[Nouveau] NVAC - BUG: unable to handle kernel NULL pointer dereference

[poma]

With lightweight desktoping,
the atomic modesetting seems far from robust.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000021
IP: dma_fence_wait_timeout+0x36/0xf0
...
Oops: 0000 [#1] SMP
Modules linked in: ... nouveau ...
CPU: 0 PID: 6895 Comm: Xorg Not tainted 4.10.5-1001.fc24.x86_64 #1
...
Call Trace:
 drm_atomic_helper_wait_for_fences+0x48/0x120 [drm_kms_helper]
 nv50_disp_atomic_commit+0x19c/0x2a0 [nouveau]
 drm_atomic_commit+0x4b/0x50 [drm]
 drm_atomic_helper_update_plane+0xec/0x150 [drm_kms_helper]
 __setplane_internal+0x1b4/0x280 [drm]
 drm_mode_cursor_universal+0x126/0x210 [drm]
 drm_mode_cursor_common+0x86/0x180 [drm]
 drm_mode_cursor_ioctl+0x50/0x70 [drm]
 drm_ioctl+0x21b/0x4c0 [drm]
 ? drm_mode_setplane+0x1a0/0x1a0 [drm]
 nouveau_drm_ioctl+0x74/0xc0 [nouveau]
 do_vfs_ioctl+0xa3/0x5f0
 SyS_ioctl+0x79/0x90
 entry_SYSCALL_64_fastpath+0x1a/0xa9
...
RIP: dma_fence_wait_timeout+0x36/0xf0 RSP: ffffc1f700723a38
...
---[ end trace a6bef2d32ed5fbbc ]---


BUG: unable to handle kernel NULL pointer dereference at 0000000000000021
IP: dma_fence_wait_timeout+0x36/0xf0
...
Oops: 0000 [#1] SMP
Modules linked in: ... nouveau ...
CPU: 3 PID: 30654 Comm: Xorg Tainted: G            E   4.11.0-0.rc3.git0.1.fc26.x86_64 #1
...
Call Trace:
 drm_atomic_helper_wait_for_fences+0x73/0x110 [drm_kms_helper]
 nv50_disp_atomic_commit+0x28a/0x2c0 [nouveau]
 ? refcount_dec_and_test+0x11/0x20
 drm_atomic_commit+0x4b/0x50 [drm]
 drm_atomic_helper_update_plane+0xf1/0x150 [drm_kms_helper]
 __setplane_internal+0x1fa/0x260 [drm]
 drm_mode_cursor_universal+0x12a/0x220 [drm]
 drm_mode_cursor_common+0x88/0x180 [drm]
 drm_mode_cursor_ioctl+0x4a/0x60 [drm]
 drm_ioctl+0x203/0x4d0 [drm]
 ? drm_mode_setplane+0x1a0/0x1a0 [drm]
 nouveau_drm_ioctl+0x72/0xc0 [nouveau]
 do_vfs_ioctl+0xa5/0x600
 ? security_inode_getsecid+0x1b/0x40
 SyS_ioctl+0x79/0x90
 entry_SYSCALL_64_fastpath+0x1a/0xa9
...
RIP: dma_fence_wait_timeout+0x36/0xf0 RSP: ffffbda700723a40
...
---[ end trace 95b0fca6a8295839 ]---


Subsequently, hardware reset is needed.