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

[Christian König]

Am 23.11.18 um 11:56 schrieb zhoucm1:
>
>
> On 2018年11月23日 18:10, Koenig, Christian wrote:
>> Am 23.11.18 um 03:36 schrieb zhoucm1:
>>>
>>> On 2018年11月22日 19:30, Christian König wrote:
>>>> Am 22.11.18 um 07:52 schrieb zhoucm1:
>>>>>
>>>>> On 2018年11月15日 19:12, Christian König wrote:
>>>>>> Implement finding the right timeline point in 
>>>>>> drm_syncobj_find_fence.
>>>>>>
>>>>>> Signed-off-by: Christian König <christian.koenig at amd.com>
>>>>>> ---
>>>>>>    drivers/gpu/drm/drm_syncobj.c | 10 +++++++++-
>>>>>>    1 file changed, 9 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/drm_syncobj.c
>>>>>> b/drivers/gpu/drm/drm_syncobj.c
>>>>>> index 589d884ccd58..d42c51520da4 100644
>>>>>> --- a/drivers/gpu/drm/drm_syncobj.c
>>>>>> +++ b/drivers/gpu/drm/drm_syncobj.c
>>>>>> @@ -307,9 +307,17 @@ int drm_syncobj_find_fence(struct drm_file
>>>>>> *file_private,
>>>>>>            return -ENOENT;
>>>>>>          *fence = drm_syncobj_fence_get(syncobj);
>>>>>> -    if (!*fence) {
>>>>>> +    if (!*fence)
>>>>>>            ret = -EINVAL;
>>>>>> +
>>>>>> +    if (!ret && point) {
>>>>>> +        dma_fence_chain_for_each(*fence) {
>>>>>> +            if (!to_dma_fence_chain(*fence) ||
>>>>>> +                (*fence)->seqno <= point)
>>>>>> +                break;
>>>>> This condition isn't enough to find proper point.
>>>>> For two examples:
>>>>> a. No garbage collection happens, the points in chain are
>>>>> 1----3----6----9----12---18---20, if user wants to get point17, then
>>>>> we should return node 18.
>>>> And that is exactly what's wrong in the original logic. In this case
>>>> we need to return 12, not 18 because point 17 could have already been
>>>> garbage collected.
>>> I don't think so, the 'a' case I already assume there isn't garbage
>>> collection. If user wants to get point17, then we should return node 
>>> 18.
>>> timeline means point[N]  must be signaled later than point[N-1].
>>> Point[12] just can make sure point[1] ~point[12] are signaled.
>>> Point[18] signal can make sure point[17] is signaled.
>>> So this case we need to return 18, not 12, which is key timeline 
>>> concept.
>> No, exactly that's incorrect. When we ask for 17 and can't find it then
>> this means it either never existed or that it is signaled already.
>>
>> Returning a lower number in this case or even a stub fence is perfectly
>> fine since we only need to wait for that one in this case.
>>
>> If we return 18 in this case then we add incorrect synchronization when
>> there shouldn't be any.
> No, That will make timeline not work at all and break timeline 
> semantics totally.
>
> If there aren't point18 and point20, the chain is 
> 1----3----6----9----12, if user wants to get point 17, you also return 
> 12? if yes, which absolutely is incorrect. The answer should be NO, 
> right? point17 should be waited on there until a bigger point is coming.

Correct, but this is a different case. In this situation we either 
return an error or wait for point 17 (or something >=17) to show up.

The key difference is if point 17 shows up then we return point 17, but 
if point 18 shows up then we need to return point 12.

> For chain is 1----3----6----9----12---18---20, if user wants to wait 
> on any one of points 13,14,15,16,17,18, we must wait for point 18, 
> this is timeline semantic.

Ah, now I understand. You are still sticking with the assumption of a 
fence number, right?

In other words what you imply here is that we have the same semantic as 
when somebody waits for a memory location to be written by number 17, 
right? In this case the semantics you describe here indeed applies.

But that is certainly not what we want to implement or otherwise we will 
never be able to garbage collect the numbers in between.

So if Vulkan has this requirement then we need to reject that.

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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