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fix(cdc): stop arming OUT endpoint with a NULL buffer #3058

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fix(cdc): stop arming OUT endpoint with a NULL buffer #3058

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cd7b8c2
fix(cdc): stop arming OUT endpoint with a NULL buffer
dldntjr407 Aug 25, 2026
22f7dd4
chore(cdc): drop the unused USBD_CDC_ClearBuffer
dldntjr407 Aug 25, 2026
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2 changes: 0 additions & 2 deletions libraries/USBDevice/inc/usbd_cdc.h
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Original file line number Diff line number Diff line change
Expand Up @@ -148,12 +148,10 @@ uint8_t USBD_CDC_RegisterInterface(USBD_HandleTypeDef *pdev,
uint8_t USBD_CDC_SetTxBuffer(USBD_HandleTypeDef *pdev, uint8_t *pbuff,
uint32_t length, uint8_t ClassId);
uint8_t USBD_CDC_TransmitPacket(USBD_HandleTypeDef *pdev, uint8_t ClassId);
uint8_t USBD_CDC_ClearBuffer(USBD_HandleTypeDef *pdev, uint8_t ClassId);
#else
uint8_t USBD_CDC_SetTxBuffer(USBD_HandleTypeDef *pdev, uint8_t *pbuff,
uint32_t length);
uint8_t USBD_CDC_TransmitPacket(USBD_HandleTypeDef *pdev);
uint8_t USBD_CDC_ClearBuffer(USBD_HandleTypeDef *pdev);
#endif /* USE_USBD_COMPOSITE */
uint8_t USBD_CDC_SetRxBuffer(USBD_HandleTypeDef *pdev, uint8_t *pbuff);
uint8_t USBD_CDC_ReceivePacket(USBD_HandleTypeDef *pdev);
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18 changes: 0 additions & 18 deletions libraries/USBDevice/src/cdc/usbd_cdc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1019,24 +1019,6 @@ uint8_t USBD_CDC_ReceivePacket(USBD_HandleTypeDef *pdev)

return (uint8_t)USBD_OK;
}
#ifdef USE_USBD_COMPOSITE
uint8_t USBD_CDC_ClearBuffer(USBD_HandleTypeDef *pdev, uint8_t ClassId)
{
/* Suspend or Resume USB Out process */
if (pdev->pClassDataCmsit[classId] != NULL) {
#else
uint8_t USBD_CDC_ClearBuffer(USBD_HandleTypeDef *pdev)
{
/* Suspend or Resume USB Out process */
if (pdev->pClassDataCmsit[pdev->classId] != NULL) {
#endif /* USE_USBD_COMPOSITE */
/* Prepare Out endpoint to receive next packet */
USBD_LL_PrepareReceive(pdev, CDC_OUT_EP, 0, 0);
return (uint8_t)USBD_OK;
} else {
return (uint8_t)USBD_FAIL;
}
}

#endif /* USBD_USE_CDC */
#endif /* USBCON */
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13 changes: 9 additions & 4 deletions libraries/USBDevice/src/cdc/usbd_cdc_if.c
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Original file line number Diff line number Diff line change
Expand Up @@ -240,10 +240,15 @@ static int8_t USBD_CDC_Receive(uint8_t *Buf, uint32_t *Len)
/* It always contains required amount of free space for writing */
CDC_ReceiveQueue_CommitBlock(&ReceiveQueue, (uint16_t)(*Len));
receivePended = false;
/* If enough space in the queue for a full buffer then continue receive */
if (!CDC_resume_receive()) {
USBD_CDC_ClearBuffer(&hUSBD_Device_CDC);
}
/*
* If there is enough space in the queue for a full packet, continue receive.
* Else leave the OUT endpoint unarmed: it then NAKs and the host retries.
* USBSerial::read() and the readBytes() family call CDC_resume_receive()
* after dequeuing, so the endpoint is armed again with a valid block as soon
* as the sketch drains data. Arming it with a NULL buffer instead makes the
* low level driver copy the retried packet to address 0.
*/
(void)CDC_resume_receive();
return ((int8_t)USBD_OK);
}

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