[PATCH v2 2/3] 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.
The next patch adds such a caller for Tesla, and without a subscriber
the consequences 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; it stays that way for
minutes until the fences time out, and a reboot clears it immediately.
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 598513f60449..1db9fbd81f5c 100644
--- a/drivers/gpu/drm/nouveau/nouveau_chan.c
+++ b/drivers/gpu/drm/nouveau/nouveau_chan.c
@@ -363,7 +363,7 @@ nouveau_channel_init(struct nouveau_channel *chan, u32 vram, u32 gart)
 	if (ret)
 		return ret;
 
-	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