[PATCH] nouveau/fence: handle cross device fences properly. (v3)

[Danilo Krummrich]

On Thu, Jan 09, 2025 at 10:55:53AM +1000, Dave Airlie wrote:
> From: Dave Airlie <airlied at redhat.com>
> 
> This is the 3rd iteration of this after talking to Ben and
> Danilo, I think this makes sense now.
> 
> The fence sync logic doesn't handle a fence sync across devices
> as it tries to write to a channel offset from one device into
> the fence bo from a different device, which won't work so well.
> 
> This patch fixes that to avoid using the sync path in the case
> where the fences come from different nouveau drm devices.
> 
> This works fine on a single device as the fence bo is shared
> across the devices, and mapped into each channels vma space,
> the channel offsets are therefore okay to pass between sides,
> so one channel can sync on the seqnos from the other by using
> the offset into it's vma.

Huh, they indeed all share and map drm->fence->bo, good catch.

> 
> Signed-off-by: Dave Airlie <airlied at redhat.com>
> Cc: stable at vger.kernel.org
> ---
>  drivers/gpu/drm/nouveau/nouveau_fence.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
> index ee5e9d40c166..a3eb1f447a29 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> @@ -367,11 +367,13 @@ nouveau_fence_sync(struct nouveau_bo *nvbo, struct nouveau_channel *chan,
>  			if (f) {
>  				struct nouveau_channel *prev;
>  				bool must_wait = true;
> +				bool local;
>  
>  				rcu_read_lock();
>  				prev = rcu_dereference(f->channel);
> -				if (prev && (prev == chan ||
> -					     fctx->sync(f, prev, chan) == 0))
> +				local = prev && prev->drm == chan->drm;

struct nouveau_channel has no pointer to a struct nouveau_drm, this should be
prev->cli->drm and chan->cli->drm instead.

No need to resend, I can fix it when applying the patch if you want.

> +				if (local && (prev == chan ||
> +					      fctx->sync(f, prev, chan) == 0))
>  					must_wait = false;
>  				rcu_read_unlock();
>  				if (!must_wait)
> -- 
> 2.43.0
>