usb: dwc3: ep0: add dwc3_ep0_prepare_one_trb()

commit 7931ec86 upstream.

For now this is just a cleanup patch, no functional
changes. We will be using the new function to fix a
bug introduced long ago by commit 0416e494
("usb: dwc3: ep0: correct cache sync issue in case
of ep0_bounced") and further worsened by commit
c0bd5456 ("usb: dwc3: ep0: handle non maxpacket
aligned transfers > 512")
Reported-by: Janusz Dziedzic <januszx.dziedzic@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
[bwh: Backported to 3.16:
 - dwc3_ep0_start_trans() doesn't have a 'chain' parameter, so don't
   give dwc3_ep0_prepare_one_trb() this parameter
 - Also delete a debug log statement, removed earlier upstream
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/dwc3/ep0.c

@@ -54,20 +54,13 @@ static const char *dwc3_ep0_state_string(enum dwc3_ep0_state state)
 	}
 }
 
-static int dwc3_ep0_start_trans(struct dwc3 *dwc, u8 epnum, dma_addr_t buf_dma,
-		u32 len, u32 type)
+static void dwc3_ep0_prepare_one_trb(struct dwc3 *dwc, u8 epnum,
+		dma_addr_t buf_dma, u32 len, u32 type)
 {
-	struct dwc3_gadget_ep_cmd_params params;
 	struct dwc3_trb			*trb;
 	struct dwc3_ep			*dep;
 
-	int				ret;
-
 	dep = dwc->eps[epnum];
-	if (dep->flags & DWC3_EP_BUSY) {
-		dev_vdbg(dwc->dev, "%s: still busy\n", dep->name);
-		return 0;
-	}
 
 	trb = dwc->ep0_trb;
 
@@ -80,6 +73,20 @@ static int dwc3_ep0_start_trans(struct dwc3 *dwc, u8 epnum, dma_addr_t buf_dma,
 			| DWC3_TRB_CTRL_LST
 			| DWC3_TRB_CTRL_IOC
 			| DWC3_TRB_CTRL_ISP_IMI);
+}
+
+static int dwc3_ep0_start_trans(struct dwc3 *dwc, u8 epnum, dma_addr_t buf_dma,
+		u32 len, u32 type)
+{
+	struct dwc3_gadget_ep_cmd_params params;
+	struct dwc3_ep			*dep;
+	int				ret;
+
+	dep = dwc->eps[epnum];
+	if (dep->flags & DWC3_EP_BUSY)
+		return 0;
+
+	dwc3_ep0_prepare_one_trb(dwc, epnum, buf_dma, len, type);
 
 	memset(&params, 0, sizeof(params));
 	params.param0 = upper_32_bits(dwc->ep0_trb_addr);