Re: [PATCH] drm/i915: Before pageflip, also wait for shared dmabuf fences.

[Mario Kleiner <mario.kleiner.de@gmail.com>]

On 09/08/2016 08:30 AM, Chris Wilson wrote:
> On Thu, Sep 08, 2016 at 02:14:43AM +0200, Mario Kleiner wrote:
>> amdgpu-kms uses shared fences for its prime exported dmabufs,
>> instead of an exclusive fence. Therefore we need to wait for
>> all fences of the dmabuf reservation object to prevent
>> unsynchronized rendering and flipping.
>
> No. Fix the root cause as this affects not just flips but copies -
> this implies that everybody using the resv object must wait for all
> fences. The resv object is not just used for prime, but all fencing, so
> this breaks the ability to schedule parallel operations across engine.
> -Chris
>

Ok. I think i now understand the difference, but let's check: The 
exclusive fence is essentially acting a bit like a write-lock, and the 
shared fences as readers-locks? So you can have multiple readers but 
only one writer at a time?

Ie.:

Writer must wait for all fences before starting write access to a 
buffer, then attach the exclusive fence and signal it on end of write 
access. E.g., write to renderbuffer, write to texture etc.

Readers must wait for exclusive fence, then attach a shared fence per 
reader and signal it on end of read access? E.g., read from texture, fb, 
scanout?

Is that correct? In that case we'd have a missing exclusive fence in 
amdgpu for the linear target dmabuf? Probably beyond my level of 
knowledge to fix this?

thanks,
-mario
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