[PATCH] drm/nouveau: idle the channel before destroying an engine object via abi16

[Alex <xnderv@yandex.com>]

Cc: dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org

nouveau_abi16_ioctl_del() destroys an engine object (ENGOBJ) created by
nouveau_abi16_ioctl_new() without waiting for the owning channel to go
idle.  The legacy path, nouveau_abi16_ioctl_gpuobj_free(), does call
nouveau_channel_idle() first ("synchronize with the user channel and
destroy the gpu object"), the new ioctl does not.

Chromium-based applications (Chrome, Yandex Browser, Electron apps)
create and immediately delete probe 3D objects at startup.  If the GPU
is busy with other work at that moment (compositor animation while the
app is launched from the GNOME dash, cursor movement), the engine
context is unbound while the channel still references it, and the GPU
faults:

  nouveau 0000:01:00.0: fifo: fault 00 [VIRT_READ] at 0000000000000000
    engine 00 [GR] client 0f [HUB/CTXCTL] reason 00 [PDE] on channel 5
  nouveau 0000:01:00.0: fifo: ... PAGE_NOT_PRESENT ... channel 5 killed!

after which the desktop freezes.  Observed on GF114 (GeForce GTX 560 Ti)
with Linux 7.0.x, reproducible almost every time by launching a Chromium
app from the dash; not seen in 10 consecutive launches with this patch.

Remember the channel an engine object was created on, drop the reference
when the channel is destroyed first, and idle the channel before
nvif_object_dtor() like the gpuobj_free path does.

Signed-off-by: Alex <xnderv@yandex.com>
---
 drivers/gpu/drm/nouveau/nouveau_abi16.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/nouveau/nouveau_abi16.c
+++ b/drivers/gpu/drm/nouveau/nouveau_abi16.c
@@ -89,6 +89,7 @@
 		ENGOBJ,
 	} type;
 	u64 object;
+	struct nouveau_abi16_chan *chan;
 
 	struct nvif_object engobj;
 
@@ -173,6 +174,13 @@
 			struct nouveau_abi16_chan *chan)
 {
 	struct nouveau_abi16_ntfy *ntfy, *temp;
+	struct nouveau_abi16_obj *obj;
+
+	/* Forget references to this channel held by engine objects. */
+	list_for_each_entry(obj, &abi16->objects, head) {
+		if (obj->chan == chan)
+			obj->chan = NULL;
+	}
 
 	/* Cancel all jobs from the entity's queue. */
 	if (chan->sched)
@@ -749,8 +757,16 @@
 
 	obj = nouveau_abi16_obj_find(abi16, ioctl->object);
 	if (obj) {
-		if (obj->type == ENGOBJ)
+		if (obj->type == ENGOBJ) {
+			/* Synchronize with the user channel before destroying the
+			 * engine object, as the legacy GPUOBJ_FREE path does.  The
+			 * channel may still have work queued that needs the engine
+			 * context which is about to be unbound.
+			 */
+			if (obj->chan && obj->chan->chan)
+				nouveau_channel_idle(obj->chan->chan);
 			nvif_object_dtor(&obj->engobj);
+		}
 		nouveau_abi16_obj_del(obj);
 	}
 
@@ -791,6 +807,7 @@
 	obj = nouveau_abi16_obj_new(abi16, ENGOBJ, args->object);
 	if (IS_ERR(obj))
 		return PTR_ERR(obj);
+	obj->chan = chan;
 
 	ret = nvif_object_ctor(&chan->chan->user, "abi16EngObj", args->handle, args->oclass,
 			       NULL, 0, &obj->engobj);