The aim of this locking rework is that ioctls which a compositor should be
might call for every frame (set_cursor, page_flip, addfb, rmfb and
getfb/create_handle) should not be able to block on kms background
activities like output detection. And since each EDID read takes about
25ms (in the best case), that always means we'll drop at least one frame.
The solution is to add per-crtc locking for these ioctls, and restrict
background activities to only use the global lock. Change-the-world type
of events (modeset, dpms, ...) need to grab all locks.
Two tricky parts arose in the conversion:
- A lot of current code assumes that a kms fb object can't disappear while
holding the global lock, since the current code serializes fb
destruction with it. Hence proper lifetime management using the already
created refcounting for fbs need to be instantiated for all ioctls and
interfaces/users.
- The rmfb ioctl removes the to-be-deleted fb from all active users. But
unconditionally taking the global kms lock to do so introduces an
unacceptable potential stall point. And obviously changing the userspace
abi isn't on the table, either. Hence this conversion opportunistically
checks whether the rmfb ioctl holds the very last reference, which
guarantees that the fb isn't in active use on any crtc or plane (thanks
to the conversion to the new lifetime rules using proper refcounting).
Only if this is not the case will the code go through the slowpath and
grab all modeset locks. Sane compositors will never hit this path and so
avoid the stall, but userspace relying on these semantics will also not
break.
All these cases are exercised by the newly added subtests for the i-g-t
kms_flip, tested on a machine where a full detect cycle takes around 100
ms. It works, and no frames are dropped any more with these patches
applied. kms_flip also contains a special case to exercise the
above-describe rmfb slowpath.
* 'drm-kms-locking' of git://people.freedesktop.org/~danvet/drm-intel: (335 commits)
drm/fb_helper: check whether fbcon is bound
drm/doc: updates for new framebuffer lifetime rules
drm: don't hold crtc mutexes for connector ->detect callbacks
drm: only grab the crtc lock for pageflips
drm: optimize drm_framebuffer_remove
drm/vmwgfx: add proper framebuffer refcounting
drm/i915: dump refcount into framebuffer debugfs file
drm: refcounting for crtc framebuffers
drm: refcounting for sprite framebuffers
drm: fb refcounting for dirtyfb_ioctl
drm: don't take modeset locks in getfb ioctl
drm: push modeset_lock_all into ->fb_create driver callbacks
drm: nest modeset locks within fpriv->fbs_lock
drm: reference framebuffers which are on the idr
drm: revamp framebuffer cleanup interfaces
drm: create drm_framebuffer_lookup
drm: revamp locking around fb creation/destruction
drm: only take the crtc lock for ->cursor_move
drm: only take the crtc lock for ->cursor_set
drm: add per-crtc locks
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