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

[Ben Skeggs]

On 9/1/25 13:42, Ben Skeggs wrote:

> On 9/1/25 10:55, 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.
>>
>> Signed-off-by: Dave Airlie <airlied at redhat.com>
> Signed-off-by: Ben Skeggs <bskeggs at nvidia.com>

Err, not sure what my fingers did there ;)

Reviewed-by: Ben Skeggs <bskeggs at nvidia.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;
>> +                if (local && (prev == chan ||
>> +                          fctx->sync(f, prev, chan) == 0))
>>                       must_wait = false;
>>                   rcu_read_unlock();
>>                   if (!must_wait)