drm/xe/display: fix ttm_bo_access() usage

ttm_bo_access() returns the size on success, account for that otherwise
the caller incorrectly thinks this is an error in
intel_atomic_prepare_plane_clear_colors().

v2 (Thomas)
 - Make sure we check for the partial copy case. Also since this api is
   easy to get wrong, wrap the whole thing in a new helper to hide the
   details and then convert the existing users over.

Fixes: b6308aa ("drm/xe/display: Update intel_bo_read_from_page to use ttm_bo_access")
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/3661
Signed-off-by: Matthew Auld <[email protected]>
Cc: Thomas Hellström <[email protected]>
Cc: Matthew Brost <[email protected]>
Cc: Jani Nikula <[email protected]>
Reviewed-by: Thomas Hellström <[email protected]>
Reviewed-by: Nirmoy Das <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: matt-auld

drivers/gpu/drm/xe/display/intel_bo.c

@@ -41,7 +41,7 @@ int intel_bo_read_from_page(struct drm_gem_object *obj, u64 offset, void *dst, i
 {
 	struct xe_bo *bo = gem_to_xe_bo(obj);
 
-	return ttm_bo_access(&bo->ttm, offset, dst, size, 0);
+	return xe_bo_read(bo, offset, dst, size);
 }
 
 struct intel_frontbuffer *intel_bo_get_frontbuffer(struct drm_gem_object *obj)

drivers/gpu/drm/xe/xe_bo.c

@@ -1320,6 +1320,30 @@ static int xe_bo_vm_access(struct vm_area_struct *vma, unsigned long addr,
 	return ret;
 }
 
+/**
+ * xe_bo_read() - Read from an xe_bo
+ * @bo: The buffer object to read from.
+ * @offset: The byte offset to start reading from.
+ * @dst: Location to store the read.
+ * @size: Size in bytes for the read.
+ *
+ * Read @size bytes from the @bo, starting from @offset, storing into @dst.
+ *
+ * Return: Zero on success, or negative error.
+ */
+int xe_bo_read(struct xe_bo *bo, u64 offset, void *dst, int size)
+{
+	int ret;
+
+	ret = ttm_bo_access(&bo->ttm, offset, dst, size, 0);
+	if (ret >= 0 && ret != size)
+		ret = -EIO;
+	else if (ret == size)
+		ret = 0;
+
+	return ret;
+}
+
 static const struct vm_operations_struct xe_gem_vm_ops = {
 	.fault = xe_gem_fault,
 	.open = ttm_bo_vm_open,

drivers/gpu/drm/xe/xe_bo.h

@@ -228,6 +228,7 @@ xe_bo_ggtt_addr(struct xe_bo *bo)
 
 int xe_bo_vmap(struct xe_bo *bo);
 void xe_bo_vunmap(struct xe_bo *bo);
+int xe_bo_read(struct xe_bo *bo, u64 offset, void *dst, int size);
 
 bool mem_type_is_vram(u32 mem_type);
 bool xe_bo_is_vram(struct xe_bo *bo);

drivers/gpu/drm/xe/xe_vm.c

@@ -3322,12 +3322,8 @@ void xe_vm_snapshot_capture_delayed(struct xe_vm_snapshot *snap)
 		}
 
 		if (bo) {
-			err = ttm_bo_access(&bo->ttm, snap->snap[i].bo_ofs,
-					    snap->snap[i].data, snap->snap[i].len, 0);
-			if (!(err < 0) && err != snap->snap[i].len)
-				err = -EIO;
-			else if (!(err < 0))
-				err = 0;
+			err = xe_bo_read(bo, snap->snap[i].bo_ofs,
+					 snap->snap[i].data, snap->snap[i].len);
 		} else {
 			void __user *userptr = (void __user *)(size_t)snap->snap[i].bo_ofs;