[Intel-gfx] [Linaro-mm-sig] [PATCH 1/2] dma-buf.rst: Document why indefinite fences are a bad idea

[Thomas Hellström (Intel)]

On 7/21/20 9:45 AM, Christian König wrote:
> Am 21.07.20 um 09:41 schrieb Daniel Vetter:
>> On Mon, Jul 20, 2020 at 01:15:17PM +0200, Thomas Hellström (Intel) 
>> wrote:
>>> Hi,
>>>
>>> On 7/9/20 2:33 PM, Daniel Vetter wrote:
>>>> Comes up every few years, gets somewhat tedious to discuss, let's
>>>> write this down once and for all.
>>>>
>>>> What I'm not sure about is whether the text should be more explicit in
>>>> flat out mandating the amdkfd eviction fences for long running compute
>>>> workloads or workloads where userspace fencing is allowed.
>>> Although (in my humble opinion) it might be possible to completely 
>>> untangle
>>> kernel-introduced fences for resource management and dma-fences used 
>>> for
>>> completion- and dependency tracking and lift a lot of restrictions 
>>> for the
>>> dma-fences, including prohibiting infinite ones, I think this makes 
>>> sense
>>> describing the current state.
>> Yeah I think a future patch needs to type up how we want to make that
>> happen (for some cross driver consistency) and what needs to be
>> considered. Some of the necessary parts are already there (with like the
>> preemption fences amdkfd has as an example), but I think some clear docs
>> on what's required from both hw, drivers and userspace would be really
>> good.
>
> I'm currently writing that up, but probably still need a few days for 
> this.

Great! I put down some (very) initial thoughts a couple of weeks ago 
building on eviction fences for various hardware complexity levels here:

https://gitlab.freedesktop.org/thomash/docs/-/blob/master/Untangling%20dma-fence%20and%20memory%20allocation.odt

/Thomas