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

[Danilo Krummrich]

On 09.01.2025 17:58, Danilo Krummrich wrote:
> 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.

Applied to drm-misc-fixes.

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