diff --git a/_audioif.c b/_audioif.c
index cca98b2..f131c08 100644
--- a/_audioif.c
+++ b/_audioif.c
@@ -37,6 +37,7 @@
 #include "py/mperrno.h"
 #include "py/mphal.h"
 #include "py/obj.h"
+#include "py/objstr.h"
 #include "py/runtime.h"
 
 #include "audiocore/__init__.h"
@@ -1760,8 +1761,18 @@ static MP_DEFINE_CONST_FUN_OBJ_0(audiopump_lock_probe_obj,
 
 #endif
 
+
+// _audioif.__revision__: which audioif this firmware was built from. The build
+// passes AUDIOIF_REVISION (git describe) as a compile definition; a build with no
+// git says "unknown" rather than guessing.
+#ifndef AUDIOIF_REVISION
+#define AUDIOIF_REVISION "unknown"
+#endif
+static const MP_DEFINE_STR_OBJ(_audioif_revision_obj, AUDIOIF_REVISION);
+
 static const mp_rom_map_elem_t audioif_driver_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR__audioif) },
+    { MP_ROM_QSTR(MP_QSTR___revision__), MP_ROM_PTR(&_audioif_revision_obj) },
     // What this driver says it is. audiopump.driver() asks the ENGINE the same
     // question, through the binding; the two agreeing is the proof the
     // binding took.
diff --git a/micropython.cmake b/micropython.cmake
index 49676ad..dd267a7 100644
--- a/micropython.cmake
+++ b/micropython.cmake
@@ -56,3 +56,16 @@ target_include_directories(usermod_audiopump INTERFACE
 target_compile_definitions(usermod_audiopump INTERFACE AUDIOIF_DRIVER_ESP32=1)
 
 target_link_libraries(usermod INTERFACE usermod_audiopump)
+
+# --- which audioif this firmware was built from ----------------------
+# Computed at build time from this repo's own git, never stored; "unknown" when
+# there is no git (a tarball). Read on a target as <module>.__revision__.
+execute_process(
+    COMMAND git -C ${AUDIOPUMP_MOD_DIR} describe --always --dirty --abbrev=7
+    OUTPUT_VARIABLE AUDIOIF_REVISION
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_QUIET)
+if(NOT AUDIOIF_REVISION)
+    set(AUDIOIF_REVISION "unknown")
+endif()
+target_compile_definitions(usermod_audiopump INTERFACE AUDIOIF_REVISION=\"${AUDIOIF_REVISION}\")
diff --git a/micropython.mk b/micropython.mk
index c36230d..7d14d88 100644
--- a/micropython.mk
+++ b/micropython.mk
@@ -13,3 +13,9 @@ AUDIODSP_DIR ?= $(abspath $(AUDIOPUMP_MOD_DIR)/../audiodsp)
 CFLAGS_USERMOD += -I$(AUDIODSP_DIR)/src
 SRC_USERMOD_C += $(AUDIOPUMP_MOD_DIR)/_audioif.c
 SRC_USERMOD_C += $(AUDIOPUMP_MOD_DIR)/audiobusio.c
+
+# --- which audioif this firmware was built from ----------------------
+# Computed at build time from this repo's own git, never stored; "unknown" when
+# there is no git (a tarball). Read on a target as <module>.__revision__.
+AUDIOIF_REVISION := $(shell git -C $(AUDIOPUMP_MOD_DIR) describe --always --dirty --abbrev=7 2>/dev/null || echo unknown)
+CFLAGS_USERMOD += -DAUDIOIF_REVISION='"$(AUDIOIF_REVISION)"'