drm: Move master pointer from drm_minor to drm_device

There can only be one current master, and it's for the overall device.
Render/control minors don't support master-based auth at all.

This simplifies the master logic a lot, at least in my eyes: All these
additional pointer chases are just confusing.

While doing the conversion I spotted some locking fail:
- drm_lock/drm_auth check dev->master without holding the
  master_mutex. This is fallout from

  commit c996fd0b956450563454e7ccc97a82ca31f9d043
  Author: Thomas Hellstrom <thellstrom@vmware.com>
  Date:   Tue Feb 25 19:57:44 2014 +0100

      drm: Protect the master management with a drm_device::master_mutex v3

  but I honestly don't care one bit about those old legacy drivers
  using this.

- debugfs name info should just grab master_mutex.

- And the fbdev helper looked at it to figure out whether someone is
  using KMS. We just need a consistent value, so READ_ONCE. Aside: We
  should probably check if anyone has opened a control node too, but I
  guess current userspace doesn't really do that yet.

v2: Balance locking, reported by Julia.

v3: Rebase on top of Chris' oops fixes.

Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466499262-18717-1-git-send-email-daniel.vetter@ffwll.ch

1 files changed, 8 insertions, 1 deletions

diff --git a/drivers/gpu/drm/drm_info.c b/drivers/gpu/drm/drm_info.c
index e2d2543d5bd0..d6cfd5340d52 100644
--- a/drivers/gpu/drm/drm_info.c
+++ b/drivers/gpu/drm/drm_info.c
@@ -50,7 +50,12 @@ int drm_name_info(struct seq_file *m, void *data)
 	struct drm_info_node *node = (struct drm_info_node *) m->private;
 	struct drm_minor *minor = node->minor;
 	struct drm_device *dev = minor->dev;
-	struct drm_master *master = minor->master;
+	struct drm_master *master;
+
+	mutex_lock(&dev->master_mutex);
+	master = dev->master;
+	if (!master)
+		goto out_unlock;
 
 	seq_printf(m, "%s", dev->driver->name);
 	if (dev->dev)
@@ -60,6 +65,8 @@ int drm_name_info(struct seq_file *m, void *data)
 	if (dev->unique)
 		seq_printf(m, " unique=%s", dev->unique);
 	seq_printf(m, "\n");
+out_unlock:
+	mutex_unlock(&dev->master_mutex);
 
 	return 0;
 }