Update dcd_stm32_fsdev_pvt_st.h

1 files changed, 7 insertions, 10 deletions

diff --git a/src/portable/st/stm32_fsdev/dcd_stm32_fsdev_pvt_st.h b/src/portable/st/stm32_fsdev/dcd_stm32_fsdev_pvt_st.h
index e0df21860..5a129ecf8 100644
--- a/src/portable/st/stm32_fsdev/dcd_stm32_fsdev_pvt_st.h
+++ b/src/portable/st/stm32_fsdev/dcd_stm32_fsdev_pvt_st.h
@@ -296,17 +296,14 @@ TU_ATTR_ALWAYS_INLINE static inline uint32_t pcd_get_ep_rx_cnt(USB_TypeDef * USB
 {
 #ifdef FSDEV_BUS_32BIT
   (void) USBx;
-  volatile uint32_t count = 10;
-  /*
-  WA: few cycles for RX PMA descriptor to update
-  This workaround is ported from stm32h5xx_hal_pcd.h : PCD_GET_EP_RX_CNT H5
-  This code fixes an issue when the code is compiled in GCC with a fast optimization(O2/O3) and device with an high frequency.
-  The function doesn't return the correct value.
-  Issue observed on Windows 10 and stm32h573i_dk and tud_task() scheduled by IT, the device is not migrated the USB device is not visible .
+  /* WA: few cycles for RX PMA descriptor to update, otherwise doesn't return the correct value.
+     Note: required for G0, U5, H5 etc. 
+  This workaround is ported from stm32h5xx_hal_pcd.h and fixes the issue when calling this function fast enough.
+  Reproduced with GCC ast optimization(O2/O3) and stm32h573i_dk with an high frequency. 
+  Observed on Windows 10 where tud_task() is scheduled by interrupt handler.
   */
-  while (count > 0U)
-  {
-    asm("NOP");
+  volatile uint32_t count = 10; // defined as PCD_RX_PMA_CNT in stm32 hal_driver
+  while (count > 0U) {
     count--;
   }
   return (pma32[2*bEpIdx + 1] & 0x03FF0000) >> 16;