linux-kernel - Re: [PATCH 2/2] dma-buf/fence: add fence

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Date:	Fri, 20 May 2016 19:53:20 +0200
From:	Christian König <deathsimple@...afone.de>
To:	Gustavo Padovan <gustavo@...ovan.org>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	daniel.vetter@...ll.ch
Subject: Re: [PATCH 2/2] dma-buf/fence: add fence_array fences v4

Am 20.05.2016 um 16:47 schrieb Gustavo Padovan:
> 2016-05-20 Christian König <deathsimple@...afone.de>:
>
>> From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
>>
>> struct fence_collection inherits from struct fence and carries a
>> collection of fences that needs to be waited together.
>>
>> It is useful to translate a sync_file to a fence to remove the complexity
>> of dealing with sync_files on DRM drivers. So even if there are many
>> fences in the sync_file that needs to waited for a commit to happen,
>> they all get added to the fence_collection and passed for DRM use as
>> a standard struct fence.
>>
>> That means that no changes needed to any driver besides supporting fences.
>>
>> fence_collection's fence doesn't belong to any timeline context, so
>> fence_is_later() and fence_later() are not meant to be called with
>> fence_collections fences.
>>
>> v2: Comments by Daniel Vetter:
>> 	- merge fence_collection_init() and fence_collection_add()
>> 	- only add callbacks at ->enable_signalling()
>> 	- remove fence_collection_put()
>> 	- check for type on to_fence_collection()
>> 	- adjust fence_is_later() and fence_later() to WARN_ON() if they
>> 	are used with collection fences.
>>
>> v3: - Initialize fence_cb.node at fence init.
>>
>>      Comments by Chris Wilson:
>> 	- return "unbound" on fence_collection_get_timeline_name()
>> 	- don't stop adding callbacks if one fails
>> 	- remove redundant !! on fence_collection_enable_signaling()
>> 	- remove redundant () on fence_collection_signaled
>> 	- use fence_default_wait() instead
>>
>> v4 (chk): Rework, simplification and cleanup:
>> 	- Drop FENCE_NO_CONTEXT handling, always allocate a context.
>> 	- Rename to fence_array.
>> 	- Return fixed driver name.
>> 	- Register only one callback at a time.
>> 	- Document that create function takes ownership of array.
> This looks good to me. Dropping NO_CONTEXT was a good idea, also
> registering only one callback makes it looks better.

Thinking about it a bit more I think we need to avoid removing the 
callback when the fence is released as well.

That stuff is just a bit to racy (see the comment on the 
fence_remove_callback function as well).

I will just grab a reference to the fence while there is any callback 
registered.

Also please note that this is only compile tested at the moment. I'm 
still working on integrating it into my code.

Regards,
Christian.

>
> 	Gustavo