Re: [PATCH 7/7] drm/syncobj: use the timeline point in drm_syncobj_find_fence

[Chunming Zhou <zhoucm1@amd.com>]

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在 2018/11/24 2:16, Christian König 写道:
Am 23.11.18 um 14:42 schrieb Chunming Zhou:


But I've came up with something which should work. Assume the original
chain is:

1----3----6----9----12---18

And we garbage collect everything but 6 and 18 then all we need to know
to return the correct node is what the original previous sequence number
was.

6 (3)----18 (12)

When then somebody asks for 17 we can still return 18 and if somebody
asks for 9 we would return 6.

then what point we return when somebody asks for 11?

In this case 6. If 9 or 12 wouldn't be signaled yet than those.



Another use case, I'm not sure if you considered:
if chain is  1----3----6----9----12---18, a wait operation is on point
17, then we return 18, another signal point 17 comes, then we still wait
on 18(assume 18 takes very long time), that looks not reseonable, but
this is just performance problem potientially. Seems the way of timeline
sw_sync.c with comparing point for signal status can sovle it.


Well I thought that we declared that signaling lower numbers is illegal?

Sorry, I forgot it, quote from spec: "
*RESOLVED*: A 64-bit unsigned integer that can only be set to monotonically
increasing values by signal operations and is not changed by wait operations."

Can we think signaling lower numbers is forbidden?

If that's true, we can directly ignore lower number and return without error, keep the larger signal point.

I've considered this as well, but came to the conclusion that we then would lose some sync fence.

Starting a new sequence when userspace does that is the better alternative, cause then we again always sync to much but never to less.
But that will lead timeline not monotonically increasing, won't it? Seems it's not good.
Which could result in some problems, e.g. if timeline is already up to 18, then userA think he can wait on 18, another signal point17 comes, the new sequence is changed to 17, but userA still send its command by appending wait on point18, which would never be signaled if no more signal comes, right?

-David

Christian.


Thanks,
David

My current solution to that is when userspace messes up the sequence
numbers and submit 1-3-6-9-12-18-17 we start a new chain with 17 and
never look back.

E.g. when somebody then asks for anything below 17 we always return 17
and if somebody asks for 18 we return an error because that is handled
as not signaled yet.

Regards,
Christian.



-David



Thanks,
Christian.



Regards,
Christian.



You can also check sw_sync.c for timeline meaning.

-David


Christian.



-David


b. garbage collection happens on point6, chain would be updated to
1---3---9---12---18---20, if user wants to get point5, then we
should return node 3, but if user wants to get point 7, then we
should return node 9.


Why? That doesn't seem to make any sense to me.



I still have no idea how to satisfy all these requirements with your
current chain-fence. All these logic just are same we encountered
before, we're walking them again. After solving these problems, I
guess all design is similar as before.

In fact, I don't know what problem previous design has, maybe there
are some bugs, can't we fix these bugs by time going? Who can make
sure his implementation never have bugs?


Well there where numerous problems with the original design. For
example we need to reject the requirement that timeline fences are in
order because that doesn't make sense in the kernel.

When userspace does something like submitting fences in the order 1,
5, 3 then it is broken and can keep the pieces. In other words the
kernel should not care about that, but rather make sure that it never
looses any synchronization no matter what.

Regards,
Christian.



-David


+        }
         }
+
         drm_syncobj_put(syncobj);
         return ret;
     }


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