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

[Christian König]

Am 22.09.2016 um 08:36 schrieb Daniel Vetter:
> On Wed, Sep 21, 2016 at 06:23:35PM +0200, Christian König wrote:
>> For a quick workaround I suggest to just serialize all accesses to BO shared
>> with different drivers, but essentially I think it is a perfectly valid
>> requirement to have multiple writers to one BO.
> It is, but it's not possible with implicit sync. If you want parallel
> write access to the same shared buffer, you _must_ carry around some
> explicit fences. Within amdgpu you can use driver-specific cookies, for
> shared buffer we now have sync_file. But multiple writers with implicit
> sync simply fundamentally doesn't work. Because you have no idea with which
> writer, touching the same subrange you want to touch.

You don't need to separate the BO into subranges which are touched by 
different engines for allowing multiple writers.

AMD hardware and I'm pretty sure others as well are perfectly capable of 
writing to the same memory from multiple engines and even multiple GPUs 
at the same time.

For a good hint of what is possible see the public AMD ISA documentation 
about atomic operations, but that is only the start of it.


The crux here is that we need to assume that we will have implicit and 
explicit sync mixed for backward compatibility.

This implies that we need some mechanism like amdgpu uses in it's sync 
implementation where every fence is associated with an owner which 
denotes the domain in which implicit sync happens. If you leave this 
domain you will automatically run into explicit sync.

Currently we define the borders of this domain in amdgpu on process 
boundary to keep things like DRI2/DRI3 working as expected.

I really don't see how you want to solve this with a single explicit 
fence for each reservation object. As long as you have multiple 
concurrently running operations accessing the same buffer you need to 
keep one fence for each operation no matter what.

Regards,
Christian.

> -Daniel