drm/i915/userptr: Enable read-only support on gen8+

On gen8 and onwards, we can mark GPU accesses through the ppGTT as being
read-only, that is cause any GPU write onto that page to be discarded
(not triggering a fault). This is all that we need to finally support
the read-only flag for userptr!

v2: Check default address space for read only support as a proxy for the
user context/ppgtt.

Testcase: igt/gem_userptr_blits/readonly*
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712191430.9269-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_object.h

@@ -267,7 +267,6 @@ struct drm_i915_gem_object {
 	union {
 		struct i915_gem_userptr {
 			uintptr_t ptr;
-			unsigned read_only :1;
 
 			struct i915_mm_struct *mm;
 			struct i915_mmu_object *mmu_object;

drivers/gpu/drm/i915/i915_gem_userptr.c

@@ -507,7 +507,7 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 		struct mm_struct *mm = obj->userptr.mm->mm;
 		unsigned int flags = 0;
 
-		if (!obj->userptr.read_only)
+		if (!i915_gem_object_is_readonly(obj))
 			flags |= FOLL_WRITE;
 
 		ret = -EFAULT;
@@ -643,7 +643,7 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 		if (pvec) /* defer to worker if malloc fails */
 			pinned = __get_user_pages_fast(obj->userptr.ptr,
 						       num_pages,
-						       !obj->userptr.read_only,
+						       !i915_gem_object_is_readonly(obj),
 						       pvec);
 	}
 
@@ -789,10 +789,15 @@ i915_gem_userptr_ioctl(struct drm_device *dev,
 		return -EFAULT;
 
 	if (args->flags & I915_USERPTR_READ_ONLY) {
-		/* On almost all of the current hw, we cannot tell the GPU that a
-		 * page is readonly, so this is just a placeholder in the uAPI.
+		struct i915_hw_ppgtt *ppgtt;
+
+		/*
+		 * On almost all of the older hw, we cannot tell the GPU that
+		 * a page is readonly.
 		 */
-		return -ENODEV;
+		ppgtt = dev_priv->kernel_context->ppgtt;
+		if (!ppgtt || !ppgtt->vm.has_read_only)
+			return -ENODEV;
 	}
 
 	obj = i915_gem_object_alloc(dev_priv);
@@ -806,7 +811,8 @@ i915_gem_userptr_ioctl(struct drm_device *dev,
 	i915_gem_object_set_cache_coherency(obj, I915_CACHE_LLC);
 
 	obj->userptr.ptr = args->user_ptr;
-	obj->userptr.read_only = !!(args->flags & I915_USERPTR_READ_ONLY);
+	if (args->flags & I915_USERPTR_READ_ONLY)
+		i915_gem_object_set_readonly(obj);
 
 	/* And keep a pointer to the current->mm for resolving the user pages
 	 * at binding. This means that we need to hook into the mmu_notifier