diff --git a/stmhal/irq.h b/stmhal/irq.h
index 3c0f13d0a7..1771d3675f 100644
--- a/stmhal/irq.h
+++ b/stmhal/irq.h
@@ -69,6 +69,7 @@ MP_DECLARE_CONST_FUN_OBJ(pyb_enable_irq_obj);
 #define IRQ_PRI_FLASH           1
 #define IRQ_SUBPRI_FLASH                    1
 
+// SDIO must be higher priority than DMA for SDIO DMA transfers to work.
 #define IRQ_PRI_SDIO            4
 #define IRQ_SUBPRI_SDIO                     0