Re: [PATCH 2/2] intel: Use I915_EXEC_NO_RELOC when available

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jan 19, 2015 at 09:58:55PM -0800, Kristian Høgsberg wrote:
> On Sat, Jan 17, 2015 at 1:49 AM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > 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@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.)
> 
> You're referring to having to set the EXEC_OBJECT_WRITE flag for
> buffers that are in any write domain?  That seems doable - we're
> already scanning all relocs before submitting.  I didn't find anything
> in i915_drm.h about the SNB pipecontrol... can you elaborate?

On gen6 if you do a pipe_control write (as e.g. used for query objects or
the nonzero flush wa) the kernel needs to apply a wa. For old w/a it does
this by matching on relocs with write_domain =
I915_GEM_DOMAIN_INSTRUCTION. With the no_reloc trick instead you need to
set the per-obj EXEC_OBJECT_NEEDS_GTT flag. Note that this is only
required on gen6, if you do it on gen7+ and have full ppgtt enabled the
kernel will yell at you.

If you go with recovering the no_reloc semantics in libdrm then you could
also recover this one (on top of checking presumed_offsets for all
relocs).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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