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


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