drm/i915/gt: Skip global serialisation of clear_range for bxt vtd

VT'd on Broxton and on Braswell require serialisation of GGTT updates.
However, it seems to only be required for insertion, so drop the
complication and heavyweight stop_machine() for clears. The range will
be serialised again before use.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200130092239.1743672-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_ggtt.c

@@ -350,31 +350,6 @@ static void bxt_vtd_ggtt_insert_entries__BKL(struct i915_address_space *vm,
 	stop_machine(bxt_vtd_ggtt_insert_entries__cb, &arg, NULL);
 }
 
-struct clear_range {
-	struct i915_address_space *vm;
-	u64 start;
-	u64 length;
-};
-
-static int bxt_vtd_ggtt_clear_range__cb(void *_arg)
-{
-	struct clear_range *arg = _arg;
-
-	gen8_ggtt_clear_range(arg->vm, arg->start, arg->length);
-	bxt_vtd_ggtt_wa(arg->vm);
-
-	return 0;
-}
-
-static void bxt_vtd_ggtt_clear_range__BKL(struct i915_address_space *vm,
-					  u64 start,
-					  u64 length)
-{
-	struct clear_range arg = { vm, start, length };
-
-	stop_machine(bxt_vtd_ggtt_clear_range__cb, &arg, NULL);
-}
-
 static void gen6_ggtt_clear_range(struct i915_address_space *vm,
 				  u64 start, u64 length)
 {
@@ -881,8 +856,6 @@ static int gen8_gmch_probe(struct i915_ggtt *ggtt)
 	    IS_CHERRYVIEW(i915) /* fails with concurrent use/update */) {
 		ggtt->vm.insert_entries = bxt_vtd_ggtt_insert_entries__BKL;
 		ggtt->vm.insert_page    = bxt_vtd_ggtt_insert_page__BKL;
-		if (ggtt->vm.clear_range != nop_clear_range)
-			ggtt->vm.clear_range = bxt_vtd_ggtt_clear_range__BKL;
 	}
 
 	ggtt->invalidate = gen8_ggtt_invalidate;