drm/i915: Disable read only ppgtt support for gen11

On gen11 writing to read only ppgtt page causes a gpu hang.
This behaviour is different than with previous gen where
read only ppgtt access is supported. On those, the write
is just dropped without visible side effects.

Disable ro ppgtt support on gen11 until a solution can
be found to bring it into line with its predecessors.

References: HSDES#1807136187
References: https://bugzilla.freedesktop.org/show_bug.cgi?id=108569
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190411083034.28311-1-mika.kuoppala@linux.intel.com

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1228,7 +1228,7 @@ static int gen8_init_scratch(struct i915_address_space *vm)
 	vm->scratch_pte =
 		gen8_pte_encode(vm->scratch_page.daddr,
 				I915_CACHE_LLC,
-				PTE_READ_ONLY);
+				vm->has_read_only);
 
 	vm->scratch_pt = alloc_pt(vm);
 	if (IS_ERR(vm->scratch_pt)) {
@@ -1548,8 +1548,13 @@ static struct i915_hw_ppgtt *gen8_ppgtt_create(struct drm_i915_private *i915)
 
 	ppgtt_init(i915, ppgtt);
 
-	/* From bdw, there is support for read-only pages in the PPGTT. */
-	ppgtt->vm.has_read_only = true;
+	/*
+	 * From bdw, there is hw support for read-only pages in the PPGTT.
+	 *
+	 * Gen11 has HSDES#:1807136187 unresolved. Disable ro support
+	 * for now.
+	 */
+	ppgtt->vm.has_read_only = INTEL_GEN(i915) != 11;
 
 	/* There are only few exceptions for gen >=6. chv and bxt.
 	 * And we are not sure about the latter so play safe for now.