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

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

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@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.

> 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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