drm: Rip out totally bogus vga_switcheroo->can_switch locking
So I just wanted to add a new field to struct drm_device and
accidentally stumbled over something. According to comments
dev->open_count is protected by dev->count_lock, but that's totally
not the case. It's protected by drm_global_mutex.
Unfortunately the vga switcheroo callbacks took this comment at face
value. The problem is that we can't just take the drm_global_mutex
because:
- It would lead to a locking inversion with the driver load/unload
paths.
- It wouldn't actually protect anything, for that we'd need to wrap
the entire vga switcheroo code in the drm_global_mutex. And I'm not
sure whether that would actually solve anything.
What we probably want is a try_to_grab_switcheroo reference kind of
thing which is used in the driver's ->open callback. Then we could
move all that ->can_switch madness into the vga switcheroo core where
it really belongs.
But since that would amount to real work take the easy way out and
just add a comment. It's definitely not going to make anything worse
since doing switcheroo state changes while restarting X just isn't
recommended. Even though the delayed switching code does exactly that.
v2:
- Simplify the ->can_switch implementations more (Thierry)
- Fix comment about the dev->open_count locking (Thierry)
Cc: Thierry Reding <treding@nvidia.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> (v1)
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 9f1fb8d..a20d882 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -1076,7 +1076,7 @@
/** \name Usage Counters */
/*@{ */
- int open_count; /**< Outstanding files open */
+ int open_count; /**< Outstanding files open, protected by drm_global_mutex. */
int buf_use; /**< Buffers in use -- cannot alloc */
atomic_t buf_alloc; /**< Buffer allocation in progress */
/*@} */