From 5a7cee9f422afca50d67ef707c292ff7db80428f Mon Sep 17 00:00:00 2001
From: 94xhn <87560781+94xhn@users.noreply.github.com>
Date: Sat, 11 Jul 2026 05:30:32 +0800
Subject: [PATCH] fix(UART_ReceptionToIdle_CircularDMA): wrap old_pos to fix
 silent data loss

HAL_UARTEx_RxEventCallback() tracks the last-processed position in the
circular DMA Rx buffer via old_pos, and compares it against Size to
detect whether new data has arrived (`if (Size != old_pos)`).

On a Transfer Complete event (a full circular buffer), the HAL reports
Size == huart->RxXferSize == RX_BUFFER_SIZE, not a wrapped 0 (confirmed
in stm32f4xx_hal_uart.c's UART_DMAReceiveCplt(), which calls
HAL_UARTEx_RxEventCallback(huart, huart->RxXferSize) for
HAL_UART_RECEPTION_TOIDLE mode). Since old_pos was previously set to
`old_pos = Size;` unwrapped, after one full-buffer cycle old_pos is
left at RX_BUFFER_SIZE. On every subsequent full-buffer Transfer
Complete event, Size == old_pos == RX_BUFFER_SIZE, so the `Size !=
old_pos` check spuriously evaluates false and the entire buffer's
worth of newly received data is silently dropped without calling
UserDataTreatment().

Wrap old_pos with `% RX_BUFFER_SIZE` so it stays a valid index into
aRXBufferUser, matching the actual wrapped position.

Fixes #196

Signed-off-by: 94xhn <87560781+94xhn@users.noreply.github.com>
---
 .../UART/UART_ReceptionToIdle_CircularDMA/Src/main.c       | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/Projects/STM32446E-Nucleo/Examples/UART/UART_ReceptionToIdle_CircularDMA/Src/main.c b/Projects/STM32446E-Nucleo/Examples/UART/UART_ReceptionToIdle_CircularDMA/Src/main.c
index 3cb27163e2..3092b9d34d 100644
--- a/Projects/STM32446E-Nucleo/Examples/UART/UART_ReceptionToIdle_CircularDMA/Src/main.c
+++ b/Projects/STM32446E-Nucleo/Examples/UART/UART_ReceptionToIdle_CircularDMA/Src/main.c
@@ -392,8 +392,11 @@ void HAL_UARTEx_RxEventCallback(UART_HandleTypeDef *huart, uint16_t Size)
     pBufferReadyForReception = ptemp;
   }
   /* Update old_pos as new reference of position in User Rx buffer that
-     indicates position to which data have been processed */
-  old_pos = Size;
+     indicates position to which data have been processed.
+     Size can be equal to RX_BUFFER_SIZE (not wrapped to 0) when the
+     Transfer Complete event reports a full circular buffer, so it must be
+     wrapped here to stay a valid index into aRXBufferUser. */
+  old_pos = Size % RX_BUFFER_SIZE;
 
 }