FAILED: patch "[PATCH] drm/i915/gtt: drop the page table optimisation" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 0abb33bfca0fb74df76aac03e90ce685016ef7be Mon Sep 17 00:00:00 2001
From: Matthew Auld <[email protected]>
Date: Tue, 13 Jul 2021 14:04:31 +0100
Subject: [PATCH] drm/i915/gtt: drop the page table optimisation

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.

index 21c8b7350b7a..da4f5eb43ac2 100644
@@ -303,10 +303,7 @@ static void __gen8_ppgtt_alloc(struct i915_address_space * const vm,
 			__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])) {