When DMAR / VT-d is enabled, the display engine uses overfetching, presumably to deal with the increased latency. To avoid display engine errors and DMAR faults, as a workaround the GGTT is populated with scatch PTEs when VT-d is enabled. However starting with gen10, Write-combined writing of scratch PTES is no longer possible and as a result, populating the full GGTT with scratch PTEs like on resume becomes very slow as uncached access is needed.
Therefore, replace filling the GGTT entirely with scratch pages with only filling surrounding scanout vma with guard pages. This eliminates the 100+ms delay in resume where we have to repopulate the GGTT with scratch.
While 100+ms might appear like a short time it's 10% to 20% of total resume time and important in some applications.
Additional considerations:
Since GPUs where VT-d might be present should have at least 2GiB worth of GGTT space, the extra guard pages should not really have a significant impact on GGTT contention.
Neither should there be a problem with not populating GGTT with scratch PTEs on unbind since that's typically not done when VT-d is not enabled either.
Finally, discrete GPUs should ideally not overfetch even with VT-d enabled, but removing the workaround for discrete GPUs needs thorough testing and will be done, if needed, as a follow up.
Patch 1 introduces accessors for vma.node.start and vma.node.size. While this patch is the most invasive, the end result is actually something we might want event without the introduced guard pages.
Patch 2 introduces the concept of guard pages to i915_vma and wraps the needed arithmetic in the accessors.
Patch 3 uses the guard pages to replace the old VT-d workaround.
v4: Completely rebase on drm-tip: - Avoid vmas in the binding backends - Make sure vma PIN flags don't clash - Add some kernedoc and rewrite cover-letter.
Chris Wilson (3): drm/i915: Wrap all access to i915_vma.node.start|size drm/i915: Introduce guard pages to i915_vma drm/i915: Refine VT-d scanout workaround
drivers/gpu/drm/i915/display/intel_fbdev.c | 2 +- drivers/gpu/drm/i915/gem/i915_gem_domain.c | 13 ++++ .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 33 ++++++----- drivers/gpu/drm/i915/gem/i915_gem_mman.c | 2 +- drivers/gpu/drm/i915/gem/i915_gem_shrinker.c | 2 +- drivers/gpu/drm/i915/gem/i915_gem_tiling.c | 4 +- .../gpu/drm/i915/gem/selftests/huge_pages.c | 2 +- .../i915/gem/selftests/i915_gem_client_blt.c | 15 ++--- .../drm/i915/gem/selftests/i915_gem_context.c | 15 +++-- .../drm/i915/gem/selftests/i915_gem_mman.c | 2 +- .../drm/i915/gem/selftests/igt_gem_utils.c | 7 ++- drivers/gpu/drm/i915/gt/gen7_renderclear.c | 2 +- drivers/gpu/drm/i915/gt/intel_ggtt.c | 39 ++++-------- drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c | 3 +- drivers/gpu/drm/i915/gt/intel_renderstate.c | 2 +- .../gpu/drm/i915/gt/intel_ring_submission.c | 2 +- drivers/gpu/drm/i915/gt/selftest_engine_cs.c | 8 +-- drivers/gpu/drm/i915/gt/selftest_execlists.c | 18 +++--- drivers/gpu/drm/i915/gt/selftest_hangcheck.c | 15 ++--- drivers/gpu/drm/i915/gt/selftest_lrc.c | 16 ++--- .../drm/i915/gt/selftest_ring_submission.c | 2 +- drivers/gpu/drm/i915/gt/selftest_rps.c | 12 ++-- .../gpu/drm/i915/gt/selftest_workarounds.c | 8 +-- drivers/gpu/drm/i915/i915_cmd_parser.c | 4 +- drivers/gpu/drm/i915/i915_debugfs.c | 2 +- drivers/gpu/drm/i915/i915_gem_gtt.h | 3 +- drivers/gpu/drm/i915/i915_perf.c | 2 +- drivers/gpu/drm/i915/i915_vma.c | 59 +++++++++++++------ drivers/gpu/drm/i915/i915_vma.h | 52 +++++++++++++++- drivers/gpu/drm/i915/i915_vma_resource.c | 4 +- drivers/gpu/drm/i915/i915_vma_resource.h | 17 ++++-- drivers/gpu/drm/i915/i915_vma_types.h | 3 +- drivers/gpu/drm/i915/selftests/i915_request.c | 20 +++---- drivers/gpu/drm/i915/selftests/igt_spinner.c | 8 +-- 34 files changed, 242 insertions(+), 156 deletions(-)