[PATCH 1/3] dma-fence: Reserve 0 as a special NO_CONTEXT token

[Chris Wilson]

On Sun, Apr 09, 2017 at 09:08:53AM +0200, Christian König wrote:
> Am 08.04.2017 um 20:00 schrieb Chris Wilson:
> >On Sat, Apr 08, 2017 at 07:49:37PM +0200, Christian König wrote:
> >>Am 08.04.2017 um 18:26 schrieb Chris Wilson:
> >>>Reserve 0 for general use a token meaning that the fence doesn't belong
> >>>to an ordered timeline (fence context).
> >>NAK, we kept context allocation cheap to avoid exactly that.
> >However, they result in very sparse mappings.
> 
> Which is perfectly fine at least for how we use this in Radeon and Amdgpu.
> 
> The fence context is used as key for a hashtable, even when the
> context is only used once we want an evenly distribution over all of
> them.

The ht is a more expensive solution.

> >>Please elaborate further why it should be necessary now.
> >Because I want to efficiently exclude them from comparisons as
> >demonstrated by this small series as there may be several hundred such
> >fences as dependencies for this job.
> 
> That would horrible break Amdgpu, Radeon and the GPU scheduler
> because all of them assume that context numbers are unique.

Why would it break if it respected the trivial notion of an unordered
timeline?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre