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.html

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/drivers/gpu/drm/nouveau/nouveau_gem.c b/drivers/gpu/drm/nouveau/nouveau_gem.c
index 30f54231694..fe3f5a821b8 100644
--- a/drivers/gpu/drm/nouveau/nouveau_gem.c
+++ b/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);