Am 20.12.24 um 15:51 schrieb Danilo Krummrich:
On Fri, Dec 20, 2024 at 03:11:34PM +0100, Philipp Stanner wrote:
On Fri, 2024-12-20 at 14:25 +0100, Christian König wrote:
Am 20.12.24 um 14:18 schrieb Danilo Krummrich:
On Fri, Dec 20, 2024 at 01:53:34PM +0100, Christian König wrote:
Am 20.12.24 um 13:45 schrieb Philipp Stanner:
diff --git a/drivers/gpu/drm/scheduler/sched_main.c
b/drivers/gpu/drm/scheduler/sched_main.c
index 7ce25281c74c..d6f8df39d848 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
+ *
+ * @sched_job: the job to run
+ *
+ * Returns: dma_fence the driver must signal once the
hardware has
+ * completed the job ("hardware fence").
+ *
+ * Note that the scheduler expects to 'inherit' its
own reference to
+ * this fence from the callback. It does not invoke an
extra
+ * dma_fence_get() on it. Consequently, this callback
must return a
+ * fence whose refcount is at least 2: One for the
scheduler's
+ * reference returned here, another one for the
reference kept by the
+ * driver.
Well the driver actually doesn't need any extra reference. The
scheduler
just needs to guarantee that this reference isn't dropped before
it is
signaled.
I think he means the reference the driver's fence context has to
have in order
to signal that thing eventually.
Yeah, but this is usually a weak reference. IIRC most drivers don't
increment the reference count for the reference they keep to signal a
fence.
It's expected that the consumers of the dma_fence keep the fence
alive
at least until it is signaled.
So are you saying that the driver having an extra reference (without
having obtained it with dma_fence_get()) is not an issue because the
driver is the one who will signal the fence [and then be done with it]?
It's never a "real" issue if you have multiple pointers to a reference counted
object as long as you can ensure that you hold at least one reference for the
time you have pointers to the object.
Well, I'm not saying that this isn't an issue. I'm just pointing out
that this is the current practice :)
But, that's bad design. For every pointer to an object a separate reference
should be taken.
Yeah, completely agree. Weak references are usually a bad idea if you
don't absolutely need them for something.
Regards,
Christian.
