[Intel-gfx] [PATCH 2/2] intel: Use I915_EXEC_NO_RELOC when available

[Chris Wilson]

On Sat, Jan 17, 2015 at 05:23:59AM +0100, Daniel Vetter wrote:
> On Fri, Jan 16, 2015 at 05:46:00PM -0800, Kristian Høgsberg wrote:
> > The I915_EXEC_NO_RELOC flag lets us tell the kernel that the offset we
> > provide in the validate list entry is what we've used in all relocations
> > to the bo in question.  If the bo hasn't moved, the kernel can skip
> > relocations completely.
> >
> > Signed-off-by: Kristian Høgsberg <krh at bitplanet.net>
> > ---
> >  intel/intel_bufmgr_gem.c | 17 ++++++++++++++++-
> >  1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/intel/intel_bufmgr_gem.c b/intel/intel_bufmgr_gem.c
> > index 8a51cea..a657a4d 100644
> > --- a/intel/intel_bufmgr_gem.c
> > +++ b/intel/intel_bufmgr_gem.c
> > @@ -131,6 +131,7 @@ typedef struct _drm_intel_bufmgr_gem {
> >   unsigned int no_exec : 1;
> >   unsigned int has_vebox : 1;
> >   unsigned int has_handle_lut : 1;
> > + unsigned int has_no_reloc : 1;
> >   bool fenced_relocs;
> >
> >   char *aub_filename;
> > @@ -504,7 +505,15 @@ drm_intel_add_validate_buffer2(drm_intel_bo *bo, int need_fence)
> >   bufmgr_gem->exec2_objects[index].relocation_count = bo_gem->reloc_count;
> >   bufmgr_gem->exec2_objects[index].relocs_ptr = (uintptr_t)bo_gem->relocs;
> >   bufmgr_gem->exec2_objects[index].alignment = 0;
> > - bufmgr_gem->exec2_objects[index].offset = 0;
> > +
> > + /* If the kernel supports I915_EXEC_NO_RELOC, it will compare
> > + * offset in struct drm_i915_gem_exec_object2 against the bos
> > + * current offset and if all bos haven't moved it will skip
> > + * relocation processing alltogether.  If I915_EXEC_NO_RELOC
> > + * is not supported, the kernel ignores the incoming value of
> > + * offset so we can set it either way.
> > + */
> > + bufmgr_gem->exec2_objects[index].offset = bo->offset64;
> >   bufmgr_gem->exec_bos[index] = bo;
> >   bufmgr_gem->exec2_objects[index].flags = 0;
> >   bufmgr_gem->exec2_objects[index].rsvd1 = 0;
> > @@ -2471,6 +2480,8 @@ do_exec2(drm_intel_bo *bo, int used, drm_intel_context *ctx,
> >
> >   if (bufmgr_gem->has_handle_lut)
> >   execbuf.flags |= I915_EXEC_HANDLE_LUT;
> > + if (bufmgr_gem->has_no_reloc)
> > + execbuf.flags |= I915_EXEC_NO_RELOC;
> 
> You need some opt-in flag to not break existing userspace: Iirc both UXA
> and libva retain reloc trees partially, which means that we might have
> different presumed offsets for the same bo in different relocs.

A bigger challenge is that you have to use execlist flags to indicate
read/write domains (actually just read or write!), and a special flag for
the SNB pipecontrol w/a. (This is because the kernel no longer even
scans the relocation trees if the buffers haven't moved and so we don't
have the chance to construct the read/write domains from the relocs.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre