libpayload: Make USB transfer functions return amount of bytes

The USB bulk and control transfer functions in libpayload currently
always return 0 for success and 1 for all errors. This is sufficient for
current use cases (essentially just mass storage), but other classes
(like certain Ethernet adapters) need to be able to tell if a transfer
reached the intended amount of bytes, or if it fell short.

This patch slightly changes that USB API to return -1 on errors, and the
amount of transferred bytes on successes. All drivers in the current
libpayload mainline are modified to conform to the new error detection
model. Any third party users of this API will need to adapt their
if (...<controller>->bulk/control(...)) checks to
if (...<controller>->bulk/control(...) < 0) as well.

The host controller drivers for OHCI and EHCI correctly implement the
new behavior. UHCI and the XHCI stub just comply with the new API by
returning 0 or -1, but do not actually count the returned bytes.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48308
Reviewed-by: Gabe Black <gabeblack@chromium.org>
Reviewed-by: Stefan Reinauer <reinauer@google.com>
Tested-by: Gabe Black <gabeblack@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>

Updated the patch to support XHCI as well.

Change-Id: Ic2ea2810c5edb992cbe185bc9711d2f8f557cae6
(cherry picked from commit e39e2d84762a3804653d950a228ed2269c651458)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6390
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

7 files changed, 91 insertions, 67 deletions

diff --git a/payloads/libpayload/drivers/usb/ehci.c b/payloads/libpayload/drivers/usb/ehci.c
index b9837017e4..0764592f33 100644
--- a/payloads/libpayload/drivers/usb/ehci.c
+++ b/payloads/libpayload/drivers/usb/ehci.c
@@ -270,6 +270,7 @@ static void free_qh_and_tds(ehci_qh_t *qh, qtd_t *cur)
 
 static int wait_for_tds(qtd_t *head)
 {
+	/* returns the amount of bytes *not* transmitted, or -1 for error */
 	int result = 0;
 	qtd_t *cur = head;
 	while (1) {
@@ -291,16 +292,18 @@ static int wait_for_tds(qtd_t *head)
 		if (timeout < 0) {
 			usb_debug("Error: ehci: queue transfer "
 				"processing timed out.\n");
-			return 1;
+			return -1;
 		}
 		if (cur->token & QTD_HALTED) {
 			usb_debug("ERROR with packet\n");
 			dump_td(virt_to_phys(cur));
 			usb_debug("-----------------\n");
-			return 1;
+			return -1;
 		}
+		result += (cur->token & QTD_TOTAL_LEN_MASK)
+				>> QTD_TOTAL_LEN_SHIFT;
 		if (cur->next_qtd & 1) {
-			return 0;
+			break;
 		}
 		if (0) dump_td(virt_to_phys(cur));
 		/* helps debugging the TD chain */
@@ -338,13 +341,13 @@ static int ehci_process_async_schedule(
 	int result;
 
 	/* make sure async schedule is disabled */
-	if (ehci_set_async_schedule(ehcic, 0)) return 1;
+	if (ehci_set_async_schedule(ehcic, 0)) return -1;
 
 	/* hook up QH */
 	ehcic->operation->asynclistaddr = virt_to_phys(qhead);
 
 	/* start async schedule */
-	if (ehci_set_async_schedule(ehcic, 1)) return 1;
+	if (ehci_set_async_schedule(ehcic, 1)) return -1;
 
 	/* wait for result */
 	result = wait_for_tds(head);
@@ -358,6 +361,7 @@ static int ehci_process_async_schedule(
 static int ehci_bulk (endpoint_t *ep, int size, u8 *data, int finalize)
 {
 	int result = 0;
+	int remaining = size;
 	int endp = ep->endpoint & 0xf;
 	int pid = (ep->direction==IN)?EHCI_IN:EHCI_OUT;
 
@@ -365,7 +369,7 @@ static int ehci_bulk (endpoint_t *ep, int size, u8 *data, int finalize)
 	if (ep->dev->speed < 2) {
 		/* we need a split transaction */
 		if (closest_usb2_hub(ep->dev, &hubaddr, &hubport))
-			return 1;
+			return -1;
 	}
 
 	qtd_t *head = memalign(64, sizeof(qtd_t));
@@ -375,12 +379,12 @@ static int ehci_bulk (endpoint_t *ep, int size, u8 *data, int finalize)
 		cur->token = QTD_ACTIVE |
 			(pid << QTD_PID_SHIFT) |
 			(0 << QTD_CERR_SHIFT);
-		u32 chunk = fill_td(cur, data, size);
-		size -= chunk;
+		u32 chunk = fill_td(cur, data, remaining);
+		remaining -= chunk;
 		data += chunk;
 
 		cur->alt_next_qtd = QTD_TERMINATE;
-		if (size == 0) {
+		if (remaining == 0) {
 			cur->next_qtd = virt_to_phys(0) | QTD_TERMINATE;
 			break;
 		} else {
@@ -411,10 +415,13 @@ static int ehci_bulk (endpoint_t *ep, int size, u8 *data, int finalize)
 
 	result = ehci_process_async_schedule(
 			EHCI_INST(ep->dev->controller), qh, head);
+	if (result >= 0)
+		result = size - result;
 
 	ep->toggle = (cur->token & QTD_TOGGLE_MASK) >> QTD_TOGGLE_SHIFT;
 
 	free_qh_and_tds(qh, head);
+
 	return result;
 }
 
@@ -432,7 +439,7 @@ static int ehci_control (usbdev_t *dev, direction_t dir, int drlen, void *devreq
 	if (dev->speed < 2) {
 		/* we need a split transaction */
 		if (closest_usb2_hub(dev, &hubaddr, &hubport))
-			return 1;
+			return -1;
 		non_hs_ctrl_ep = 1;
 	}
 
@@ -499,6 +506,8 @@ static int ehci_control (usbdev_t *dev, direction_t dir, int drlen, void *devreq
 
 	result = ehci_process_async_schedule(
 			EHCI_INST(dev->controller), qh, head);
+	if (result >= 0)
+		result = dalen - result;
 
 	free_qh_and_tds(qh, head);
 	return result;
diff --git a/payloads/libpayload/drivers/usb/ohci.c b/payloads/libpayload/drivers/usb/ohci.c
index 0683de3c5d..723b0dbadf 100644
--- a/payloads/libpayload/drivers/usb/ohci.c
+++ b/payloads/libpayload/drivers/usb/ohci.c
@@ -44,7 +44,7 @@ static int ohci_control (usbdev_t *dev, direction_t dir, int drlen, void *devreq
 static void* ohci_create_intr_queue (endpoint_t *ep, int reqsize, int reqcount, int reqtiming);
 static void ohci_destroy_intr_queue (endpoint_t *ep, void *queue);
 static u8* ohci_poll_intr_queue (void *queue);
-static void ohci_process_done_queue(ohci_t *ohci, int spew_debug);
+static int ohci_process_done_queue(ohci_t *ohci, int spew_debug);
 
 #ifdef USB_DEBUG
 static void
@@ -321,13 +321,13 @@ wait_for_ed(usbdev_t *dev, ed_t *head, int pages)
 		usb_debug("Error: ohci: endpoint "
 			"descriptor processing timed out.\n");
 	/* Clear the done queue. */
-	ohci_process_done_queue(OHCI_INST(dev->controller), 1);
+	int result = ohci_process_done_queue(OHCI_INST(dev->controller), 1);
 
 	if (head->head_pointer & 1) {
 		usb_debug("HALTED!\n");
-		return 1;
+		return -1;
 	}
-	return 0;
+	return result;
 }
 
 static void
@@ -355,6 +355,7 @@ static int
 ohci_control (usbdev_t *dev, direction_t dir, int drlen, void *devreq, int dalen,
 	      unsigned char *data)
 {
+	int remaining = dalen;
 	td_t *cur;
 
 	// pages are specified as 4K in OHCI, so don't use getpagesize()
@@ -392,21 +393,21 @@ ohci_control (usbdev_t *dev, direction_t dir, int drlen, void *devreq, int dalen
 		cur->current_buffer_pointer = virt_to_phys(data);
 		pages--;
 		int consumed = (4096 - ((unsigned long)data % 4096));
-		if (consumed >= dalen) {
+		if (consumed >= remaining) {
 			// end of data is within same page
-			cur->buffer_end = virt_to_phys(data + dalen - 1);
-			dalen = 0;
+			cur->buffer_end = virt_to_phys(data + remaining - 1);
+			remaining = 0;
 			/* assert(pages == 0); */
 		} else {
-			dalen -= consumed;
+			remaining -= consumed;
 			data += consumed;
 			pages--;
-			int second_page_size = dalen;
-			if (dalen > 4096) {
+			int second_page_size = remaining;
+			if (remaining > 4096) {
 				second_page_size = 4096;
 			}
 			cur->buffer_end = virt_to_phys(data + second_page_size - 1);
-			dalen -= second_page_size;
+			remaining -= second_page_size;
 			data += second_page_size;
 		}
 	}
@@ -454,7 +455,7 @@ ohci_control (usbdev_t *dev, direction_t dir, int drlen, void *devreq, int dalen
 	OHCI_INST(dev->controller)->opreg->HcControl |= ControlListEnable;
 	OHCI_INST(dev->controller)->opreg->HcCommandStatus = ControlListFilled;
 
-	int failure = wait_for_ed(dev, head,
+	int result = wait_for_ed(dev, head,
 			(dalen==0)?0:(last_page - first_page + 1));
 	/* Wait some frames before and one after disabling list access. */
 	mdelay(4);
@@ -464,7 +465,10 @@ ohci_control (usbdev_t *dev, direction_t dir, int drlen, void *devreq, int dalen
 	/* free memory */
 	ohci_free_ed(head);
 
-	return failure;
+	if (result >= 0)
+		result = dalen - result;
+
+	return result;
 }
 
 /* finalize == 1: if data is of packet aligned size, add a zero length packet */
@@ -472,6 +476,7 @@ static int
 ohci_bulk (endpoint_t *ep, int dalen, u8 *data, int finalize)
 {
 	int i;
+	int remaining = dalen;
 	usb_debug("bulk: %x bytes from %x, finalize: %x, maxpacketsize: %x\n", dalen, data, finalize, ep->maxpacketsize);
 
 	td_t *cur, *next;
@@ -501,28 +506,28 @@ ohci_bulk (endpoint_t *ep, int dalen, u8 *data, int finalize)
                         TD_CC_NOACCESS;
 		cur->current_buffer_pointer = virt_to_phys(data);
 		pages--;
-		if (dalen == 0) {
+		if (remaining == 0) {
 			/* magic TD for empty packet transfer */
 			cur->current_buffer_pointer = 0;
 			cur->buffer_end = 0;
 			/* assert((pages == 0) && finalize); */
 		}
 		int consumed = (4096 - ((unsigned long)data % 4096));
-		if (consumed >= dalen) {
+		if (consumed >= remaining) {
 			// end of data is within same page
-			cur->buffer_end = virt_to_phys(data + dalen - 1);
-			dalen = 0;
+			cur->buffer_end = virt_to_phys(data + remaining - 1);
+			remaining = 0;
 			/* assert(pages == finalize); */
 		} else {
-			dalen -= consumed;
+			remaining -= consumed;
 			data += consumed;
 			pages--;
-			int second_page_size = dalen;
-			if (dalen > 4096) {
+			int second_page_size = remaining;
+			if (remaining > 4096) {
 				second_page_size = 4096;
 			}
 			cur->buffer_end = virt_to_phys(data + second_page_size - 1);
-			dalen -= second_page_size;
+			remaining -= second_page_size;
 			data += second_page_size;
 		}
 		/* One more TD. */
@@ -558,7 +563,7 @@ ohci_bulk (endpoint_t *ep, int dalen, u8 *data, int finalize)
 	OHCI_INST(ep->dev->controller)->opreg->HcControl |= BulkListEnable;
 	OHCI_INST(ep->dev->controller)->opreg->HcCommandStatus = BulkListFilled;
 
-	int failure = wait_for_ed(ep->dev, head,
+	int result = wait_for_ed(ep->dev, head,
 			(dalen==0)?0:(last_page - first_page + 1));
 	/* Wait some frames before and one after disabling list access. */
 	mdelay(4);
@@ -570,12 +575,12 @@ ohci_bulk (endpoint_t *ep, int dalen, u8 *data, int finalize)
 	/* free memory */
 	ohci_free_ed(head);
 
-	if (failure) {
-		/* try cleanup */
+	if (result >= 0)
+		result = dalen - result;
+	else
 		clear_stall(ep);
-	}
 
-	return failure;
+	return result;
 }
 
 
@@ -780,9 +785,11 @@ ohci_poll_intr_queue(void *const q_)
 	return data;
 }
 
-static void
+static int
 ohci_process_done_queue(ohci_t *const ohci, const int spew_debug)
 {
+	/* returns the amount of bytes *not* transmitted for short packets */
+	int result = 0;
 	int i, j;
 
 	/* Temporary queue of interrupt queue TDs (to reverse order). */
@@ -790,7 +797,7 @@ ohci_process_done_queue(ohci_t *const ohci, const int spew_debug)
 
 	/* Check if done head has been written. */
 	if (!(ohci->opreg->HcInterruptStatus & WritebackDoneHead))
-		return;
+		return 0;
 	/* Fetch current done head.
 	   Lsb is only interesting for hw interrupts. */
 	u32 phys_done_queue = ohci->hcca->HccaDoneHead & ~1;
@@ -807,7 +814,11 @@ ohci_process_done_queue(ohci_t *const ohci, const int spew_debug)
 
 		switch (done_td->config & TD_QUEUETYPE_MASK) {
 		case TD_QUEUETYPE_ASYNC:
-			/* Free processed async TDs. */
+			/* Free processed async TDs and count short transfer. */
+			if (done_td->current_buffer_pointer)
+				result += (done_td->buffer_end & 0xfff) -
+						(done_td->current_buffer_pointer
+						& 0xfff) + 1;
 			free((void *)done_td);
 			break;
 		case TD_QUEUETYPE_INTR: {
@@ -864,5 +875,7 @@ ohci_process_done_queue(ohci_t *const ohci, const int spew_debug)
 	}
 	if (spew_debug)
 		usb_debug("processed %d done tds, %d intr tds thereof.\n", i, j);
+
+	return result;
 }
 
diff --git a/payloads/libpayload/drivers/usb/uhci.c b/payloads/libpayload/drivers/usb/uhci.c
index b81b911eef..3eff612129 100644
--- a/payloads/libpayload/drivers/usb/uhci.c
+++ b/payloads/libpayload/drivers/usb/uhci.c
@@ -386,7 +386,7 @@ uhci_control (usbdev_t *dev, direction_t dir, int drlen, void *devreq, int dalen
 	} else {
 		usb_debug ("control packet, req %x\n", req);
 		td_dump (td);
-		result = 1;
+		result = -1;
 	}
 	free (tds);
 	return result;
@@ -468,7 +468,7 @@ uhci_bulk (endpoint_t *ep, int size, u8 *data, int finalize)
 		usb_debug("Stalled. Trying to clean up.\n");
 		clear_stall (ep);
 		free (tds);
-		return 1;
+		return -1;
 	}
 	ep->toggle = toggle;
 	free (tds);
diff --git a/payloads/libpayload/drivers/usb/usb.c b/payloads/libpayload/drivers/usb/usb.c
index 6b54dbbada..d9727f01b0 100644
--- a/payloads/libpayload/drivers/usb/usb.c
+++ b/payloads/libpayload/drivers/usb/usb.c
@@ -157,7 +157,7 @@ get_descriptor (usbdev_t *dev, unsigned char bmRequestType, int descType,
 	dr.wValue = (descType << 8) | descIdx;
 	dr.wIndex = langID;
 	dr.wLength = 8;
-	if (dev->controller->control (dev, IN, sizeof (dr), &dr, 8, buf)) {
+	if (dev->controller->control (dev, IN, sizeof (dr), &dr, 8, buf) < 0) {
 		usb_debug ("getting descriptor size (type %x) failed\n",
 			descType);
 	}
@@ -181,7 +181,7 @@ get_descriptor (usbdev_t *dev, unsigned char bmRequestType, int descType,
 	memset (result, 0, size);
 	dr.wLength = size;
 	if (dev->controller->
-	    control (dev, IN, sizeof (dr), &dr, size, result)) {
+	    control (dev, IN, sizeof (dr), &dr, size, result) < 0) {
 		usb_debug ("getting descriptor (type %x, size %x) failed\n",
 			descType, size);
 	}
@@ -213,7 +213,7 @@ clear_feature (usbdev_t *dev, int endp, int feature, int rtype)
 	dr.wValue = feature;
 	dr.wIndex = endp;
 	dr.wLength = 0;
-	return dev->controller->control (dev, OUT, sizeof (dr), &dr, 0, 0);
+	return dev->controller->control (dev, OUT, sizeof (dr), &dr, 0, 0) < 0;
 }
 
 int
@@ -270,7 +270,7 @@ generic_set_address (hci_t *controller, int speed, int hubport, int hubaddr)
 	dev->endpoints[0].toggle = 0;
 	dev->endpoints[0].direction = SETUP;
 	mdelay (50);
-	if (dev->controller->control (dev, OUT, sizeof (dr), &dr, 0, 0)) {
+	if (dev->controller->control (dev, OUT, sizeof (dr), &dr, 0, 0) < 0) {
 		return -1;
 	}
 	mdelay (50);
diff --git a/payloads/libpayload/drivers/usb/usbmsc.c b/payloads/libpayload/drivers/usb/usbmsc.c
index f1de483170..2d28fbd12f 100644
--- a/payloads/libpayload/drivers/usb/usbmsc.c
+++ b/payloads/libpayload/drivers/usb/usbmsc.c
@@ -184,7 +184,7 @@ reset_transport (usbdev_t *dev)
 	dr.wLength = 0;
 
 	/* if any of these fails, detach device, as we are lost */
-	if (dev->controller->control (dev, OUT, sizeof (dr), &dr, 0, 0) ||
+	if (dev->controller->control (dev, OUT, sizeof (dr), &dr, 0, 0) < 0 ||
 			clear_stall (MSC_INST (dev)->bulk_in) ||
 			clear_stall (MSC_INST (dev)->bulk_out)) {
 		usb_debug ("Detaching unresponsive device.\n");
@@ -211,9 +211,8 @@ get_max_luns (usbdev_t *dev)
 	dr.wValue = 0;
 	dr.wIndex = 0;
 	dr.wLength = 1;
-	if (dev->controller->control (dev, IN, sizeof (dr), &dr, 1, &luns)) {
+	if (dev->controller->control (dev, IN, sizeof (dr), &dr, 1, &luns) < 0)
 		luns = 0;	// assume only 1 lun if req fails
-	}
 	return luns;
 }
 
@@ -239,10 +238,10 @@ wrap_cbw (cbw_t *cbw, int datalen, cbw_direction dir, const u8 *cmd,
 static int
 get_csw (endpoint_t *ep, csw_t *csw)
 {
-	if (ep->dev->controller->bulk (ep, sizeof (csw_t), (u8 *) csw, 1)) {
+	if (ep->dev->controller->bulk (ep, sizeof (csw_t), (u8 *) csw, 1) < 0) {
 		clear_stall (ep);
 		if (ep->dev->controller->bulk
-				(ep, sizeof (csw_t), (u8 *) csw, 1)) {
+				(ep, sizeof (csw_t), (u8 *) csw, 1) < 0) {
 			return reset_transport (ep->dev);
 		}
 	}
@@ -265,17 +264,17 @@ execute_command (usbdev_t *dev, cbw_direction dir, const u8 *cb, int cblen,
 	}
 	wrap_cbw (&cbw, buflen, dir, cb, cblen);
 	if (dev->controller->
-	    bulk (MSC_INST (dev)->bulk_out, sizeof (cbw), (u8 *) &cbw, 0)) {
+	    bulk (MSC_INST (dev)->bulk_out, sizeof (cbw), (u8 *) &cbw, 0) < 0) {
 		return reset_transport (dev);
 	}
 	if (buflen > 0) {
 		if (dir == cbw_direction_data_in) {
 			if (dev->controller->
-			    bulk (MSC_INST (dev)->bulk_in, buflen, buf, 0))
+			    bulk (MSC_INST (dev)->bulk_in, buflen, buf, 0) < 0)
 				clear_stall (MSC_INST (dev)->bulk_in);
 		} else {
 			if (dev->controller->
-			    bulk (MSC_INST (dev)->bulk_out, buflen, buf, 0))
+			    bulk (MSC_INST (dev)->bulk_out, buflen, buf, 0) < 0)
 				clear_stall (MSC_INST (dev)->bulk_out);
 		}
 	}
diff --git a/payloads/libpayload/drivers/usb/xhci.c b/payloads/libpayload/drivers/usb/xhci.c
index d31a8e2a8e..184a370936 100644
--- a/payloads/libpayload/drivers/usb/xhci.c
+++ b/payloads/libpayload/drivers/usb/xhci.c
@@ -588,14 +588,14 @@ xhci_control(usbdev_t *const dev, const direction_t dir,
 	const size_t off = (size_t)data & 0xffff;
 	if ((off + dalen) > ((TRANSFER_RING_SIZE - 4) << 16)) {
 		xhci_debug("Unsupported transfer size\n");
-		return 1;
+		return -1;
 	}
 
 	/* Reset endpoint if it's halted */
 	const unsigned ep_state = EC_GET(STATE, di->devctx.ep0);
 	if (ep_state == 2 || ep_state == 4) {
 		if (xhci_reset_endpoint(dev, NULL, 0))
-			return 1;
+			return -1;
 	}
 
 	/* Fill and enqueue setup TRB */
@@ -631,11 +631,12 @@ xhci_control(usbdev_t *const dev, const direction_t dir,
 	xhci->dbreg[dev->address] = 1;
 
 	/* Wait for transfer events */
-	int i;
+	int i, transferred = 0;
 	const int n_stages = 2 + !!dalen;
 	for (i = 0; i < n_stages; ++i) {
 		const int ret = xhci_wait_for_transfer(xhci, dev->address, 1);
-		if (ret != CC_SUCCESS) {
+		transferred += ret;
+		if (ret < 0) {
 			if (ret == TIMEOUT) {
 				xhci_debug("Stopping ID %d EP 1\n",
 					   dev->address);
@@ -651,11 +652,11 @@ xhci_control(usbdev_t *const dev, const direction_t dir,
 				   tr->ring, setup, status,
 				   ep_state, EC_GET(STATE, di->devctx.ep0),
 				   xhci->opreg->usbsts);
-			return 1;
+			return ret;
 		}
 	}
 
-	return 0;
+	return transferred;
 }
 
 /* finalize == 1: if data is of packet aligned size, add a zero length packet */
@@ -675,14 +676,14 @@ xhci_bulk(endpoint_t *const ep,
 	const size_t off = (size_t)data & 0xffff;
 	if ((off + size) > ((TRANSFER_RING_SIZE - 2) << 16)) {
 		xhci_debug("Unsupported transfer size\n");
-		return 1;
+		return -1;
 	}
 
 	/* Reset endpoint if it's halted */
 	const unsigned ep_state = EC_GET(STATE, di->devctx.eps[ep_id]);
 	if (ep_state == 2 || ep_state == 4) {
 		if (xhci_reset_endpoint(ep->dev, ep, 0))
-			return 1;
+			return -1;
 	}
 
 	/* Enqueue transfer and ring doorbell */
@@ -693,12 +694,12 @@ xhci_bulk(endpoint_t *const ep,
 
 	/* Wait for transfer event */
 	const int ret = xhci_wait_for_transfer(xhci, ep->dev->address, ep_id);
-	if (ret != CC_SUCCESS) {
+	if (ret < 0) {
 		if (ret == TIMEOUT) {
 			xhci_debug("Stopping ID %d EP %d\n",
 				   ep->dev->address, ep_id);
 			xhci_cmd_stop_endpoint(xhci, ep->dev->address, ep_id);
-		} else if (ret == CC_STALL_ERROR) {
+		} else if (ret == -CC_STALL_ERROR) {
 			xhci_reset_endpoint(ep->dev, ep, 1);
 		}
 		xhci_debug("Bulk transfer failed: %d\n"
@@ -707,10 +708,10 @@ xhci_bulk(endpoint_t *const ep,
 			   ret, ep_state,
 			   EC_GET(STATE, di->devctx.eps[ep_id]),
 			   xhci->opreg->usbsts);
-		return 1;
+		return ret;
 	}
 
-	return 0;
+	return ret;
 }
 
 static trb_t *
diff --git a/payloads/libpayload/drivers/usb/xhci_events.c b/payloads/libpayload/drivers/usb/xhci_events.c
index b04ecda267..ab90c59fe3 100644
--- a/payloads/libpayload/drivers/usb/xhci_events.c
+++ b/payloads/libpayload/drivers/usb/xhci_events.c
@@ -308,7 +308,7 @@ xhci_wait_for_command_done(xhci_t *const xhci,
 	return cc;
 }
 
-/* returns cc of transfer for given slot/endpoint pair */
+/* returns amount of bytes transferred on success, negative CC on error */
 int
 xhci_wait_for_transfer(xhci_t *const xhci, const int slot_id, const int ep_id)
 {
@@ -319,7 +319,9 @@ xhci_wait_for_transfer(xhci_t *const xhci, const int slot_id, const int ep_id)
 	while (xhci_wait_for_event_type(xhci, TRB_EV_TRANSFER, &timeout_us)) {
 		if (TRB_GET(ID, xhci->er.cur) == slot_id &&
 				TRB_GET(EP, xhci->er.cur) == ep_id) {
-			cc = TRB_GET(CC, xhci->er.cur);
+			cc = -TRB_GET(CC, xhci->er.cur);
+			if (cc == -CC_SUCCESS || cc == -CC_SHORT_PACKET)
+				cc = TRB_GET(EVTL, xhci->er.cur);
 			xhci_advance_event_ring(xhci);
 			break;
 		}