[Mesa-dev] [PATCH] i965: Share the workaround bo between all contexts

[Chris Wilson]

On Thu, Jan 26, 2017 at 09:39:51AM -0800, Chad Versace wrote:
> On Thu 26 Jan 2017, Chris Wilson wrote:
> > Since the workaround bo is used strictly as a write-only buffer, we need
> > only allocate one per screen and use the same one from all contexts.
> > 
> > (The caveat here is during extension initialisation, where we write into
> > and read back register values from the buffer, but that is performed only
> > once for the first context - and baring synchronisation issues should not
> > be a problem. Safer would be to move that also to the screen.)
> > 
> > v2: Give the workaround bo its own init function and don't piggy back
> > intel_bufmgr_init() since it is not that related.
> > 
> > v3: Drop the reference count of the workaround bo for the context since
> > the context itself is owned by the screen (and so we can rely on the bo
> > existing for the lifetime of the context).
> 
> I like this idea, but I have questions and comments about the details.
> More questions than comments, really.
> 
> Today, with only Mesa changes, could we effectively do the same as
>   drm_intel_gem_bo_disable_implicit_sync(screen->workaround_bo);
> by hacking Mesa to set no read/write domain when emitting relocs for the
> workaround_bo? (I admit I don't fully understand the kernel's domain
> tracking). If that does work, then it just would require a small hack to
> brw_emit_pipe_control_write().

However... There is a hack that requires the write hazard for gen6
pipecontrols unless you use the noreloc patches (hw limitation causing
pipecontrols to always use ggtt offsets not the ppgtt you have normally).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre