[PATCH v3 4/4] drm/nouveau: subscribe to channel-kill events on NV50 and newer

[Marek Czernohous <mczernohous@gmail.com>]

From: Marek Czernohous <marek@czernohous.de>

nouveau_channel_init() only subscribes to the channel-killed event for
FERMI_CHANNEL_GPFIFO and newer. On NV50/Tesla the subscription therefore
never happens, and nvkm_chan_error()'s NVKM_CHAN_EVENT_ERRORED is
delivered into an empty notifier list.

Today that is harmless, because nothing kills a channel on Tesla: the
only nvkm_chan_error() callers are the Fermi and newer recovery paths.
So this patch changes no observable behaviour on its own, and that is
deliberate: it removes a latent trap before anything can fall into it.
I am carrying a Tesla recovery path that does add such a caller and will
send it separately once it is ready. Without a subscriber in place the
consequences there are severe: nouveau_channel_killed() never runs, so
nouveau_fence_context_kill() never runs either, and the pending fences
of the killed channel are never signalled. Everything waiting on them
waits forever: drm_atomic_helper_wait_for_fences() in the display commit
tail waits uninterruptibly and without a timeout, and the TTM delayed
delete workers wait in TASK_UNINTERRUPTIBLE. The user sees a frozen
desktop on a machine that is otherwise alive, and nothing in the kernel
ends that state: both waits pass MAX_SCHEDULE_TIMEOUT, so the fences
cannot time out. They are signalled only when the fence context is torn
down, that is when the DRM client owning the channel closes its fd and
nouveau_fence_context_del() runs. Killing the client, or rebooting,
clears it; waiting does not.
That is also a dma-fence contract violation: a fence must always be
signalled, with an error if necessary.

Lower the class gate to NV50_CHANNEL_GPFIFO. The nvkm side is already
class neutral: the KILLED case hangs the notifier on runl->chid->event,
which every fifo owns since the runlist rework, and nvkm_uchan_uevent()
does not discriminate by class. Pre-NV50 chips keep the old behaviour,
so NV04 to NV40 are unaffected.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Marek Czernohous <marek@czernohous.de>
---
 drivers/gpu/drm/nouveau/nouveau_chan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_chan.c b/drivers/gpu/drm/nouveau/nouveau_chan.c
index 07b0bd1bc519..5c2f4b9342b7 100644
--- a/drivers/gpu/drm/nouveau/nouveau_chan.c
+++ b/drivers/gpu/drm/nouveau/nouveau_chan.c
@@ -485,7 +485,7 @@ nouveau_channel_init(struct nouveau_channel *chan, u32 vram, u32 gart)
 	 * lock and the pending list, so an event arriving in between would
 	 * find a non-NULL but unusable context and walk a NULL list head.
 	 */
-	if (chan->user.oclass >= FERMI_CHANNEL_GPFIFO) {
+	if (chan->user.oclass >= NV50_CHANNEL_GPFIFO) {
 		DEFINE_RAW_FLEX(struct nvif_event_v0, args, data,
 				sizeof(struct nvif_chan_event_v0));
 		struct nvif_chan_event_v0 *host =
-- 
2.54.0