usb: gadget: dwc2: fix zlp handling

The patch 10209abe
usb: dwc2: gadget: Add scatter-gather mode

avoided a NULL pointer dereference (hs_ep->req == NULL) by
calling dwc2_gadget_fill_nonisoc_xfer_dma_one() directly instead of through
the dwc2_gadget_config_nonisoc_xfer_ddma() wrapper, which unconditionally
dereferenced the said pointer.

However, this was based on an incorrect assumption that in the context of
dwc2_hsotg_program_zlp() the pointer is always NULL, which is not the case.
The result were SB CV MSC tests failing starting from Test Case 6.

Instead, this patch reverts to calling the wrapper and adds a check for
the pointer being NULL inside the wrapper.

Fixes: 10209abe (usb: dwc2: gadget: Add scatter-gather mode)
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc2/gadget.c

@@ -835,19 +835,22 @@ static void dwc2_gadget_fill_nonisoc_xfer_ddma_one(struct dwc2_hsotg_ep *hs_ep,
  * with corresponding information based on transfer data.
  */
 static void dwc2_gadget_config_nonisoc_xfer_ddma(struct dwc2_hsotg_ep *hs_ep,
-						 struct usb_request *ureq,
-						 unsigned int offset,
+						 dma_addr_t dma_buff,
 						 unsigned int len)
 {
+	struct usb_request *ureq = NULL;
 	struct dwc2_dma_desc *desc = hs_ep->desc_list;
 	struct scatterlist *sg;
 	int i;
 	u8 desc_count = 0;
 
+	if (hs_ep->req)
+		ureq = &hs_ep->req->req;
+
 	/* non-DMA sg buffer */
-	if (!ureq->num_sgs) {
+	if (!ureq || !ureq->num_sgs) {
 		dwc2_gadget_fill_nonisoc_xfer_ddma_one(hs_ep, &desc,
-			ureq->dma + offset, len, true);
+			dma_buff, len, true);
 		return;
 	}
 
@@ -1135,7 +1138,7 @@ static void dwc2_hsotg_start_req(struct dwc2_hsotg *hsotg,
 			offset = ureq->actual;
 
 		/* Fill DDMA chain entries */
-		dwc2_gadget_config_nonisoc_xfer_ddma(hs_ep, ureq, offset,
+		dwc2_gadget_config_nonisoc_xfer_ddma(hs_ep, ureq->dma + offset,
 						     length);
 
 		/* write descriptor chain address to control register */
@@ -2037,12 +2040,13 @@ static void dwc2_hsotg_program_zlp(struct dwc2_hsotg *hsotg,
 		dev_dbg(hsotg->dev, "Receiving zero-length packet on ep%d\n",
 			index);
 	if (using_desc_dma(hsotg)) {
+		/* Not specific buffer needed for ep0 ZLP */
+		dma_addr_t dma = hs_ep->desc_list_dma;
+
 		if (!index)
 			dwc2_gadget_set_ep0_desc_chain(hsotg, hs_ep);
 
-		/* Not specific buffer needed for ep0 ZLP */
-		dwc2_gadget_fill_nonisoc_xfer_ddma_one(hs_ep, &hs_ep->desc_list,
-			hs_ep->desc_list_dma, 0, true);
+		dwc2_gadget_config_nonisoc_xfer_ddma(hs_ep, dma, 0);
 	} else {
 		dwc2_writel(hsotg, DXEPTSIZ_MC(1) | DXEPTSIZ_PKTCNT(1) |
 			    DXEPTSIZ_XFERSIZE(0),