Restore errata context comment on inline delay helper

Agent-Logs-Url: https://github.com/hathach/tinyusb/sessions/8588a09c-d4e5-4284-8944-47ae4e2a2a1f

Co-authored-by: HiFiPhile <4375114+HiFiPhile@users.noreply.github.com>

Author: Copilot

src/portable/st/stm32_fsdev/fsdev_common.h

@@ -467,8 +467,13 @@ uint16_t pma_align_buffer_size(uint16_t size, uint8_t *blsize, uint8_t *num_bloc
 // Set RX buffer size
 void btable_set_rx_bufsize(uint32_t ep_id, uint8_t buf_id, uint16_t wCount);
 
-// Delay helper for STM32 FSDEV PMA Buffer Description Table errata (ES0561/ES0587).
-// Low-speed path uses CFG_TUSB_FSDEV_BTABLE_LS_DELAY_COUNT, otherwise full-speed count.
+/* STM32 FSDEV PMA Buffer Description Table errata workaround:
+ * - ES0561 (STM32H503), ES0587 (STM32U535/U545)
+ * - CTR may trigger before final PMA SRAM accesses complete on OUT transfers.
+ * - Insert delay before reading PMA count/data.
+ *
+ * Low-speed path uses CFG_TUSB_FSDEV_BTABLE_LS_DELAY_COUNT, otherwise full-speed count.
+ */
 TU_ATTR_ALWAYS_INLINE static inline void fsdev_btable_workaround_delay(bool low_speed) {
 #if defined(TUP_USBIP_FSDEV_STM32) && defined(CFG_TUSB_FSDEV_32BIT)
   uint32_t cycle_count = low_speed ? CFG_TUSB_FSDEV_BTABLE_LS_DELAY_COUNT : CFG_TUSB_FSDEV_BTABLE_FS_DELAY_COUNT;