drm_sched with panfrost crash on T820

[Neil Armstrong]

Hi Christian,

In v5.3, running dEQP triggers the following kernel crash :

[   20.224982] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000038
[...]
[   20.291064] Hardware name: Khadas VIM2 (DT)
[   20.295217] Workqueue: events drm_sched_job_timedout
[...]
[   20.304867] pc : drm_sched_increase_karma+0x5c/0xf0
[   20.309696] lr : drm_sched_increase_karma+0x44/0xf0
[...]
[   20.396720] Call trace:
[   20.399138]  drm_sched_increase_karma+0x5c/0xf0
[   20.403623]  panfrost_job_timedout+0x12c/0x1e0
[   20.408021]  drm_sched_job_timedout+0x48/0xa0
[   20.412336]  process_one_work+0x1e0/0x320
[   20.416300]  worker_thread+0x40/0x450
[   20.419924]  kthread+0x124/0x128
[   20.423116]  ret_from_fork+0x10/0x18
[   20.426653] Code: f9400001 540001c0 f9400a83 f9402402 (f9401c64)
[   20.432690] ---[ end trace bd02f890139096a7 ]---

Which never happens, at all, on v5.2.

I did a (very) long (7 days, ~100runs) bisect run using our LAVA lab (thanks tomeu !), but
bisecting was not easy since the bad commit landed on drm-misc-next after v5.1-rc6, and
then v5.2-rc1 was backmerged into drm-misc-next at:
[1] 374ed5429346 Merge drm/drm-next into drm-misc-next

Thus bisecting between [1] ang v5.2-rc1 leads to commit based on v5.2-rc1... where panfrost was
not enabled in the Khadas VIM2 DT.

Anyway, I managed to identify 3 possibly breaking commits :
[2] 290764af7e36 drm/sched: Keep s_fence->parent pointer
[3] 5918045c4ed4 drm/scheduler: rework job destruction
[4] a5343b8a2ca5 drm/scheduler: Add flag to hint the release of guilty job.

But [1] and [2] doesn't crash the same way :
[   16.257912] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000060
[...]
[   16.308307] CPU: 4 PID: 80 Comm: kworker/4:1 Not tainted 5.1.0-rc2-01185-g290764af7e36-dirty #378
[   16.317099] Hardware name: Khadas VIM2 (DT)
[...])
[   16.330907] pc : refcount_sub_and_test_checked+0x4/0xb0
[   16.336078] lr : refcount_dec_and_test_checked+0x14/0x20
[...]
[   16.423533] Process kworker/4:1 (pid: 80, stack limit = 0x(____ptrval____))
[   16.430431] Call trace:
[   16.432851]  refcount_sub_and_test_checked+0x4/0xb0
[   16.437681]  drm_sched_job_cleanup+0x24/0x58
[   16.441908]  panfrost_job_free+0x14/0x28
[   16.445787]  drm_sched_job_timedout+0x6c/0xa0
[   16.450102]  process_one_work+0x1e0/0x320
[   16.454067]  worker_thread+0x40/0x450
[   16.457690]  kthread+0x124/0x128
[   16.460882]  ret_from_fork+0x10/0x18
[   16.464421] Code: 52800000 d65f03c0 d503201f aa0103e3 (b9400021)
[   16.470456] ---[ end trace 39a67412ee1b64b5 ]---

and [3] fails like on v5.3 (in drm_sched_increase_karma):
[   33.830080] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000038
[...]
[   33.871946] Internal error: Oops: 96000004 [#1] PREEMPT SMP
[   33.877450] Modules linked in:
[   33.880474] CPU: 6 PID: 81 Comm: kworker/6:1 Not tainted 5.1.0-rc2-01186-ga5343b8a2ca5-dirty #380
[   33.889265] Hardware name: Khadas VIM2 (DT)
[   33.893419] Workqueue: events drm_sched_job_timedout
[...]
[   33.903069] pc : drm_sched_increase_karma+0x5c/0xf0
[   33.907898] lr : drm_sched_increase_karma+0x44/0xf0
[...]
[   33.994924] Process kworker/6:1 (pid: 81, stack limit = 0x(____ptrval____))
[   34.001822] Call trace:
[   34.004242]  drm_sched_increase_karma+0x5c/0xf0
[   34.008726]  panfrost_job_timedout+0x12c/0x1e0
[   34.013122]  drm_sched_job_timedout+0x48/0xa0
[   34.017438]  process_one_work+0x1e0/0x320
[   34.021402]  worker_thread+0x40/0x450
[   34.025026]  kthread+0x124/0x128
[   34.028218]  ret_from_fork+0x10/0x18
[   34.031755] Code: f9400001 540001c0 f9400a83 f9402402 (f9401c64)
[   34.037792] ---[ end trace be3fd6f77f4df267 ]---


When I revert [3] on [1], i get the same crash as [2], meaning
the commit [3] masks the failure [2] introduced.

Do you know how to solve this ?

Thanks,
Neil