linux-kernel - [PATCH 5.10 011/243] drm/i915/gtt: drop the page table optimisation

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Date:   Mon, 19 Jul 2021 16:50:40 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Matthew Auld <matthew.auld@...el.com>,
        Jon Bloomfield <jon.bloomfield@...el.com>,
        Chris Wilson <chris.p.wilson@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Subject: [PATCH 5.10 011/243] drm/i915/gtt: drop the page table optimisation

From: Matthew Auld <matthew.auld@...el.com>

commit 0abb33bfca0fb74df76aac03e90ce685016ef7be upstream.

We skip filling out the pt with scratch entries if the va range covers
the entire pt, since we later have to fill it with the PTEs for the
object pages anyway. However this might leave open a small window where
the PTEs don't point to anything valid for the HW to consume.

When for example using 2M GTT pages this fill_px() showed up as being
quite significant in perf measurements, and ends up being completely
wasted since we ignore the pt and just use the pde directly.

Anyway, currently we have our PTE construction split between alloc and
insert, which is probably slightly iffy nowadays, since the alloc
doesn't actually allocate anything anymore, instead it just sets up the
page directories and points the PTEs at the scratch page. Later when we
do the insert step we re-program the PTEs again. Better might be to
squash the alloc and insert into a single step, then bringing back this
optimisation(along with some others) should be possible.

Fixes: 14826673247e ("drm/i915: Only initialize partially filled pagetables")
Signed-off-by: Matthew Auld <matthew.auld@...el.com>
Cc: Jon Bloomfield <jon.bloomfield@...el.com>
Cc: Chris Wilson <chris.p.wilson@...el.com>
Cc: Daniel Vetter <daniel@...ll.ch>
Cc: <stable@...r.kernel.org> # v4.15+
Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210713130431.2392740-1-matthew.auld@intel.com
(cherry picked from commit 8f88ca76b3942d82e2c1cea8735ec368d89ecc15)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@...el.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/gpu/drm/i915/gt/gen8_ppgtt.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

--- a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
+++ b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
@@ -299,10 +299,7 @@ static void __gen8_ppgtt_alloc(struct i9
 			__i915_gem_object_pin_pages(pt->base);
 			i915_gem_object_make_unshrinkable(pt->base);

-			if (lvl ||
-			    gen8_pt_count(*start, end) < I915_PDES ||
-			    intel_vgpu_active(vm->i915))
-				fill_px(pt, vm->scratch[lvl]->encode);
+			fill_px(pt, vm->scratch[lvl]->encode);

 			spin_lock(&pd->lock);
 			if (likely(!pd->entry[idx])) {