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

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Date:	Mon, 23 May 2016 09:41:49 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Gustavo Padovan <gustavo@...ovan.org>,
	Christian König <deathsimple@...afone.de>,
	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

On Fri, May 20, 2016 at 11:47:28AM -0300, Gustavo Padovan wrote:
> 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.

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. 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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch