At driver init no one can access modeset objects and we're
single-threaded. So locking is just cargo-culting here. Worse, with
the new ww mutexes and ww mutex slowpath debugging the mutex_lock
might actually fail, and we don't have the full-blown ww recovery
dance.
Which then leads to fireworks when we try to unlock the not-locked
crtc lock.
An audit of all the functions called from here shows that none of them
contain locking checks, so there's also no reason to keep the locking
around just for consistency of caller contexts. Besides that I have
the rule (at least in i915) that such places where we take locks just
to simplify locking checks and not for correctness always require a
comment.
This regression was introduced in
commit 51fd371bba
Author: Rob Clark <robdclark@gmail.com>
Date: Tue Nov 19 12:10:12 2013 -0500
drm: convert crtc and connection_mutex to ww_mutex (v5)
v2: Don't drop the lock_init call, spotted by the 0day builder.
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=83341
Cc: Rob Clark <robdclark@gmail.com>
Cc: thellstrom@vmware.com
Cc: maarten.lankhorst@canonical.com
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
************************************************************
* For the very latest on DRI development, please see: *
* http://dri.freedesktop.org/ *
************************************************************
The Direct Rendering Manager (drm) is a device-independent kernel-level
device driver that provides support for the XFree86 Direct Rendering
Infrastructure (DRI).
The DRM supports the Direct Rendering Infrastructure (DRI) in four major
ways:
1. The DRM provides synchronized access to the graphics hardware via
the use of an optimized two-tiered lock.
2. The DRM enforces the DRI security policy for access to the graphics
hardware by only allowing authenticated X11 clients access to
restricted regions of memory.
3. The DRM provides a generic DMA engine, complete with multiple
queues and the ability to detect the need for an OpenGL context
switch.
4. The DRM is extensible via the use of small device-specific modules
that rely extensively on the API exported by the DRM module.
Documentation on the DRI is available from:
http://dri.freedesktop.org/wiki/Documentation
http://sourceforge.net/project/showfiles.php?group_id=387
http://dri.sourceforge.net/doc/
For specific information about kernel-level support, see:
The Direct Rendering Manager, Kernel Support for the Direct Rendering
Infrastructure
http://dri.sourceforge.net/doc/drm_low_level.html
Hardware Locking for the Direct Rendering Infrastructure
http://dri.sourceforge.net/doc/hardware_locking_low_level.html
A Security Analysis of the Direct Rendering Infrastructure
http://dri.sourceforge.net/doc/security_low_level.html
d0fa1af40e