drm/amdgpu/fence: Fix oops due to non-matching drm_sched init/fini

Currently amdgpu calls drm_sched_fini() from the fence driver sw fini
routine - such function is expected to be called only after the
respective init function - drm_sched_init() - was executed successfully.

Happens that we faced a driver probe failure in the Steam Deck
recently, and the function drm_sched_fini() was called even without
its counter-part had been previously called, causing the following oops:

amdgpu: probe of 0000:04:00.0 failed with error -110
BUG: kernel NULL pointer dereference, address: 0000000000000090
PGD 0 P4D 0
Oops: 0002 [zen-kernel#1] PREEMPT SMP NOPTI
CPU: 0 PID: 609 Comm: systemd-udevd Not tainted 6.2.0-rc3-gpiccoli zen-kernel#338
Hardware name: Valve Jupiter/Jupiter, BIOS F7A0113 11/04/2022
RIP: 0010:drm_sched_fini+0x84/0xa0 [gpu_sched]
[...]
Call Trace:
 <TASK>
 amdgpu_fence_driver_sw_fini+0xc8/0xd0 [amdgpu]
 amdgpu_device_fini_sw+0x2b/0x3b0 [amdgpu]
 amdgpu_driver_release_kms+0x16/0x30 [amdgpu]
 devm_drm_dev_init_release+0x49/0x70
 [...]

To prevent that, check if the drm_sched was properly initialized for a
given ring before calling its fini counter-part.

Notice ideally we'd use sched.ready for that; such field is set as the latest
thing on drm_sched_init(). But amdgpu seems to "override" the meaning of such
field - in the above oops for example, it was a GFX ring causing the crash, and
the sched.ready field was set to true in the ring init routine, regardless of
the state of the DRM scheduler. Hence, we ended-up using sched.ops as per
Christian's suggestion [0].

[0] https://lore.kernel.org/amd-gfx/[email protected]/

Fixes: 067f44c ("drm/amdgpu: avoid over-handle of fence driver fini in s3 test (v2)")
Suggested-by: Christian König <[email protected]>
Cc: Guchun Chen <[email protected]>
Cc: Luben Tuikov <[email protected]>
Cc: Mario Limonciello <[email protected]>
Signed-off-by: Guilherme G. Piccoli <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Luben Tuikov <[email protected]>

drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c

@@ -618,7 +618,13 @@ void amdgpu_fence_driver_sw_fini(struct amdgpu_device *adev)
 		if (!ring || !ring->fence_drv.initialized)
 			continue;
 
-		if (!ring->no_scheduler)
+		/*
+		 * Notice we check for sched.ops since there's some
+		 * override on the meaning of sched.ready by amdgpu.
+		 * The natural check would be sched.ready, which is
+		 * set as drm_sched_init() finishes...
+		 */
+		if (!ring->no_scheduler && ring->sched.ops)
 			drm_sched_fini(&ring->sched);
 
 		for (j = 0; j <= ring->fence_drv.num_fences_mask; ++j)