[Intel-gfx] [PATCH 2/2] dma-buf: fix dma-fence-chain out of order test

[Lionel Landwerlin]

On 25/06/2020 16:47, Chris Wilson wrote:
> Quoting Lionel Landwerlin (2020-06-25 14:23:25)
>> On 25/06/2020 16:18, Chris Wilson wrote:
>>> Quoting Lionel Landwerlin (2020-06-25 13:34:43)
>>>> There was probably a misunderstand on how the dma-fence-chain is
>>>> supposed to work or what dma_fence_chain_find_seqno() is supposed to
>>>> return.
>>>>
>>>> dma_fence_chain_find_seqno() is here to give us the fence to wait upon
>>>> for a particular point in the timeline. The timeline progresses only
>>>> when all the points prior to a given number have completed.
>>> Hmm, the question was what point is it supposed to wait for.
>>>
>>> For the simple chain of [1, 3], does 1 being signaled imply that all
>>> points up to 3 are signaled, or does 3 not being signaled imply that all
>>> points after 1 are not. If that's mentioned already somewhere, my bad.
>>> If not, could you put the answer somewhere.
>>> -Chris
>> In [1, 3], if 1 is signaled, the timeline value is 1. And find_seqno(2)
>> should return NULL.
>>
>>
>> In the out_of_order selftest the chain was [1, 2, 3], 2 was signaled and
>> the test was expecting no fence to be returned by find_seqno(2).
>>
>> But we still have to wait on 1 to complete before find_seqno(2) can
>> return NULL (as in you don't have to wait on anything).
> * scratches head
>
> I thought it was meant to be expecting fc.chain[1] to still be present
> as the chain at that point was not yet signaled.


You're right that the point is not yet signaled.

But it doesn't need to stay in the chain if you can wait on a previous 
point.


chain[1] gets removed as we walk the chain backward in dma_fence_chain_walk.


-Lionel


>
> Oh well, a mistake compounded. :|
> -Chris