MicroPython port: stm32 builds with single-precision floats link

ports/micropython/micropython.mk

@@ -53,6 +53,14 @@ endif
 
 SOURCES += $(LVMP_DIR)/src/lv_mem_core_micropython.c
 
+# An stm32 firmware with single-precision floats links MicroPython's
+# single-precision libm, which has no double sqrt() for LVGL's gesture code.
+ifneq ($(findstring /ports/stm32,$(LVMP_PORT_DIR)),)
+ifeq ($(MICROPY_FLOAT_IMPL),single)
+SOURCES += $(LVMP_DIR)/src/lv_sqrt_single.c
+endif
+endif
+
 $(if $(wildcard $(LVMP_C)),,$(error $(LVMP_C) not found. Run $(BINDINGS_DIR)/regenerate_all.sh --target micropython))
 
 # -Wno-unused-function here is not enough on ports that append -Werror after py.mk

ports/micropython/src/lv_sqrt_single.c

@@ -0,0 +1,20 @@
+// SPDX-FileCopyrightText: 2026 Brad Barnett
+//
+// SPDX-License-Identifier: MIT
+
+// LVGL's gesture recogniser (src/indev/lv_indev_gesture.c) calls the double
+// sqrt(). An stm32 firmware built with single-precision floats links only
+// MicroPython's single-precision libm, which has sqrtf() and no sqrt(), so the
+// link fails. Gesture distances are pixels: single precision is plenty.
+
+#include "py/mpconfig.h"
+
+#if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
+
+float sqrtf(float x);
+
+double sqrt(double x) {
+    return sqrtf((float)x);
+}
+
+#endif

ports/micropython/tests/test_micropython_port.py

@@ -32,3 +32,11 @@ def test_fs_driver_is_frozen_from_the_one_copy():
 
 def test_no_consumer_smoke_wrapper_remains():
     assert not (ROOT / "tools" / "test_lvgl_unix.py").exists()
+
+
+def test_stm32_single_float_builds_get_a_double_sqrt():
+    make = (ROOT / "micropython.mk").read_text()
+    assert "src/lv_sqrt_single.c" in make
+    assert "/ports/stm32" in make
+    shim = (ROOT / "src" / "lv_sqrt_single.c").read_text()
+    assert "MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT" in shim