Am 13.12.18 um 17:01 schrieb Daniel Vetter:
> On Thu, Dec 13, 2018 at 12:24:57PM +0000, Koenig, Christian wrote:
>> Am 13.12.18 um 13:21 schrieb Chris Wilson:
>>> Quoting Koenig, Christian (2018-12-13 12:11:10)
>>>> Am 13.12.18 um 12:37 schrieb Chris Wilson:
>>>>> Quoting Chunming Zhou (2018-12-11 10:34:45)
>>>>>> From: Christian König <ckoenig.leichtzumerken@xxxxxxxxx>
>>>>>>
>>>>>> Implement finding the right timeline point in drm_syncobj_find_fence.
>>>>>>
>>>>>> v2: return -EINVAL when the point is not submitted yet.
>>>>>> v3: fix reference counting bug, add flags handling as well
>>>>>>
>>>>>> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
>>>>>> ---
>>>>>> drivers/gpu/drm/drm_syncobj.c | 43 ++++++++++++++++++++++++++++++++---
>>>>>> 1 file changed, 40 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
>>>>>> index 76ce13dafc4d..d964b348ecba 100644
>>>>>> --- a/drivers/gpu/drm/drm_syncobj.c
>>>>>> +++ b/drivers/gpu/drm/drm_syncobj.c
>>>>>> @@ -231,16 +231,53 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
>>>>>> struct dma_fence **fence)
>>>>>> {
>>>>>> struct drm_syncobj *syncobj = drm_syncobj_find(file_private, handle);
>>>>>> - int ret = 0;
>>>>>> + struct syncobj_wait_entry wait;
>>>>>> + int ret;
>>>>>>
>>>>>> if (!syncobj)
>>>>>> return -ENOENT;
>>>>>>
>>>>>> *fence = drm_syncobj_fence_get(syncobj);
>>>>>> - if (!*fence) {
>>>>>> + drm_syncobj_put(syncobj);
>>>>>> +
>>>>>> + if (*fence) {
>>>>>> + ret = dma_fence_chain_find_seqno(fence, point);
>>>>>> + if (!ret)
>>>>>> + return 0;
>>>>>> + dma_fence_put(*fence);
>>>>>> + } else {
>>>>>> ret = -EINVAL;
>>>>>> }
>>>>>> - drm_syncobj_put(syncobj);
>>>>>> +
>>>>>> + if (!(flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT))
>>>>>> + return ret;
>>>>>> +
>>>>>> + memset(&wait, 0, sizeof(wait));
>>>>>> + wait.task = current;
>>>>>> + wait.point = point;
>>>>>> + drm_syncobj_fence_add_wait(syncobj, &wait);
>>>>>> +
>>>>>> + do {
>>>>>> + set_current_state(TASK_INTERRUPTIBLE);
>>>>>> + if (wait.fence) {
>>>>>> + ret = 0;
>>>>>> + break;
>>>>>> + }
>>>>>> +
>>>>>> + if (signal_pending(current)) {
>>>>>> + ret = -ERESTARTSYS;
>>>>>> + break;
>>>>>> + }
>>>>>> +
>>>>>> + schedule();
>>>>>> + } while (1);
>>>>> I've previously used a dma_fence_proxy so that we could do nonblocking
>>>>> waits on future submits. That would be preferrable (a requirement for
>>>>> our stupid BKL-driven code).
>>>> That is exactly what I would definitely NAK.
>>>>
>>>> I would rather say we should come up with a wait_multiple_events() macro
>>>> and completely nuke the custom implementation of this in:
>>>> 1. dma_fence_default_wait and dma_fence_wait_any_timeout
>>>> 2. the radeon fence implementation
>>>> 3. the nouveau fence implementation
>>>> 4. the syncobj code
>>>>
>>>> Cause all of them do exactly the same. The dma_fence implementation
>>>> unfortunately came up with a custom event handling mechanism instead of
>>>> extending the core Linux wait_event() system.
>>> I don't want a blocking wait at all.
>> Ok I wasn't clear enough :) That is exactly what I would NAK!
>>
>> The wait must be blocking or otherwise you would allow wait-before-signal.
> Well the current implementation is pulling a rather big trick on readers
> in this regard: It looks like a dma_fence, it's implemented as one even,
> heck you even open-code a dma_fence_wait here.
>
> Except the semantics are completely different.
>
> So aside from the discussion whether we really want to fully chain them I
> think it just doesn't make sense to implement the "wait for fence submit"
> as a dma_fence wait. And I'd outright remove that part from the uapi, and
> force the wait. The current radv/anv plans I discussed with Jason was that
> we'd have a separate submit thread, and hence unconditionally blocking
> until the fence has materialized is the right thing to do. Even allowing
> that option, either through a flag, or making these things look like
> dma_fences (they are _not_) just tricks folks into misunderstanding what's
> going on.
Good, that sounds strongly like something I can agree on as well.
> Code sharing just because the code looks similar is imo a really
> bad idea, when the semantics are entirely different (that was also the
> reason behind not reusing all the cpu event stuff for dma_fence, they're
> not normal cpu events).
Ok, the last sentence is what I don't understand.
What exactly is the semantic difference between the dma_fence_wait and
the wait_event interface?
I mean the wait_event interface was introduced to prevent drivers from
openly coding an event interface and getting it wrong all the time.
So a good part of the bugs we have seen around waiting for dma-fences
are exactly why wait_event was invented in the first place.
The only big thing I can see missing in the wait_event interface is
waiting for many events at the same time, but that should be a rather
easy addition.
Regards,
Christian.
> -Daniel
>
>> Christian.
>>
>>> -Chris
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