can remove inode with our i_data */
+		ihold(container_of(dev->dev_mapping, struct inode, i_data));
+		inode->i_mapping = dev->dev_mapping;
+		filp->f_mapping = dev->dev_mapping;
 		mutex_unlock(&dev->struct_mutex);
 	}
 
@@ -509,6 +509,9 @@ int drm_release(struct inode *inode, struct file *filp)
 		}
 	}
 
+	BUG_ON(dev->dev_mapping == NULL);
+	iput(container_of(dev->dev_mapping, struct inode, i_data));
+
 	/* drop the reference held my the file priv */
 	drm_master_put(&file_priv->master);
 	file_priv->is_master = 0;

drivers/gpu/drm/nouveau/nouveau_gem.c

@@ -207,8 +207,7 @@ nouveau_gem_ioctl_new(struct drm_device *dev, void *data,
 	struct nouveau_bo *nvbo = NULL;
 	int ret = 0;
 
-	if (unlikely(dev_priv->ttm.bdev.dev_mapping == NULL))
-		dev_priv->ttm.bdev.dev_mapping = dev_priv->dev->dev_mapping;
+	dev_priv->ttm.bdev.dev_mapping = dev->dev_mapping;
 
 	if (!dev_priv->engine.vram.flags_valid(dev, req->info.tile_flags)) {
 		NV_ERROR(dev, "bad page flags: 0x%08x\n", req->info.tile_flags);

drivers/gpu/drm/radeon/radeon_object.c

@@ -115,9 +115,7 @@ int radeon_bo_create(struct radeon_device *rdev,
 
 	size = ALIGN(size, PAGE_SIZE);
 
-	if (unlikely(rdev->mman.bdev.dev_mapping == NULL)) {
 	rdev->mman.bdev.dev_mapping = rdev->ddev->dev_mapping;
-	}
 	if (kernel) {
 		type = ttm_bo_type_kernel;
 	} else if (sg) {

drivers/gpu/drm/radeon/radeon_ttm.c

@@ -734,9 +734,7 @@ int radeon_ttm_init(struct radeon_device *rdev)
 	}
 	DRM_INFO("radeon: %uM of GTT memory ready.\n",
 		 (unsigned)(rdev->mc.gtt_size / (1024 * 1024)));
-	if (unlikely(rdev->mman.bdev.dev_mapping == NULL)) {
 	rdev->mman.bdev.dev_mapping = rdev->ddev->dev_mapping;
-	}
 
 	r = radeon_ttm_debugfs_init(rdev);
 	if (r) {

drivers/gpu/drm/vmwgfx/vmwgfx_drv.c

@@ -769,10 +769,7 @@ static int vmw_driver_open(struct drm_device *dev, struct drm_file *file_priv)
 		goto out_no_tfile;
 
 	file_priv->driver_priv = vmw_fp;
-
-	if (unlikely(dev_priv->bdev.dev_mapping == NULL))
-		dev_priv->bdev.dev_mapping =
-			file_priv->filp->f_path.dentry->d_inode->i_mapping;
+	dev_priv->bdev.dev_mapping = dev->dev_mapping;
 
 	return 0;