Danilo Krummrich
2023-Aug-29 22:38 UTC
[Nouveau] [PATCH drm-misc-next] drm/nouveau: fence: fix undefined fence state after emit
nouveau_fence_emit() can fail before and after initializing the
dma-fence and hence before and after initializing the dma-fence' kref.
In order to avoid nouveau_fence_emit() potentially failing before
dma-fence initialization pass the channel to nouveau_fence_new() already
and perform the required check before even allocating the fence.
While at it, restore the original behavior of nouveau_fence_new() and
add nouveau_fence_create() for separate (pre-)allocation instead. Always
splitting up allocation end emit wasn't a good idea in the first place.
Hence, limit it to the places where we actually need to pre-allocate.
Fixes: 7f2a0b50b2b2 ("drm/nouveau: fence: separate fence alloc and
emit")
Signed-off-by: Danilo Krummrich <dakr at redhat.com>
---
drivers/gpu/drm/nouveau/dispnv04/crtc.c | 9 +------
drivers/gpu/drm/nouveau/nouveau_bo.c | 8 +------
drivers/gpu/drm/nouveau/nouveau_chan.c | 6 ++---
drivers/gpu/drm/nouveau/nouveau_dmem.c | 9 +++----
drivers/gpu/drm/nouveau/nouveau_exec.c | 11 ++++++---
drivers/gpu/drm/nouveau/nouveau_fence.c | 32 ++++++++++++++++++++-----
drivers/gpu/drm/nouveau/nouveau_fence.h | 5 ++--
drivers/gpu/drm/nouveau/nouveau_gem.c | 5 +---
8 files changed, 45 insertions(+), 40 deletions(-)
diff --git a/drivers/gpu/drm/nouveau/dispnv04/crtc.c
b/drivers/gpu/drm/nouveau/dispnv04/crtc.c
index a34924523133..a34917b048f9 100644
--- a/drivers/gpu/drm/nouveau/dispnv04/crtc.c
+++ b/drivers/gpu/drm/nouveau/dispnv04/crtc.c
@@ -1122,18 +1122,11 @@ nv04_page_flip_emit(struct nouveau_channel *chan,
PUSH_NVSQ(push, NV_SW, NV_SW_PAGE_FLIP, 0x00000000);
PUSH_KICK(push);
- ret = nouveau_fence_new(pfence);
+ ret = nouveau_fence_new(pfence, chan);
if (ret)
goto fail;
- ret = nouveau_fence_emit(*pfence, chan);
- if (ret)
- goto fail_fence_unref;
-
return 0;
-
-fail_fence_unref:
- nouveau_fence_unref(pfence);
fail:
spin_lock_irqsave(&dev->event_lock, flags);
list_del(&s->head);
diff --git a/drivers/gpu/drm/nouveau/nouveau_bo.c
b/drivers/gpu/drm/nouveau/nouveau_bo.c
index 64f50adb2856..56427b6a00a4 100644
--- a/drivers/gpu/drm/nouveau/nouveau_bo.c
+++ b/drivers/gpu/drm/nouveau/nouveau_bo.c
@@ -875,16 +875,10 @@ nouveau_bo_move_m2mf(struct ttm_buffer_object *bo, int
evict,
if (ret)
goto out_unlock;
- ret = nouveau_fence_new(&fence);
+ ret = nouveau_fence_new(&fence, chan);
if (ret)
goto out_unlock;
- ret = nouveau_fence_emit(fence, chan);
- if (ret) {
- nouveau_fence_unref(&fence);
- goto out_unlock;
- }
-
/* TODO: figure out a better solution here
*
* wait on the fence here explicitly as going through
diff --git a/drivers/gpu/drm/nouveau/nouveau_chan.c
b/drivers/gpu/drm/nouveau/nouveau_chan.c
index 1fd5ccf41128..bb3d6e5c122f 100644
--- a/drivers/gpu/drm/nouveau/nouveau_chan.c
+++ b/drivers/gpu/drm/nouveau/nouveau_chan.c
@@ -70,11 +70,9 @@ nouveau_channel_idle(struct nouveau_channel *chan)
struct nouveau_fence *fence = NULL;
int ret;
- ret = nouveau_fence_new(&fence);
+ ret = nouveau_fence_new(&fence, chan);
if (!ret) {
- ret = nouveau_fence_emit(fence, chan);
- if (!ret)
- ret = nouveau_fence_wait(fence, false, false);
+ ret = nouveau_fence_wait(fence, false, false);
nouveau_fence_unref(&fence);
}
diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c
b/drivers/gpu/drm/nouveau/nouveau_dmem.c
index 61e84562094a..12feecf71e75 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
@@ -209,8 +209,7 @@ static vm_fault_t nouveau_dmem_migrate_to_ram(struct
vm_fault *vmf)
goto done;
}
- if (!nouveau_fence_new(&fence))
- nouveau_fence_emit(fence, dmem->migrate.chan);
+ nouveau_fence_new(&fence, dmem->migrate.chan);
migrate_vma_pages(&args);
nouveau_dmem_fence_done(&fence);
dma_unmap_page(drm->dev->dev, dma_addr, PAGE_SIZE, DMA_BIDIRECTIONAL);
@@ -403,8 +402,7 @@ nouveau_dmem_evict_chunk(struct nouveau_dmem_chunk *chunk)
}
}
- if (!nouveau_fence_new(&fence))
- nouveau_fence_emit(fence, chunk->drm->dmem->migrate.chan);
+ nouveau_fence_new(&fence, chunk->drm->dmem->migrate.chan);
migrate_device_pages(src_pfns, dst_pfns, npages);
nouveau_dmem_fence_done(&fence);
migrate_device_finalize(src_pfns, dst_pfns, npages);
@@ -677,8 +675,7 @@ static void nouveau_dmem_migrate_chunk(struct nouveau_drm
*drm,
addr += PAGE_SIZE;
}
- if (!nouveau_fence_new(&fence))
- nouveau_fence_emit(fence, drm->dmem->migrate.chan);
+ nouveau_fence_new(&fence, drm->dmem->migrate.chan);
migrate_vma_pages(args);
nouveau_dmem_fence_done(&fence);
nouveau_pfns_map(svmm, args->vma->vm_mm, args->start, pfns, i);
diff --git a/drivers/gpu/drm/nouveau/nouveau_exec.c
b/drivers/gpu/drm/nouveau/nouveau_exec.c
index 98a7a94cec5a..72f6543a0790 100644
--- a/drivers/gpu/drm/nouveau/nouveau_exec.c
+++ b/drivers/gpu/drm/nouveau/nouveau_exec.c
@@ -91,7 +91,8 @@ nouveau_exec_job_submit(struct nouveau_job *job)
struct nouveau_uvmm *uvmm = nouveau_cli_uvmm(cli);
int ret;
- ret = nouveau_fence_new(&exec_job->fence);
+ /* Create a new fence, but do not emit yet. */
+ ret = nouveau_fence_create(&exec_job->fence, exec_job->chan);
if (ret)
return ret;
@@ -143,13 +144,17 @@ nouveau_exec_job_run(struct nouveau_job *job)
nv50_dma_push(chan, p->va, p->va_len, no_prefetch);
}
- ret = nouveau_fence_emit(fence, chan);
+ ret = nouveau_fence_emit(fence);
if (ret) {
+ nouveau_fence_unref(&exec_job->fence);
NV_PRINTK(err, job->cli, "error fencing pushbuf: %d\n", ret);
WIND_RING(chan);
return ERR_PTR(ret);
}
+ /* The fence was emitted successfully, set the job's fence pointer to
+ * NULL in order to avoid freeing it up when the job is cleaned up.
+ */
exec_job->fence = NULL;
return &fence->base;
@@ -162,7 +167,7 @@ nouveau_exec_job_free(struct nouveau_job *job)
nouveau_job_free(job);
- nouveau_fence_unref(&exec_job->fence);
+ kfree(exec_job->fence);
kfree(exec_job->push.s);
kfree(exec_job);
}
diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c
b/drivers/gpu/drm/nouveau/nouveau_fence.c
index 77c739a55b19..61d9e70da9fd 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.c
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
@@ -205,16 +205,13 @@ nouveau_fence_context_new(struct nouveau_channel *chan,
struct nouveau_fence_cha
}
int
-nouveau_fence_emit(struct nouveau_fence *fence, struct nouveau_channel *chan)
+nouveau_fence_emit(struct nouveau_fence *fence)
{
+ struct nouveau_channel *chan = fence->channel;
struct nouveau_fence_chan *fctx = chan->fence;
struct nouveau_fence_priv *priv = (void*)chan->drm->fence;
int ret;
- if (unlikely(!chan->fence))
- return -ENODEV;
-
- fence->channel = chan;
fence->timeout = jiffies + (15 * HZ);
if (priv->uevent)
@@ -406,18 +403,41 @@ nouveau_fence_unref(struct nouveau_fence **pfence)
}
int
-nouveau_fence_new(struct nouveau_fence **pfence)
+nouveau_fence_create(struct nouveau_fence **pfence,
+ struct nouveau_channel *chan)
{
struct nouveau_fence *fence;
+ if (unlikely(!chan->fence))
+ return -ENODEV;
+
fence = kzalloc(sizeof(*fence), GFP_KERNEL);
if (!fence)
return -ENOMEM;
+ fence->channel = chan;
+
*pfence = fence;
return 0;
}
+int
+nouveau_fence_new(struct nouveau_fence **pfence,
+ struct nouveau_channel *chan)
+{
+ int ret = 0;
+
+ ret = nouveau_fence_create(pfence, chan);
+ if (ret)
+ return ret;
+
+ ret = nouveau_fence_emit(*pfence);
+ if (ret)
+ nouveau_fence_unref(pfence);
+
+ return ret;
+}
+
static const char *nouveau_fence_get_get_driver_name(struct dma_fence *fence)
{
return "nouveau";
diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.h
b/drivers/gpu/drm/nouveau/nouveau_fence.h
index 2c72d96ef17d..64d33ae7f356 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.h
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.h
@@ -17,10 +17,11 @@ struct nouveau_fence {
unsigned long timeout;
};
-int nouveau_fence_new(struct nouveau_fence **);
+int nouveau_fence_create(struct nouveau_fence **, struct nouveau_channel *);
+int nouveau_fence_new(struct nouveau_fence **, struct nouveau_channel *);
void nouveau_fence_unref(struct nouveau_fence **);
-int nouveau_fence_emit(struct nouveau_fence *, struct nouveau_channel *);
+int nouveau_fence_emit(struct nouveau_fence *);
bool nouveau_fence_done(struct nouveau_fence *);
int nouveau_fence_wait(struct nouveau_fence *, bool lazy, bool intr);
int nouveau_fence_sync(struct nouveau_bo *, struct nouveau_channel *, bool
exclusive, bool intr);
diff --git a/drivers/gpu/drm/nouveau/nouveau_gem.c
b/drivers/gpu/drm/nouveau/nouveau_gem.c
index a03fe3e92059..254524fee4fb 100644
--- a/drivers/gpu/drm/nouveau/nouveau_gem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_gem.c
@@ -914,11 +914,8 @@ nouveau_gem_ioctl_pushbuf(struct drm_device *dev, void
*data,
}
}
- ret = nouveau_fence_new(&fence);
- if (!ret)
- ret = nouveau_fence_emit(fence, chan);
+ ret = nouveau_fence_new(&fence, chan);
if (ret) {
- nouveau_fence_unref(&fence);
NV_PRINTK(err, cli, "error fencing pushbuf: %d\n", ret);
WIND_RING(chan);
goto out;
--
2.41.0
Dave Airlie
2023-Aug-29 22:47 UTC
[Nouveau] [PATCH drm-misc-next] drm/nouveau: fence: fix undefined fence state after emit
On Wed, 30 Aug 2023 at 08:38, Danilo Krummrich <dakr at redhat.com> wrote:> > nouveau_fence_emit() can fail before and after initializing the > dma-fence and hence before and after initializing the dma-fence' kref. > > In order to avoid nouveau_fence_emit() potentially failing before > dma-fence initialization pass the channel to nouveau_fence_new() already > and perform the required check before even allocating the fence. > > While at it, restore the original behavior of nouveau_fence_new() and > add nouveau_fence_create() for separate (pre-)allocation instead. Always > splitting up allocation end emit wasn't a good idea in the first place. > Hence, limit it to the places where we actually need to pre-allocate. > > Fixes: 7f2a0b50b2b2 ("drm/nouveau: fence: separate fence alloc and emit") > Signed-off-by: Danilo Krummrich <dakr at redhat.com>nice find, Reviewed-by: Dave Airlie <airlied at redhat.com>> --- > drivers/gpu/drm/nouveau/dispnv04/crtc.c | 9 +------ > drivers/gpu/drm/nouveau/nouveau_bo.c | 8 +------ > drivers/gpu/drm/nouveau/nouveau_chan.c | 6 ++--- > drivers/gpu/drm/nouveau/nouveau_dmem.c | 9 +++---- > drivers/gpu/drm/nouveau/nouveau_exec.c | 11 ++++++--- > drivers/gpu/drm/nouveau/nouveau_fence.c | 32 ++++++++++++++++++++----- > drivers/gpu/drm/nouveau/nouveau_fence.h | 5 ++-- > drivers/gpu/drm/nouveau/nouveau_gem.c | 5 +--- > 8 files changed, 45 insertions(+), 40 deletions(-) > > diff --git a/drivers/gpu/drm/nouveau/dispnv04/crtc.c b/drivers/gpu/drm/nouveau/dispnv04/crtc.c > index a34924523133..a34917b048f9 100644 > --- a/drivers/gpu/drm/nouveau/dispnv04/crtc.c > +++ b/drivers/gpu/drm/nouveau/dispnv04/crtc.c > @@ -1122,18 +1122,11 @@ nv04_page_flip_emit(struct nouveau_channel *chan, > PUSH_NVSQ(push, NV_SW, NV_SW_PAGE_FLIP, 0x00000000); > PUSH_KICK(push); > > - ret = nouveau_fence_new(pfence); > + ret = nouveau_fence_new(pfence, chan); > if (ret) > goto fail; > > - ret = nouveau_fence_emit(*pfence, chan); > - if (ret) > - goto fail_fence_unref; > - > return 0; > - > -fail_fence_unref: > - nouveau_fence_unref(pfence); > fail: > spin_lock_irqsave(&dev->event_lock, flags); > list_del(&s->head); > diff --git a/drivers/gpu/drm/nouveau/nouveau_bo.c b/drivers/gpu/drm/nouveau/nouveau_bo.c > index 64f50adb2856..56427b6a00a4 100644 > --- a/drivers/gpu/drm/nouveau/nouveau_bo.c > +++ b/drivers/gpu/drm/nouveau/nouveau_bo.c > @@ -875,16 +875,10 @@ nouveau_bo_move_m2mf(struct ttm_buffer_object *bo, int evict, > if (ret) > goto out_unlock; > > - ret = nouveau_fence_new(&fence); > + ret = nouveau_fence_new(&fence, chan); > if (ret) > goto out_unlock; > > - ret = nouveau_fence_emit(fence, chan); > - if (ret) { > - nouveau_fence_unref(&fence); > - goto out_unlock; > - } > - > /* TODO: figure out a better solution here > * > * wait on the fence here explicitly as going through > diff --git a/drivers/gpu/drm/nouveau/nouveau_chan.c b/drivers/gpu/drm/nouveau/nouveau_chan.c > index 1fd5ccf41128..bb3d6e5c122f 100644 > --- a/drivers/gpu/drm/nouveau/nouveau_chan.c > +++ b/drivers/gpu/drm/nouveau/nouveau_chan.c > @@ -70,11 +70,9 @@ nouveau_channel_idle(struct nouveau_channel *chan) > struct nouveau_fence *fence = NULL; > int ret; > > - ret = nouveau_fence_new(&fence); > + ret = nouveau_fence_new(&fence, chan); > if (!ret) { > - ret = nouveau_fence_emit(fence, chan); > - if (!ret) > - ret = nouveau_fence_wait(fence, false, false); > + ret = nouveau_fence_wait(fence, false, false); > nouveau_fence_unref(&fence); > } > > diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c > index 61e84562094a..12feecf71e75 100644 > --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c > +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c > @@ -209,8 +209,7 @@ static vm_fault_t nouveau_dmem_migrate_to_ram(struct vm_fault *vmf) > goto done; > } > > - if (!nouveau_fence_new(&fence)) > - nouveau_fence_emit(fence, dmem->migrate.chan); > + nouveau_fence_new(&fence, dmem->migrate.chan); > migrate_vma_pages(&args); > nouveau_dmem_fence_done(&fence); > dma_unmap_page(drm->dev->dev, dma_addr, PAGE_SIZE, DMA_BIDIRECTIONAL); > @@ -403,8 +402,7 @@ nouveau_dmem_evict_chunk(struct nouveau_dmem_chunk *chunk) > } > } > > - if (!nouveau_fence_new(&fence)) > - nouveau_fence_emit(fence, chunk->drm->dmem->migrate.chan); > + nouveau_fence_new(&fence, chunk->drm->dmem->migrate.chan); > migrate_device_pages(src_pfns, dst_pfns, npages); > nouveau_dmem_fence_done(&fence); > migrate_device_finalize(src_pfns, dst_pfns, npages); > @@ -677,8 +675,7 @@ static void nouveau_dmem_migrate_chunk(struct nouveau_drm *drm, > addr += PAGE_SIZE; > } > > - if (!nouveau_fence_new(&fence)) > - nouveau_fence_emit(fence, drm->dmem->migrate.chan); > + nouveau_fence_new(&fence, drm->dmem->migrate.chan); > migrate_vma_pages(args); > nouveau_dmem_fence_done(&fence); > nouveau_pfns_map(svmm, args->vma->vm_mm, args->start, pfns, i); > diff --git a/drivers/gpu/drm/nouveau/nouveau_exec.c b/drivers/gpu/drm/nouveau/nouveau_exec.c > index 98a7a94cec5a..72f6543a0790 100644 > --- a/drivers/gpu/drm/nouveau/nouveau_exec.c > +++ b/drivers/gpu/drm/nouveau/nouveau_exec.c > @@ -91,7 +91,8 @@ nouveau_exec_job_submit(struct nouveau_job *job) > struct nouveau_uvmm *uvmm = nouveau_cli_uvmm(cli); > int ret; > > - ret = nouveau_fence_new(&exec_job->fence); > + /* Create a new fence, but do not emit yet. */ > + ret = nouveau_fence_create(&exec_job->fence, exec_job->chan); > if (ret) > return ret; > > @@ -143,13 +144,17 @@ nouveau_exec_job_run(struct nouveau_job *job) > nv50_dma_push(chan, p->va, p->va_len, no_prefetch); > } > > - ret = nouveau_fence_emit(fence, chan); > + ret = nouveau_fence_emit(fence); > if (ret) { > + nouveau_fence_unref(&exec_job->fence); > NV_PRINTK(err, job->cli, "error fencing pushbuf: %d\n", ret); > WIND_RING(chan); > return ERR_PTR(ret); > } > > + /* The fence was emitted successfully, set the job's fence pointer to > + * NULL in order to avoid freeing it up when the job is cleaned up. > + */ > exec_job->fence = NULL; > > return &fence->base; > @@ -162,7 +167,7 @@ nouveau_exec_job_free(struct nouveau_job *job) > > nouveau_job_free(job); > > - nouveau_fence_unref(&exec_job->fence); > + kfree(exec_job->fence); > kfree(exec_job->push.s); > kfree(exec_job); > } > diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c > index 77c739a55b19..61d9e70da9fd 100644 > --- a/drivers/gpu/drm/nouveau/nouveau_fence.c > +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c > @@ -205,16 +205,13 @@ nouveau_fence_context_new(struct nouveau_channel *chan, struct nouveau_fence_cha > } > > int > -nouveau_fence_emit(struct nouveau_fence *fence, struct nouveau_channel *chan) > +nouveau_fence_emit(struct nouveau_fence *fence) > { > + struct nouveau_channel *chan = fence->channel; > struct nouveau_fence_chan *fctx = chan->fence; > struct nouveau_fence_priv *priv = (void*)chan->drm->fence; > int ret; > > - if (unlikely(!chan->fence)) > - return -ENODEV; > - > - fence->channel = chan; > fence->timeout = jiffies + (15 * HZ); > > if (priv->uevent) > @@ -406,18 +403,41 @@ nouveau_fence_unref(struct nouveau_fence **pfence) > } > > int > -nouveau_fence_new(struct nouveau_fence **pfence) > +nouveau_fence_create(struct nouveau_fence **pfence, > + struct nouveau_channel *chan) > { > struct nouveau_fence *fence; > > + if (unlikely(!chan->fence)) > + return -ENODEV; > + > fence = kzalloc(sizeof(*fence), GFP_KERNEL); > if (!fence) > return -ENOMEM; > > + fence->channel = chan; > + > *pfence = fence; > return 0; > } > > +int > +nouveau_fence_new(struct nouveau_fence **pfence, > + struct nouveau_channel *chan) > +{ > + int ret = 0; > + > + ret = nouveau_fence_create(pfence, chan); > + if (ret) > + return ret; > + > + ret = nouveau_fence_emit(*pfence); > + if (ret) > + nouveau_fence_unref(pfence); > + > + return ret; > +} > + > static const char *nouveau_fence_get_get_driver_name(struct dma_fence *fence) > { > return "nouveau"; > diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.h b/drivers/gpu/drm/nouveau/nouveau_fence.h > index 2c72d96ef17d..64d33ae7f356 100644 > --- a/drivers/gpu/drm/nouveau/nouveau_fence.h > +++ b/drivers/gpu/drm/nouveau/nouveau_fence.h > @@ -17,10 +17,11 @@ struct nouveau_fence { > unsigned long timeout; > }; > > -int nouveau_fence_new(struct nouveau_fence **); > +int nouveau_fence_create(struct nouveau_fence **, struct nouveau_channel *); > +int nouveau_fence_new(struct nouveau_fence **, struct nouveau_channel *); > void nouveau_fence_unref(struct nouveau_fence **); > > -int nouveau_fence_emit(struct nouveau_fence *, struct nouveau_channel *); > +int nouveau_fence_emit(struct nouveau_fence *); > bool nouveau_fence_done(struct nouveau_fence *); > int nouveau_fence_wait(struct nouveau_fence *, bool lazy, bool intr); > int nouveau_fence_sync(struct nouveau_bo *, struct nouveau_channel *, bool exclusive, bool intr); > diff --git a/drivers/gpu/drm/nouveau/nouveau_gem.c b/drivers/gpu/drm/nouveau/nouveau_gem.c > index a03fe3e92059..254524fee4fb 100644 > --- a/drivers/gpu/drm/nouveau/nouveau_gem.c > +++ b/drivers/gpu/drm/nouveau/nouveau_gem.c > @@ -914,11 +914,8 @@ nouveau_gem_ioctl_pushbuf(struct drm_device *dev, void *data, > } > } > > - ret = nouveau_fence_new(&fence); > - if (!ret) > - ret = nouveau_fence_emit(fence, chan); > + ret = nouveau_fence_new(&fence, chan); > if (ret) { > - nouveau_fence_unref(&fence); > NV_PRINTK(err, cli, "error fencing pushbuf: %d\n", ret); > WIND_RING(chan); > goto out; > -- > 2.41.0 >
Possibly Parallel Threads
- [RFC PATCH v1 07/16] drm/nouveau: rework to new fence interface
- [PATCH drm-misc-next v2] drm/nouveau: uapi: don't pass NO_PREFETCH flag implicitly
- [PATCH drm-misc-next] drm/nouveau: uapi: don't pass NO_PREFETCH flag implicitly
- [PATCH drm-misc-next] drm/nouveau: uapi: don't pass NO_PREFETCH flag implicitly
- [PATCH 1/2] gallium/nouveau: decouple nouveau_fence implementation from screen