drm: add pseudo filesystem for shared inodes

Our current DRM design uses a single address_space for all users of the
same DRM device. However, there is no way to create an anonymous
address_space without an underlying inode. Therefore, we wait for the
first ->open() callback on a registered char-dev and take-over the inode
of the char-dev. This worked well so far, but has several drawbacks:
 - We screw with FS internals and rely on some non-obvious invariants like
   inode->i_mapping being the same as inode->i_data for char-devs.
 - We don't have any address_space prior to the first ->open() from
   user-space. This leads to ugly fallback code and we cannot allocate
   global objects early.

As pointed out by Al-Viro, fs/anon_inode.c is *not* supposed to be used by
drivers for anonymous inode-allocation. Therefore, this patch follows the
proposed alternative solution and adds a pseudo filesystem mount-point to
DRM. We can then allocate private inodes including a private address_space
for each DRM device at initialization time.

Note that we could use:
  sysfs_get_inode(sysfs_mnt->mnt_sb, drm_device->dev->kobj.sd);
to get access to the underlying sysfs-inode of a "struct device" object.
However, most of this information is currently hidden and it's not clear
whether this address_space is suitable for driver access. Thus, unless
linux allows anonymous address_space objects or driver-core provides a
public inode per device, we're left with our own private internal mount
point.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
(cherry picked from commit 31bbe16f6d88622d6731fa2cb4ab38d57d844ac1)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 43661277720..8d1c8697ebd 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -3115,6 +3115,7 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
 		end = ERR_PTR(-ENAMETOOLONG);
 	return end;
 }
+EXPORT_SYMBOL(simple_dname);
 
 /*
  * Write full pathname from the root of the filesystem into the buffer.