Re: [PATCH 2/2] dma-buf/fence: add fence_array fences v4

["Christian König" <deathsimple@vodafone.de>]

Am 23.05.2016 um 09:41 schrieb Daniel Vetter:
> On Fri, May 20, 2016 at 11:47:28AM -0300, Gustavo Padovan wrote:
>> 2016-05-20 Christian König <deathsimple@vodafone.de>:
>>
>>> From: Gustavo Padovan <gustavo.padovan@collabora.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.
> This will make it even harder to eventually add a real fence_context
> structure for tracking and debugging. I know you don't care for amdgpu
> since you have amdgpu-specific debug files, and there's some lifetime fun
> that makes it not immediately obvious how to resolve it.

Completely independent of my work on amdgpu I still think that it's not 
such a good idea to use a complex structure for the fence context.

Especially on SoCs and small embedded systems you probably don't want to 
overhead associated with that only for debugging purposes in a 
production environment.

> But on "lots of
> shitty little drivers" systems aka SoCs generic debugging information is
> crucial I think. Not liking too much where this is going.

Yeah I agree that generic debugging information is usually crucial, but 
the lifetime issues indeed can't be solved without reference counting 
and a hole bunch of overhead.

How about V5 of the patch I've just send out? Apart from fixing a few 
issues I've made the context and sequence number parameters of the 
fence_array object.

This way you don't need to always allocate a new context for each 
object, but just enough to keep your timelines straight.

E.g. you don't get a lot of contexts only used once. This is at least 
sufficient for my amdgpu use case.

Regards,
Christian.

> -Daniel