diff --git a/Documentation/building.md b/Documentation/building.md
index aabb45b18..81daeb069 100644
--- a/Documentation/building.md
+++ b/Documentation/building.md
@@ -48,6 +48,18 @@ ninja -C builddir install
 ninja -C builddir test
 ```
 
+### Run unit tests with code coverage
+#### gcov
+Due to a bug in lcov, the html file will only be correct if lcov is not
+installed and gcovr is installed. See
+https://github.com/linux-test-project/lcov/issues/168
+https://github.com/mesonbuild/meson/issues/6747
+```
+meson setup buildcov -Db_coverage=true
+ninja -C buildcov test
+ninja -C buildcov coverage
+```
+
 ## System specific information
 ### Ubuntu / Debian (Linux)
   * __linux-headers__ are version specific
diff --git a/tests/tests.c b/tests/tests.c
index 41972ba25..b11f64210 100644
--- a/tests/tests.c
+++ b/tests/tests.c
@@ -17,6 +17,7 @@
 #include "io_mock.h"
 #include "tests.h"
 #include "wraps.h"
+#include "io_real.h"
 
 #include <stdio.h>
 #include <string.h>
@@ -77,6 +78,7 @@ uint8_t __wrap_sio_read(uint16_t port, uint8_t reg)
 
 static int mock_open(const char *pathname, int flags, mode_t mode)
 {
+	maybe_unmock_io(pathname);
 	if (get_io() && get_io()->iom_open)
 		return get_io()->iom_open(get_io()->state, pathname, flags, mode);