drm/fb-helper: improve drm_fb_helper_initial_config locking
The locking in drm_fb_helper_initial_config is a bit troublesome for a
few reasons:
- We can't just wrap the entire function up into modeset locks since
the fbdev registration might call down into fbcon code, which then
through our ->set_par implementation needs to be able to grab all
modeset locks. So we'd have a neat deadlock.
- This implies though that all current callers don't hold any modeset
locks by necessity, so we have free reign to grab any modeset locks
we need to grab.
- The private state of the fbdev helper doesn't need any protection
through locks, since once we have the fbdev registered it is mostly
invariant or protected through the modeset locking in ->set_par and
other callbacks. We can fully rely on driver having non-racy setup
sequences here. For the initial config computation we actually may
not grab locks since drivers which provide their own magic sauce
(like i915) might need to grab locks themselves.
- We should grab locks though when we probe outputs. Currently there's
not much risk, but already now userspace could start poking at sysfs
files and so probe concurrently. I expect that in the future driver
init will be much more async, and since probing is really
time-consuming this is a prime candidate.
- We must not hold any crtc->mutex locks while calling probe functions
since those might need to lock a crtc for e.g. load detection. i915
is such a driver.
Also it's the probing calls which hit upon piles of new locking
asserts I've recently added in
commit 62ff94a5492175759546f8bc61383189d6b49122
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Thu Jan 23 22:18:47 2014 +0100
drm/crtc-helper: remove LOCKING from kerneldoc
and
commit 63951385052f7974155fa38f962f0f4e9847f90a
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Thu Jan 23 15:14:15 2014 +0100
drm/doc: Repleace LOCKING kerneldoc sections in drm_modes.c
Hence the right fix is to grab the mode_config mutex, but only that
and only right around the probe calls.
It seems to be sufficient to shut up all the locking WARNINGs I see on
i915 and nouveau in drm_fb_helper_initial_config.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
1 file changed