usb: xhci: Add DbC support in xHCI driver

xHCI compatible USB host controllers(i.e. super-speed USB3 controllers)
can be implemented with the Debug Capability(DbC). It presents a debug
device which is fully compliant with the USB framework and provides the
equivalent of a very high performance full-duplex serial link. The debug
capability operation model and registers interface are defined in 7.6.8
of the xHCI specification, revision 1.1.

The DbC debug device shares a root port with the xHCI host. By default,
the debug capability is disabled and the root port is assigned to xHCI.
When the DbC is enabled, the root port will be assigned to the DbC debug
device, and the xHCI sees nothing on this port. This implementation uses
a sysfs node named <dbc> under the xHCI device to manage the enabling
and disabling of the debug capability.

When the debug capability is enabled, it will present a debug device
through the debug port. This debug device is fully compliant with the
USB3 framework, and it can be enumerated by a debug host on the other
end of the USB link. As soon as the debug device is configured, a TTY
serial device named /dev/ttyDBC0 will be created.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

8 files changed, 1804 insertions, 0 deletions

diff --git a/drivers/usb/host/Kconfig b/drivers/usb/host/Kconfig
index b80a94e632af..6150bed7cfa8 100644
--- a/drivers/usb/host/Kconfig
+++ b/drivers/usb/host/Kconfig
@@ -27,6 +27,14 @@ config USB_XHCI_HCD
 	  module will be called xhci-hcd.
 
 if USB_XHCI_HCD
+config USB_XHCI_DBGCAP
+	bool "xHCI support for debug capability"
+	depends on TTY
+	---help---
+	  Say 'Y' to enable the support for the xHCI debug capability. Make
+	  sure that your xHCI host supports the extended debug capability and
+	  you want a TTY serial device based on the xHCI debug capability
+	  before enabling this option. If unsure, say 'N'.
 
 config USB_XHCI_PCI
        tristate
diff --git a/drivers/usb/host/Makefile b/drivers/usb/host/Makefile
index 32b036e2ffef..4ede4ce12366 100644
--- a/drivers/usb/host/Makefile
+++ b/drivers/usb/host/Makefile
@@ -14,6 +14,11 @@ fhci-$(CONFIG_FHCI_DEBUG) += fhci-dbg.o
 xhci-hcd-y := xhci.o xhci-mem.o
 xhci-hcd-y += xhci-ring.o xhci-hub.o xhci-dbg.o
 xhci-hcd-y += xhci-trace.o
+
+ifneq ($(CONFIG_USB_XHCI_DBGCAP), )
+	xhci-hcd-y += xhci-dbgcap.o xhci-dbgtty.o
+endif
+
 ifneq ($(CONFIG_USB_XHCI_MTK), )
 	xhci-hcd-y += xhci-mtk-sch.o
 endif
diff --git a/drivers/usb/host/xhci-dbgcap.c b/drivers/usb/host/xhci-dbgcap.c
new file mode 100644
index 000000000000..671e5023e683
--- /dev/null
+++ b/drivers/usb/host/xhci-dbgcap.c
@@ -0,0 +1,996 @@
+/**
+ * xhci-dbgcap.c - xHCI debug capability support
+ *
+ * Copyright (C) 2017 Intel Corporation
+ *
+ * Author: Lu Baolu <baolu.lu@linux.intel.com>
+ */
+#include <linux/dma-mapping.h>
+#include <linux/slab.h>
+#include <linux/nls.h>
+
+#include "xhci.h"
+#include "xhci-trace.h"
+#include "xhci-dbgcap.h"
+
+static inline void *
+dbc_dma_alloc_coherent(struct xhci_hcd *xhci, size_t size,
+		       dma_addr_t *dma_handle, gfp_t flags)
+{
+	void		*vaddr;
+
+	vaddr = dma_alloc_coherent(xhci_to_hcd(xhci)->self.sysdev,
+				   size, dma_handle, flags);
+	memset(vaddr, 0, size);
+	return vaddr;
+}
+
+static inline void
+dbc_dma_free_coherent(struct xhci_hcd *xhci, size_t size,
+		      void *cpu_addr, dma_addr_t dma_handle)
+{
+	if (cpu_addr)
+		dma_free_coherent(xhci_to_hcd(xhci)->self.sysdev,
+				  size, cpu_addr, dma_handle);
+}
+
+static u32 xhci_dbc_populate_strings(struct dbc_str_descs *strings)
+{
+	struct usb_string_descriptor	*s_desc;
+	u32				string_length;
+
+	/* Serial string: */
+	s_desc = (struct usb_string_descriptor *)strings->serial;
+	utf8s_to_utf16s(DBC_STRING_SERIAL, strlen(DBC_STRING_SERIAL),
+			UTF16_LITTLE_ENDIAN, (wchar_t *)s_desc->wData,
+			DBC_MAX_STRING_LENGTH);
+
+	s_desc->bLength		= (strlen(DBC_STRING_SERIAL) + 1) * 2;
+	s_desc->bDescriptorType	= USB_DT_STRING;
+	string_length		= s_desc->bLength;
+	string_length		<<= 8;
+
+	/* Product string: */
+	s_desc = (struct usb_string_descriptor *)strings->product;
+	utf8s_to_utf16s(DBC_STRING_PRODUCT, strlen(DBC_STRING_PRODUCT),
+			UTF16_LITTLE_ENDIAN, (wchar_t *)s_desc->wData,
+			DBC_MAX_STRING_LENGTH);
+
+	s_desc->bLength		= (strlen(DBC_STRING_PRODUCT) + 1) * 2;
+	s_desc->bDescriptorType	= USB_DT_STRING;
+	string_length		+= s_desc->bLength;
+	string_length		<<= 8;
+
+	/* Manufacture string: */
+	s_desc = (struct usb_string_descriptor *)strings->manufacturer;
+	utf8s_to_utf16s(DBC_STRING_MANUFACTURER,
+			strlen(DBC_STRING_MANUFACTURER),
+			UTF16_LITTLE_ENDIAN, (wchar_t *)s_desc->wData,
+			DBC_MAX_STRING_LENGTH);
+
+	s_desc->bLength		= (strlen(DBC_STRING_MANUFACTURER) + 1) * 2;
+	s_desc->bDescriptorType	= USB_DT_STRING;
+	string_length		+= s_desc->bLength;
+	string_length		<<= 8;
+
+	/* String0: */
+	strings->string0[0]	= 4;
+	strings->string0[1]	= USB_DT_STRING;
+	strings->string0[2]	= 0x09;
+	strings->string0[3]	= 0x04;
+	string_length		+= 4;
+
+	return string_length;
+}
+
+static void xhci_dbc_init_contexts(struct xhci_hcd *xhci, u32 string_length)
+{
+	struct xhci_dbc		*dbc;
+	struct dbc_info_context	*info;
+	struct xhci_ep_ctx	*ep_ctx;
+	u32			dev_info;
+	dma_addr_t		deq, dma;
+	unsigned int		max_burst;
+
+	dbc = xhci->dbc;
+	if (!dbc)
+		return;
+
+	/* Populate info Context: */
+	info			= (struct dbc_info_context *)dbc->ctx->bytes;
+	dma			= dbc->string_dma;
+	info->string0		= cpu_to_le64(dma);
+	info->manufacturer	= cpu_to_le64(dma + DBC_MAX_STRING_LENGTH);
+	info->product		= cpu_to_le64(dma + DBC_MAX_STRING_LENGTH * 2);
+	info->serial		= cpu_to_le64(dma + DBC_MAX_STRING_LENGTH * 3);
+	info->length		= cpu_to_le32(string_length);
+
+	/* Populate bulk out endpoint context: */
+	ep_ctx			= dbc_bulkout_ctx(dbc);
+	max_burst		= DBC_CTRL_MAXBURST(readl(&dbc->regs->control));
+	deq			= dbc_bulkout_enq(dbc);
+	ep_ctx->ep_info		= 0;
+	ep_ctx->ep_info2	= dbc_epctx_info2(BULK_OUT_EP, 1024, max_burst);
+	ep_ctx->deq		= cpu_to_le64(deq | dbc->ring_out->cycle_state);
+
+	/* Populate bulk in endpoint context: */
+	ep_ctx			= dbc_bulkin_ctx(dbc);
+	deq			= dbc_bulkin_enq(dbc);
+	ep_ctx->ep_info		= 0;
+	ep_ctx->ep_info2	= dbc_epctx_info2(BULK_IN_EP, 1024, max_burst);
+	ep_ctx->deq		= cpu_to_le64(deq | dbc->ring_in->cycle_state);
+
+	/* Set DbC context and info registers: */
+	xhci_write_64(xhci, dbc->ctx->dma, &dbc->regs->dccp);
+
+	dev_info = cpu_to_le32((DBC_VENDOR_ID << 16) | DBC_PROTOCOL);
+	writel(dev_info, &dbc->regs->devinfo1);
+
+	dev_info = cpu_to_le32((DBC_DEVICE_REV << 16) | DBC_PRODUCT_ID);
+	writel(dev_info, &dbc->regs->devinfo2);
+}
+
+static void xhci_dbc_giveback(struct dbc_request *req, int status)
+	__releases(&dbc->lock)
+	__acquires(&dbc->lock)
+{
+	struct dbc_ep		*dep = req->dep;
+	struct xhci_dbc		*dbc = dep->dbc;
+	struct xhci_hcd		*xhci = dbc->xhci;
+	struct device		*dev = xhci_to_hcd(dbc->xhci)->self.sysdev;
+
+	list_del_init(&req->list_pending);
+	req->trb_dma = 0;
+	req->trb = NULL;
+
+	if (req->status == -EINPROGRESS)
+		req->status = status;
+
+	trace_xhci_dbc_giveback_request(req);
+
+	dma_unmap_single(dev,
+			 req->dma,
+			 req->length,
+			 dbc_ep_dma_direction(dep));
+
+	/* Give back the transfer request: */
+	spin_unlock(&dbc->lock);
+	req->complete(xhci, req);
+	spin_lock(&dbc->lock);
+}
+
+static void xhci_dbc_flush_single_request(struct dbc_request *req)
+{
+	union xhci_trb	*trb = req->trb;
+
+	trb->generic.field[0]	= 0;
+	trb->generic.field[1]	= 0;
+	trb->generic.field[2]	= 0;
+	trb->generic.field[3]	&= cpu_to_le32(TRB_CYCLE);
+	trb->generic.field[3]	|= cpu_to_le32(TRB_TYPE(TRB_TR_NOOP));
+
+	xhci_dbc_giveback(req, -ESHUTDOWN);
+}
+
+static void xhci_dbc_flush_endpoint_requests(struct dbc_ep *dep)
+{
+	struct dbc_request	*req, *tmp;
+
+	list_for_each_entry_safe(req, tmp, &dep->list_pending, list_pending)
+		xhci_dbc_flush_single_request(req);
+}
+
+static void xhci_dbc_flush_reqests(struct xhci_dbc *dbc)
+{
+	xhci_dbc_flush_endpoint_requests(&dbc->eps[BULK_OUT]);
+	xhci_dbc_flush_endpoint_requests(&dbc->eps[BULK_IN]);
+}
+
+struct dbc_request *
+dbc_alloc_request(struct dbc_ep *dep, gfp_t gfp_flags)
+{
+	struct dbc_request	*req;
+
+	req = kzalloc(sizeof(*req), gfp_flags);
+	if (!req)
+		return NULL;
+
+	req->dep = dep;
+	INIT_LIST_HEAD(&req->list_pending);
+	INIT_LIST_HEAD(&req->list_pool);
+	req->direction = dep->direction;
+
+	trace_xhci_dbc_alloc_request(req);
+
+	return req;
+}
+
+void
+dbc_free_request(struct dbc_ep *dep, struct dbc_request *req)
+{
+	trace_xhci_dbc_free_request(req);
+
+	kfree(req);
+}
+
+static void
+xhci_dbc_queue_trb(struct xhci_ring *ring, u32 field1,
+		   u32 field2, u32 field3, u32 field4)
+{
+	union xhci_trb		*trb, *next;
+
+	trb = ring->enqueue;
+	trb->generic.field[0]	= cpu_to_le32(field1);
+	trb->generic.field[1]	= cpu_to_le32(field2);
+	trb->generic.field[2]	= cpu_to_le32(field3);
+	trb->generic.field[3]	= cpu_to_le32(field4);
+
+	trace_xhci_dbc_gadget_ep_queue(ring, &trb->generic);
+
+	ring->num_trbs_free--;
+	next = ++(ring->enqueue);
+	if (TRB_TYPE_LINK_LE32(next->link.control)) {
+		next->link.control ^= cpu_to_le32(TRB_CYCLE);
+		ring->enqueue = ring->enq_seg->trbs;
+		ring->cycle_state ^= 1;
+	}
+}
+
+static int xhci_dbc_queue_bulk_tx(struct dbc_ep *dep,
+				  struct dbc_request *req)
+{
+	u64			addr;
+	union xhci_trb		*trb;
+	unsigned int		num_trbs;
+	struct xhci_dbc		*dbc = dep->dbc;
+	struct xhci_ring	*ring = dep->ring;
+	u32			length, control, cycle;
+
+	num_trbs = count_trbs(req->dma, req->length);
+	WARN_ON(num_trbs != 1);
+	if (ring->num_trbs_free < num_trbs)
+		return -EBUSY;
+
+	addr	= req->dma;
+	trb	= ring->enqueue;
+	cycle	= ring->cycle_state;
+	length	= TRB_LEN(req->length);
+	control	= TRB_TYPE(TRB_NORMAL) | TRB_IOC;
+
+	if (cycle)
+		control &= cpu_to_le32(~TRB_CYCLE);
+	else
+		control |= cpu_to_le32(TRB_CYCLE);
+
+	req->trb = ring->enqueue;
+	req->trb_dma = xhci_trb_virt_to_dma(ring->enq_seg, ring->enqueue);
+	xhci_dbc_queue_trb(ring,
+			   lower_32_bits(addr),
+			   upper_32_bits(addr),
+			   length, control);
+
+	/*
+	 * Add a barrier between writes of trb fields and flipping
+	 * the cycle bit:
+	 */
+	wmb();
+
+	if (cycle)
+		trb->generic.field[3] |= cpu_to_le32(TRB_CYCLE);
+	else
+		trb->generic.field[3] &= cpu_to_le32(~TRB_CYCLE);
+
+	writel(DBC_DOOR_BELL_TARGET(dep->direction), &dbc->regs->doorbell);
+
+	return 0;
+}
+
+static int
+dbc_ep_do_queue(struct dbc_ep *dep, struct dbc_request *req)
+{
+	int			ret;
+	struct device		*dev;
+	struct xhci_dbc		*dbc = dep->dbc;
+	struct xhci_hcd		*xhci = dbc->xhci;
+
+	dev = xhci_to_hcd(xhci)->self.sysdev;
+
+	if (!req->length || !req->buf)
+		return -EINVAL;
+
+	req->actual		= 0;
+	req->status		= -EINPROGRESS;
+
+	req->dma = dma_map_single(dev,
+				  req->buf,
+				  req->length,
+				  dbc_ep_dma_direction(dep));
+	if (dma_mapping_error(dev, req->dma)) {
+		xhci_err(xhci, "failed to map buffer\n");
+		return -EFAULT;
+	}
+
+	ret = xhci_dbc_queue_bulk_tx(dep, req);
+	if (ret) {
+		xhci_err(xhci, "failed to queue trbs\n");
+		dma_unmap_single(dev,
+				 req->dma,
+				 req->length,
+				 dbc_ep_dma_direction(dep));
+		return -EFAULT;
+	}
+
+	list_add_tail(&req->list_pending, &dep->list_pending);
+
+	return 0;
+}
+
+int dbc_ep_queue(struct dbc_ep *dep, struct dbc_request *req,
+		 gfp_t gfp_flags)
+{
+	struct xhci_dbc		*dbc = dep->dbc;
+	int			ret = -ESHUTDOWN;
+
+	spin_lock(&dbc->lock);
+	if (dbc->state == DS_CONFIGURED)
+		ret = dbc_ep_do_queue(dep, req);
+	spin_unlock(&dbc->lock);
+
+	mod_delayed_work(system_wq, &dbc->event_work, 0);
+
+	trace_xhci_dbc_queue_request(req);
+
+	return ret;
+}
+
+static inline void xhci_dbc_do_eps_init(struct xhci_hcd *xhci, bool direction)
+{
+	struct dbc_ep		*dep;
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	dep			= &dbc->eps[direction];
+	dep->dbc		= dbc;
+	dep->direction		= direction;
+	dep->ring		= direction ? dbc->ring_in : dbc->ring_out;
+
+	INIT_LIST_HEAD(&dep->list_pending);
+}
+
+static void xhci_dbc_eps_init(struct xhci_hcd *xhci)
+{
+	xhci_dbc_do_eps_init(xhci, BULK_OUT);
+	xhci_dbc_do_eps_init(xhci, BULK_IN);
+}
+
+static void xhci_dbc_eps_exit(struct xhci_hcd *xhci)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	memset(dbc->eps, 0, ARRAY_SIZE(dbc->eps));
+}
+
+static int xhci_dbc_mem_init(struct xhci_hcd *xhci, gfp_t flags)
+{
+	int			ret;
+	dma_addr_t		deq;
+	u32			string_length;
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	/* Allocate various rings for events and transfers: */
+	dbc->ring_evt = xhci_ring_alloc(xhci, 1, 1, TYPE_EVENT, 0, flags);
+	if (!dbc->ring_evt)
+		goto evt_fail;
+
+	dbc->ring_in = xhci_ring_alloc(xhci, 1, 1, TYPE_BULK, 0, flags);
+	if (!dbc->ring_in)
+		goto in_fail;
+
+	dbc->ring_out = xhci_ring_alloc(xhci, 1, 1, TYPE_BULK, 0, flags);
+	if (!dbc->ring_out)
+		goto out_fail;
+
+	/* Allocate and populate ERST: */
+	ret = xhci_alloc_erst(xhci, dbc->ring_evt, &dbc->erst, flags);
+	if (ret)
+		goto erst_fail;
+
+	/* Allocate context data structure: */
+	dbc->ctx = xhci_alloc_container_ctx(xhci, XHCI_CTX_TYPE_DEVICE, flags);
+	if (!dbc->ctx)
+		goto ctx_fail;
+
+	/* Allocate the string table: */
+	dbc->string_size = sizeof(struct dbc_str_descs);
+	dbc->string = dbc_dma_alloc_coherent(xhci,
+					     dbc->string_size,
+					     &dbc->string_dma,
+					     flags);
+	if (!dbc->string)
+		goto string_fail;
+
+	/* Setup ERST register: */
+	writel(dbc->erst.erst_size, &dbc->regs->ersts);
+	xhci_write_64(xhci, dbc->erst.erst_dma_addr, &dbc->regs->erstba);
+	deq = xhci_trb_virt_to_dma(dbc->ring_evt->deq_seg,
+				   dbc->ring_evt->dequeue);
+	xhci_write_64(xhci, deq, &dbc->regs->erdp);
+
+	/* Setup strings and contexts: */
+	string_length = xhci_dbc_populate_strings(dbc->string);
+	xhci_dbc_init_contexts(xhci, string_length);
+
+	mmiowb();
+
+	xhci_dbc_eps_init(xhci);
+	dbc->state = DS_INITIALIZED;
+
+	return 0;
+
+string_fail:
+	xhci_free_container_ctx(xhci, dbc->ctx);
+	dbc->ctx = NULL;
+ctx_fail:
+	xhci_free_erst(xhci, &dbc->erst);
+erst_fail:
+	xhci_ring_free(xhci, dbc->ring_out);
+	dbc->ring_out = NULL;
+out_fail:
+	xhci_ring_free(xhci, dbc->ring_in);
+	dbc->ring_in = NULL;
+in_fail:
+	xhci_ring_free(xhci, dbc->ring_evt);
+	dbc->ring_evt = NULL;
+evt_fail:
+	return -ENOMEM;
+}
+
+static void xhci_dbc_mem_cleanup(struct xhci_hcd *xhci)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	if (!dbc)
+		return;
+
+	xhci_dbc_eps_exit(xhci);
+
+	if (dbc->string) {
+		dbc_dma_free_coherent(xhci,
+				      dbc->string_size,
+				      dbc->string, dbc->string_dma);
+		dbc->string = NULL;
+	}
+
+	xhci_free_container_ctx(xhci, dbc->ctx);
+	dbc->ctx = NULL;
+
+	xhci_free_erst(xhci, &dbc->erst);
+	xhci_ring_free(xhci, dbc->ring_out);
+	xhci_ring_free(xhci, dbc->ring_in);
+	xhci_ring_free(xhci, dbc->ring_evt);
+	dbc->ring_in = NULL;
+	dbc->ring_out = NULL;
+	dbc->ring_evt = NULL;
+}
+
+static int xhci_do_dbc_start(struct xhci_hcd *xhci)
+{
+	int			ret;
+	u32			ctrl;
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	if (dbc->state != DS_DISABLED)
+		return -EINVAL;
+
+	writel(0, &dbc->regs->control);
+	ret = xhci_handshake(&dbc->regs->control,
+			     DBC_CTRL_DBC_ENABLE,
+			     0, 1000);
+	if (ret)
+		return ret;
+
+	ret = xhci_dbc_mem_init(xhci, GFP_ATOMIC);
+	if (ret)
+		return ret;
+
+	ctrl = readl(&dbc->regs->control);
+	writel(ctrl | DBC_CTRL_DBC_ENABLE | DBC_CTRL_PORT_ENABLE,
+	       &dbc->regs->control);
+	ret = xhci_handshake(&dbc->regs->control,
+			     DBC_CTRL_DBC_ENABLE,
+			     DBC_CTRL_DBC_ENABLE, 1000);
+	if (ret)
+		return ret;
+
+	dbc->state = DS_ENABLED;
+
+	return 0;
+}
+
+static void xhci_do_dbc_stop(struct xhci_hcd *xhci)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	if (dbc->state == DS_DISABLED)
+		return;
+
+	writel(0, &dbc->regs->control);
+	xhci_dbc_mem_cleanup(xhci);
+	dbc->state = DS_DISABLED;
+}
+
+static int xhci_dbc_start(struct xhci_hcd *xhci)
+{
+	int			ret;
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	WARN_ON(!dbc);
+
+	pm_runtime_get_sync(xhci_to_hcd(xhci)->self.controller);
+
+	spin_lock(&dbc->lock);
+	ret = xhci_do_dbc_start(xhci);
+	spin_unlock(&dbc->lock);
+
+	if (ret) {
+		pm_runtime_put(xhci_to_hcd(xhci)->self.controller);
+		return ret;
+	}
+
+	return mod_delayed_work(system_wq, &dbc->event_work, 1);
+}
+
+static void xhci_dbc_stop(struct xhci_hcd *xhci)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+	struct dbc_port		*port = &dbc->port;
+
+	WARN_ON(!dbc);
+
+	cancel_delayed_work_sync(&dbc->event_work);
+
+	if (port->registered)
+		xhci_dbc_tty_unregister_device(xhci);
+
+	spin_lock(&dbc->lock);
+	xhci_do_dbc_stop(xhci);
+	spin_unlock(&dbc->lock);
+
+	pm_runtime_put_sync(xhci_to_hcd(xhci)->self.controller);
+}
+
+static void
+dbc_handle_port_status(struct xhci_hcd *xhci, union xhci_trb *event)
+{
+	u32			portsc;
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	portsc = readl(&dbc->regs->portsc);
+	if (portsc & DBC_PORTSC_CONN_CHANGE)
+		xhci_info(xhci, "DbC port connect change\n");
+
+	if (portsc & DBC_PORTSC_RESET_CHANGE)
+		xhci_info(xhci, "DbC port reset change\n");
+
+	if (portsc & DBC_PORTSC_LINK_CHANGE)
+		xhci_info(xhci, "DbC port link status change\n");
+
+	if (portsc & DBC_PORTSC_CONFIG_CHANGE)
+		xhci_info(xhci, "DbC config error change\n");
+
+	/* Port reset change bit will be cleared in other place: */
+	writel(portsc & ~DBC_PORTSC_RESET_CHANGE, &dbc->regs->portsc);
+}
+
+static void dbc_handle_xfer_event(struct xhci_hcd *xhci, union xhci_trb *event)
+{
+	struct dbc_ep		*dep;
+	struct xhci_ring	*ring;
+	int			ep_id;
+	int			status;
+	u32			comp_code;
+	size_t			remain_length;
+	struct dbc_request	*req = NULL, *r;
+
+	comp_code	= GET_COMP_CODE(le32_to_cpu(event->generic.field[2]));
+	remain_length	= EVENT_TRB_LEN(le32_to_cpu(event->generic.field[2]));
+	ep_id		= TRB_TO_EP_ID(le32_to_cpu(event->generic.field[3]));
+	dep		= (ep_id == EPID_OUT) ?
+				get_out_ep(xhci) : get_in_ep(xhci);
+	ring		= dep->ring;
+
+	switch (comp_code) {
+	case COMP_SUCCESS:
+		remain_length = 0;
+	/* FALLTHROUGH */
+	case COMP_SHORT_PACKET:
+		status = 0;
+		break;
+	case COMP_TRB_ERROR:
+	case COMP_BABBLE_DETECTED_ERROR:
+	case COMP_USB_TRANSACTION_ERROR:
+	case COMP_STALL_ERROR:
+		xhci_warn(xhci, "tx error %d detected\n", comp_code);
+		status = -comp_code;
+		break;
+	default:
+		xhci_err(xhci, "unknown tx error %d\n", comp_code);
+		status = -comp_code;
+		break;
+	}
+
+	/* Match the pending request: */
+	list_for_each_entry(r, &dep->list_pending, list_pending) {
+		if (r->trb_dma == event->trans_event.buffer) {
+			req = r;
+			break;
+		}
+	}
+
+	if (!req) {
+		xhci_warn(xhci, "no matched request\n");
+		return;
+	}
+
+	trace_xhci_dbc_handle_transfer(ring, &req->trb->generic);
+
+	ring->num_trbs_free++;
+	req->actual = req->length - remain_length;
+	xhci_dbc_giveback(req, status);
+}
+
+static enum evtreturn xhci_dbc_do_handle_events(struct xhci_dbc *dbc)
+{
+	dma_addr_t		deq;
+	struct dbc_ep		*dep;
+	union xhci_trb		*evt;
+	u32			ctrl, portsc;
+	struct xhci_hcd		*xhci = dbc->xhci;
+	bool			update_erdp = false;
+
+	/* DbC state machine: */
+	switch (dbc->state) {
+	case DS_DISABLED:
+	case DS_INITIALIZED:
+
+		return EVT_ERR;
+	case DS_ENABLED:
+		portsc = readl(&dbc->regs->portsc);
+		if (portsc & DBC_PORTSC_CONN_STATUS) {
+			dbc->state = DS_CONNECTED;
+			xhci_info(xhci, "DbC connected\n");
+		}
+
+		return EVT_DONE;
+	case DS_CONNECTED:
+		ctrl = readl(&dbc->regs->control);
+		if (ctrl & DBC_CTRL_DBC_RUN) {
+			dbc->state = DS_CONFIGURED;
+			xhci_info(xhci, "DbC configured\n");
+			portsc = readl(&dbc->regs->portsc);
+			writel(portsc, &dbc->regs->portsc);
+			return EVT_GSER;
+		}
+
+		return EVT_DONE;
+	case DS_CONFIGURED:
+		/* Handle cable unplug event: */
+		portsc = readl(&dbc->regs->portsc);
+		if (!(portsc & DBC_PORTSC_PORT_ENABLED) &&
+		    !(portsc & DBC_PORTSC_CONN_STATUS)) {
+			xhci_info(xhci, "DbC cable unplugged\n");
+			dbc->state = DS_ENABLED;
+			xhci_dbc_flush_reqests(dbc);
+
+			return EVT_DISC;
+		}
+
+		/* Handle debug port reset event: */
+		if (portsc & DBC_PORTSC_RESET_CHANGE) {
+			xhci_info(xhci, "DbC port reset\n");
+			writel(portsc, &dbc->regs->portsc);
+			dbc->state = DS_ENABLED;
+			xhci_dbc_flush_reqests(dbc);
+
+			return EVT_DISC;
+		}
+
+		/* Handle endpoint stall event: */
+		ctrl = readl(&dbc->regs->control);
+		if ((ctrl & DBC_CTRL_HALT_IN_TR) ||
+		    (ctrl & DBC_CTRL_HALT_OUT_TR)) {
+			xhci_info(xhci, "DbC Endpoint stall\n");
+			dbc->state = DS_STALLED;
+
+			if (ctrl & DBC_CTRL_HALT_IN_TR) {
+				dep = get_in_ep(xhci);
+				xhci_dbc_flush_endpoint_requests(dep);
+			}
+
+			if (ctrl & DBC_CTRL_HALT_OUT_TR) {
+				dep = get_out_ep(xhci);
+				xhci_dbc_flush_endpoint_requests(dep);
+			}
+
+			return EVT_DONE;
+		}
+
+		/* Clear DbC run change bit: */
+		if (ctrl & DBC_CTRL_DBC_RUN_CHANGE) {
+			writel(ctrl, &dbc->regs->control);
+			ctrl = readl(&dbc->regs->control);
+		}
+
+		break;
+	case DS_STALLED:
+		ctrl = readl(&dbc->regs->control);
+		if (!(ctrl & DBC_CTRL_HALT_IN_TR) &&
+		    !(ctrl & DBC_CTRL_HALT_OUT_TR) &&
+		    (ctrl & DBC_CTRL_DBC_RUN)) {
+			dbc->state = DS_CONFIGURED;
+			break;
+		}
+
+		return EVT_DONE;
+	default:
+		xhci_err(xhci, "Unknown DbC state %d\n", dbc->state);
+		break;
+	}
+
+	/* Handle the events in the event ring: */
+	evt = dbc->ring_evt->dequeue;
+	while ((le32_to_cpu(evt->event_cmd.flags) & TRB_CYCLE) ==
+			dbc->ring_evt->cycle_state) {
+		/*
+		 * Add a barrier between reading the cycle flag and any
+		 * reads of the event's flags/data below:
+		 */
+		rmb();
+
+		trace_xhci_dbc_handle_event(dbc->ring_evt, &evt->generic);
+
+		switch (le32_to_cpu(evt->event_cmd.flags) & TRB_TYPE_BITMASK) {
+		case TRB_TYPE(TRB_PORT_STATUS):
+			dbc_handle_port_status(xhci, evt);
+			break;
+		case TRB_TYPE(TRB_TRANSFER):
+			dbc_handle_xfer_event(xhci, evt);
+			break;
+		default:
+			break;
+		}
+
+		inc_deq(xhci, dbc->ring_evt);
+		evt = dbc->ring_evt->dequeue;
+		update_erdp = true;
+	}
+
+	/* Update event ring dequeue pointer: */
+	if (update_erdp) {
+		deq = xhci_trb_virt_to_dma(dbc->ring_evt->deq_seg,
+					   dbc->ring_evt->dequeue);
+		xhci_write_64(xhci, deq, &dbc->regs->erdp);
+	}
+
+	return EVT_DONE;
+}
+
+static void xhci_dbc_handle_events(struct work_struct *work)
+{
+	int			ret;
+	enum evtreturn		evtr;
+	struct xhci_dbc		*dbc;
+	struct xhci_hcd		*xhci;
+
+	dbc = container_of(to_delayed_work(work), struct xhci_dbc, event_work);
+	xhci = dbc->xhci;
+
+	spin_lock(&dbc->lock);
+	evtr = xhci_dbc_do_handle_events(dbc);
+	spin_unlock(&dbc->lock);
+
+	switch (evtr) {
+	case EVT_GSER:
+		ret = xhci_dbc_tty_register_device(xhci);
+		if (ret) {
+			xhci_err(xhci, "failed to alloc tty device\n");
+			break;
+		}
+
+		xhci_info(xhci, "DbC now attached to /dev/ttyDBC0\n");
+		break;
+	case EVT_DISC:
+		xhci_dbc_tty_unregister_device(xhci);
+		break;
+	case EVT_DONE:
+		break;
+	default:
+		xhci_info(xhci, "stop handling dbc events\n");
+		return;
+	}
+
+	mod_delayed_work(system_wq, &dbc->event_work, 1);
+}
+
+static void xhci_do_dbc_exit(struct xhci_hcd *xhci)
+{
+	unsigned long		flags;
+
+	spin_lock_irqsave(&xhci->lock, flags);
+	kfree(xhci->dbc);
+	xhci->dbc = NULL;
+	spin_unlock_irqrestore(&xhci->lock, flags);
+}
+
+static int xhci_do_dbc_init(struct xhci_hcd *xhci)
+{
+	u32			reg;
+	struct xhci_dbc		*dbc;
+	unsigned long		flags;
+	void __iomem		*base;
+	int			dbc_cap_offs;
+
+	base = &xhci->cap_regs->hc_capbase;
+	dbc_cap_offs = xhci_find_next_ext_cap(base, 0, XHCI_EXT_CAPS_DEBUG);
+	if (!dbc_cap_offs)
+		return -ENODEV;
+
+	dbc = kzalloc(sizeof(*dbc), GFP_KERNEL);
+	if (!dbc)
+		return -ENOMEM;
+
+	dbc->regs = base + dbc_cap_offs;
+
+	/* We will avoid using DbC in xhci driver if it's in use. */
+	reg = readl(&dbc->regs->control);
+	if (reg & DBC_CTRL_DBC_ENABLE) {
+		kfree(dbc);
+		return -EBUSY;
+	}
+
+	spin_lock_irqsave(&xhci->lock, flags);
+	if (xhci->dbc) {
+		spin_unlock_irqrestore(&xhci->lock, flags);
+		kfree(dbc);
+		return -EBUSY;
+	}
+	xhci->dbc = dbc;
+	spin_unlock_irqrestore(&xhci->lock, flags);
+
+	dbc->xhci = xhci;
+	INIT_DELAYED_WORK(&dbc->event_work, xhci_dbc_handle_events);
+	spin_lock_init(&dbc->lock);
+
+	return 0;
+}
+
+static ssize_t dbc_show(struct device *dev,
+			struct device_attribute *attr,
+			char *buf)
+{
+	const char		*p;
+	struct xhci_dbc		*dbc;
+	struct xhci_hcd		*xhci;
+
+	xhci = hcd_to_xhci(dev_get_drvdata(dev));
+	dbc = xhci->dbc;
+
+	switch (dbc->state) {
+	case DS_DISABLED:
+		p = "disabled";
+		break;
+	case DS_INITIALIZED:
+		p = "initialized";
+		break;
+	case DS_ENABLED:
+		p = "enabled";
+		break;
+	case DS_CONNECTED:
+		p = "connected";
+		break;
+	case DS_CONFIGURED:
+		p = "configured";
+		break;
+	case DS_STALLED:
+		p = "stalled";
+		break;
+	default:
+		p = "unknown";
+	}
+
+	return sprintf(buf, "%s\n", p);
+}
+
+static ssize_t dbc_store(struct device *dev,
+			 struct device_attribute *attr,
+			 const char *buf, size_t count)
+{
+	struct xhci_dbc		*dbc;
+	struct xhci_hcd		*xhci;
+
+	xhci = hcd_to_xhci(dev_get_drvdata(dev));
+	dbc = xhci->dbc;
+
+	if (!strncmp(buf, "enable", 6))
+		xhci_dbc_start(xhci);
+	else if (!strncmp(buf, "disable", 7))
+		xhci_dbc_stop(xhci);
+	else
+		return -EINVAL;
+
+	return count;
+}
+
+static DEVICE_ATTR(dbc, 0644, dbc_show, dbc_store);
+
+int xhci_dbc_init(struct xhci_hcd *xhci)
+{
+	int			ret;
+	struct device		*dev = xhci_to_hcd(xhci)->self.controller;
+
+	ret = xhci_do_dbc_init(xhci);
+	if (ret)
+		goto init_err3;
+
+	ret = xhci_dbc_tty_register_driver(xhci);
+	if (ret)
+		goto init_err2;
+
+	ret = device_create_file(dev, &dev_attr_dbc);
+	if (ret)
+		goto init_err1;
+
+	return 0;
+
+init_err1:
+	xhci_dbc_tty_unregister_driver();
+init_err2:
+	xhci_do_dbc_exit(xhci);
+init_err3:
+	return ret;
+}
+
+void xhci_dbc_exit(struct xhci_hcd *xhci)
+{
+	struct device		*dev = xhci_to_hcd(xhci)->self.controller;
+
+	if (!xhci->dbc)
+		return;
+
+	device_remove_file(dev, &dev_attr_dbc);
+	xhci_dbc_tty_unregister_driver();
+	xhci_dbc_stop(xhci);
+	xhci_do_dbc_exit(xhci);
+}
+
+#ifdef CONFIG_PM
+int xhci_dbc_suspend(struct xhci_hcd *xhci)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	if (!dbc)
+		return 0;
+
+	if (dbc->state == DS_CONFIGURED)
+		dbc->resume_required = 1;
+
+	xhci_dbc_stop(xhci);
+
+	return 0;
+}
+
+int xhci_dbc_resume(struct xhci_hcd *xhci)
+{
+	int			ret = 0;
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	if (!dbc)
+		return 0;
+
+	if (dbc->resume_required) {
+		dbc->resume_required = 0;
+		xhci_dbc_start(xhci);
+	}
+
+	return ret;
+}
+#endif /* CONFIG_PM */
diff --git a/drivers/usb/host/xhci-dbgcap.h b/drivers/usb/host/xhci-dbgcap.h
new file mode 100644
index 000000000000..e66ea0748ba3
--- /dev/null
+++ b/drivers/usb/host/xhci-dbgcap.h
@@ -0,0 +1,229 @@
+
+/**
+ * xhci-dbgcap.h - xHCI debug capability support
+ *
+ * Copyright (C) 2017 Intel Corporation
+ *
+ * Author: Lu Baolu <baolu.lu@linux.intel.com>
+ */
+#ifndef __LINUX_XHCI_DBGCAP_H
+#define __LINUX_XHCI_DBGCAP_H
+
+#include <linux/tty.h>
+#include <linux/kfifo.h>
+
+struct dbc_regs {
+	__le32	capability;
+	__le32	doorbell;
+	__le32	ersts;		/* Event Ring Segment Table Size*/
+	__le32	__reserved_0;	/* 0c~0f reserved bits */
+	__le64	erstba;		/* Event Ring Segment Table Base Address */
+	__le64	erdp;		/* Event Ring Dequeue Pointer */
+	__le32	control;
+	__le32	status;
+	__le32	portsc;		/* Port status and control */
+	__le32	__reserved_1;	/* 2b~28 reserved bits */
+	__le64	dccp;		/* Debug Capability Context Pointer */
+	__le32	devinfo1;	/* Device Descriptor Info Register 1 */
+	__le32	devinfo2;	/* Device Descriptor Info Register 2 */
+};
+
+struct dbc_info_context {
+	__le64	string0;
+	__le64	manufacturer;
+	__le64	product;
+	__le64	serial;
+	__le32	length;
+	__le32	__reserved_0[7];
+};
+
+#define DBC_CTRL_DBC_RUN		BIT(0)
+#define DBC_CTRL_PORT_ENABLE		BIT(1)
+#define DBC_CTRL_HALT_OUT_TR		BIT(2)
+#define DBC_CTRL_HALT_IN_TR		BIT(3)
+#define DBC_CTRL_DBC_RUN_CHANGE		BIT(4)
+#define DBC_CTRL_DBC_ENABLE		BIT(31)
+#define DBC_CTRL_MAXBURST(p)		(((p) >> 16) & 0xff)
+#define DBC_DOOR_BELL_TARGET(p)		(((p) & 0xff) << 8)
+
+#define DBC_MAX_PACKET			1024
+#define DBC_MAX_STRING_LENGTH		64
+#define DBC_STRING_MANUFACTURER		"Linux Foundation"
+#define DBC_STRING_PRODUCT		"Linux USB Debug Target"
+#define DBC_STRING_SERIAL		"0001"
+#define	DBC_CONTEXT_SIZE		64
+
+/*
+ * Port status:
+ */
+#define DBC_PORTSC_CONN_STATUS		BIT(0)
+#define DBC_PORTSC_PORT_ENABLED		BIT(1)
+#define DBC_PORTSC_CONN_CHANGE		BIT(17)
+#define DBC_PORTSC_RESET_CHANGE		BIT(21)
+#define DBC_PORTSC_LINK_CHANGE		BIT(22)
+#define DBC_PORTSC_CONFIG_CHANGE	BIT(23)
+
+struct dbc_str_descs {
+	char	string0[DBC_MAX_STRING_LENGTH];
+	char	manufacturer[DBC_MAX_STRING_LENGTH];
+	char	product[DBC_MAX_STRING_LENGTH];
+	char	serial[DBC_MAX_STRING_LENGTH];
+};
+
+#define DBC_PROTOCOL			1	/* GNU Remote Debug Command */
+#define DBC_VENDOR_ID			0x1d6b	/* Linux Foundation 0x1d6b */
+#define DBC_PRODUCT_ID			0x0010	/* device 0010 */
+#define DBC_DEVICE_REV			0x0010	/* 0.10 */
+
+enum dbc_state {
+	DS_DISABLED = 0,
+	DS_INITIALIZED,
+	DS_ENABLED,
+	DS_CONNECTED,
+	DS_CONFIGURED,
+	DS_STALLED,
+};
+
+struct dbc_request {
+	void				*buf;
+	unsigned int			length;
+	dma_addr_t			dma;
+	void				(*complete)(struct xhci_hcd *xhci,
+						    struct dbc_request *req);
+	struct list_head		list_pool;
+	int				status;
+	unsigned int			actual;
+
+	struct dbc_ep			*dep;
+	struct list_head		list_pending;
+	dma_addr_t			trb_dma;
+	union xhci_trb			*trb;
+	unsigned			direction:1;
+};
+
+struct dbc_ep {
+	struct xhci_dbc			*dbc;
+	struct list_head		list_pending;
+	struct xhci_ring		*ring;
+	unsigned			direction:1;
+};
+
+#define DBC_QUEUE_SIZE			16
+#define DBC_WRITE_BUF_SIZE		8192
+
+/*
+ * Private structure for DbC hardware state:
+ */
+struct dbc_port {
+	struct tty_port			port;
+	spinlock_t			port_lock;	/* port access */
+
+	struct list_head		read_pool;
+	struct list_head		read_queue;
+	unsigned int			n_read;
+	struct tasklet_struct		push;
+
+	struct list_head		write_pool;
+	struct kfifo			write_fifo;
+
+	bool				registered;
+	struct dbc_ep			*in;
+	struct dbc_ep			*out;
+};
+
+struct xhci_dbc {
+	spinlock_t			lock;		/* device access */
+	struct xhci_hcd			*xhci;
+	struct dbc_regs __iomem		*regs;
+	struct xhci_ring		*ring_evt;
+	struct xhci_ring		*ring_in;
+	struct xhci_ring		*ring_out;
+	struct xhci_erst		erst;
+	struct xhci_container_ctx	*ctx;
+
+	struct dbc_str_descs		*string;
+	dma_addr_t			string_dma;
+	size_t				string_size;
+
+	enum dbc_state			state;
+	struct delayed_work		event_work;
+	unsigned			resume_required:1;
+	struct dbc_ep			eps[2];
+
+	struct dbc_port			port;
+};
+
+#define dbc_bulkout_ctx(d)		\
+	((struct xhci_ep_ctx *)((d)->ctx->bytes + DBC_CONTEXT_SIZE))
+#define dbc_bulkin_ctx(d)		\
+	((struct xhci_ep_ctx *)((d)->ctx->bytes + DBC_CONTEXT_SIZE * 2))
+#define dbc_bulkout_enq(d)		\
+	xhci_trb_virt_to_dma((d)->ring_out->enq_seg, (d)->ring_out->enqueue)
+#define dbc_bulkin_enq(d)		\
+	xhci_trb_virt_to_dma((d)->ring_in->enq_seg, (d)->ring_in->enqueue)
+#define dbc_epctx_info2(t, p, b)	\
+	cpu_to_le32(EP_TYPE(t) | MAX_PACKET(p) | MAX_BURST(b))
+#define dbc_ep_dma_direction(d)		\
+	((d)->direction ? DMA_FROM_DEVICE : DMA_TO_DEVICE)
+
+#define BULK_OUT			0
+#define BULK_IN				1
+#define EPID_OUT			2
+#define EPID_IN				3
+
+enum evtreturn {
+	EVT_ERR	= -1,
+	EVT_DONE,
+	EVT_GSER,
+	EVT_DISC,
+};
+
+static inline struct dbc_ep *get_in_ep(struct xhci_hcd *xhci)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	return &dbc->eps[BULK_IN];
+}
+
+static inline struct dbc_ep *get_out_ep(struct xhci_hcd *xhci)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	return &dbc->eps[BULK_OUT];
+}
+
+#ifdef CONFIG_USB_XHCI_DBGCAP
+int xhci_dbc_init(struct xhci_hcd *xhci);
+void xhci_dbc_exit(struct xhci_hcd *xhci);
+int xhci_dbc_tty_register_driver(struct xhci_hcd *xhci);
+void xhci_dbc_tty_unregister_driver(void);
+int xhci_dbc_tty_register_device(struct xhci_hcd *xhci);
+void xhci_dbc_tty_unregister_device(struct xhci_hcd *xhci);
+struct dbc_request *dbc_alloc_request(struct dbc_ep *dep, gfp_t gfp_flags);
+void dbc_free_request(struct dbc_ep *dep, struct dbc_request *req);
+int dbc_ep_queue(struct dbc_ep *dep, struct dbc_request *req, gfp_t gfp_flags);
+#ifdef CONFIG_PM
+int xhci_dbc_suspend(struct xhci_hcd *xhci);
+int xhci_dbc_resume(struct xhci_hcd *xhci);
+#endif /* CONFIG_PM */
+#else
+static inline int xhci_dbc_init(struct xhci_hcd *xhci)
+{
+	return 0;
+}
+
+static inline void xhci_dbc_exit(struct xhci_hcd *xhci)
+{
+}
+
+static inline int xhci_dbc_suspend(struct xhci_hcd *xhci)
+{
+	return 0;
+}
+
+static inline int xhci_dbc_resume(struct xhci_hcd *xhci)
+{
+	return 0;
+}
+#endif /* CONFIG_USB_XHCI_DBGCAP */
+#endif /* __LINUX_XHCI_DBGCAP_H */
diff --git a/drivers/usb/host/xhci-dbgtty.c b/drivers/usb/host/xhci-dbgtty.c
new file mode 100644
index 000000000000..48811d72a94c
--- /dev/null
+++ b/drivers/usb/host/xhci-dbgtty.c
@@ -0,0 +1,497 @@
+/**
+ * xhci-dbgtty.c - tty glue for xHCI debug capability
+ *
+ * Copyright (C) 2017 Intel Corporation
+ *
+ * Author: Lu Baolu <baolu.lu@linux.intel.com>
+ */
+
+#include <linux/slab.h>
+#include <linux/tty.h>
+#include <linux/tty_flip.h>
+
+#include "xhci.h"
+#include "xhci-dbgcap.h"
+
+static unsigned int
+dbc_send_packet(struct dbc_port *port, char *packet, unsigned int size)
+{
+	unsigned int		len;
+
+	len = kfifo_len(&port->write_fifo);
+	if (len < size)
+		size = len;
+	if (size != 0)
+		size = kfifo_out(&port->write_fifo, packet, size);
+	return size;
+}
+
+static int dbc_start_tx(struct dbc_port *port)
+	__releases(&port->port_lock)
+	__acquires(&port->port_lock)
+{
+	int			len;
+	struct dbc_request	*req;
+	int			status = 0;
+	bool			do_tty_wake = false;
+	struct list_head	*pool = &port->write_pool;
+
+	while (!list_empty(pool)) {
+		req = list_entry(pool->next, struct dbc_request, list_pool);
+		len = dbc_send_packet(port, req->buf, DBC_MAX_PACKET);
+		if (len == 0)
+			break;
+		do_tty_wake = true;
+
+		req->length = len;
+		list_del(&req->list_pool);
+
+		spin_unlock(&port->port_lock);
+		status = dbc_ep_queue(port->out, req, GFP_ATOMIC);
+		spin_lock(&port->port_lock);
+
+		if (status) {
+			list_add(&req->list_pool, pool);
+			break;
+		}
+	}
+
+	if (do_tty_wake && port->port.tty)
+		tty_wakeup(port->port.tty);
+
+	return status;
+}
+
+static void dbc_start_rx(struct dbc_port *port)
+	__releases(&port->port_lock)
+	__acquires(&port->port_lock)
+{
+	struct dbc_request	*req;
+	int			status;
+	struct list_head	*pool = &port->read_pool;
+
+	while (!list_empty(pool)) {
+		if (!port->port.tty)
+			break;
+
+		req = list_entry(pool->next, struct dbc_request, list_pool);
+		list_del(&req->list_pool);
+		req->length = DBC_MAX_PACKET;
+
+		spin_unlock(&port->port_lock);
+		status = dbc_ep_queue(port->in, req, GFP_ATOMIC);
+		spin_lock(&port->port_lock);
+
+		if (status) {
+			list_add(&req->list_pool, pool);
+			break;
+		}
+	}
+}
+
+static void
+dbc_read_complete(struct xhci_hcd *xhci, struct dbc_request *req)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+	struct dbc_port		*port = &dbc->port;
+
+	spin_lock(&port->port_lock);
+	list_add_tail(&req->list_pool, &port->read_queue);
+	tasklet_schedule(&port->push);
+	spin_unlock(&port->port_lock);
+}
+
+static void dbc_write_complete(struct xhci_hcd *xhci, struct dbc_request *req)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+	struct dbc_port		*port = &dbc->port;
+
+	spin_lock(&port->port_lock);
+	list_add(&req->list_pool, &port->write_pool);
+	switch (req->status) {
+	case 0:
+		dbc_start_tx(port);
+		break;
+	case -ESHUTDOWN:
+		break;
+	default:
+		xhci_warn(xhci, "unexpected write complete status %d\n",
+			  req->status);
+		break;
+	}
+	spin_unlock(&port->port_lock);
+}
+
+void xhci_dbc_free_req(struct dbc_ep *dep, struct dbc_request *req)
+{
+	kfree(req->buf);
+	dbc_free_request(dep, req);
+}
+
+static int
+xhci_dbc_alloc_requests(struct dbc_ep *dep, struct list_head *head,
+			void (*fn)(struct xhci_hcd *, struct dbc_request *))
+{
+	int			i;
+	struct dbc_request	*req;
+
+	for (i = 0; i < DBC_QUEUE_SIZE; i++) {
+		req = dbc_alloc_request(dep, GFP_ATOMIC);
+		if (!req)
+			break;
+
+		req->length = DBC_MAX_PACKET;
+		req->buf = kmalloc(req->length, GFP_KERNEL);
+		if (!req->buf) {
+			xhci_dbc_free_req(dep, req);
+			break;
+		}
+
+		req->complete = fn;
+		list_add_tail(&req->list_pool, head);
+	}
+
+	return list_empty(head) ? -ENOMEM : 0;
+}
+
+static void
+xhci_dbc_free_requests(struct dbc_ep *dep, struct list_head *head)
+{
+	struct dbc_request	*req;
+
+	while (!list_empty(head)) {
+		req = list_entry(head->next, struct dbc_request, list_pool);
+		list_del(&req->list_pool);
+		xhci_dbc_free_req(dep, req);
+	}
+}
+
+static int dbc_tty_install(struct tty_driver *driver, struct tty_struct *tty)
+{
+	struct dbc_port		*port = driver->driver_state;
+
+	tty->driver_data = port;
+
+	return tty_port_install(&port->port, driver, tty);
+}
+
+static int dbc_tty_open(struct tty_struct *tty, struct file *file)
+{
+	struct dbc_port		*port = tty->driver_data;
+
+	return tty_port_open(&port->port, tty, file);
+}
+
+static void dbc_tty_close(struct tty_struct *tty, struct file *file)
+{
+	struct dbc_port		*port = tty->driver_data;
+
+	tty_port_close(&port->port, tty, file);
+}
+
+static int dbc_tty_write(struct tty_struct *tty,
+			 const unsigned char *buf,
+			 int count)
+{
+	struct dbc_port		*port = tty->driver_data;
+	unsigned long		flags;
+
+	spin_lock_irqsave(&port->port_lock, flags);
+	if (count)
+		count = kfifo_in(&port->write_fifo, buf, count);
+	dbc_start_tx(port);
+	spin_unlock_irqrestore(&port->port_lock, flags);
+
+	return count;
+}
+
+static int dbc_tty_put_char(struct tty_struct *tty, unsigned char ch)
+{
+	struct dbc_port		*port = tty->driver_data;
+	unsigned long		flags;
+	int			status;
+
+	spin_lock_irqsave(&port->port_lock, flags);
+	status = kfifo_put(&port->write_fifo, ch);
+	spin_unlock_irqrestore(&port->port_lock, flags);
+
+	return status;
+}
+
+static void dbc_tty_flush_chars(struct tty_struct *tty)
+{
+	struct dbc_port		*port = tty->driver_data;
+	unsigned long		flags;
+
+	spin_lock_irqsave(&port->port_lock, flags);
+	dbc_start_tx(port);
+	spin_unlock_irqrestore(&port->port_lock, flags);
+}
+
+static int dbc_tty_write_room(struct tty_struct *tty)
+{
+	struct dbc_port		*port = tty->driver_data;
+	unsigned long		flags;
+	int			room = 0;
+
+	spin_lock_irqsave(&port->port_lock, flags);
+	room = kfifo_avail(&port->write_fifo);
+	spin_unlock_irqrestore(&port->port_lock, flags);
+
+	return room;
+}
+
+static int dbc_tty_chars_in_buffer(struct tty_struct *tty)
+{
+	struct dbc_port		*port = tty->driver_data;
+	unsigned long		flags;
+	int			chars = 0;
+
+	spin_lock_irqsave(&port->port_lock, flags);
+	chars = kfifo_len(&port->write_fifo);
+	spin_unlock_irqrestore(&port->port_lock, flags);
+
+	return chars;
+}
+
+static void dbc_tty_unthrottle(struct tty_struct *tty)
+{
+	struct dbc_port		*port = tty->driver_data;
+	unsigned long		flags;
+
+	spin_lock_irqsave(&port->port_lock, flags);
+	tasklet_schedule(&port->push);
+	spin_unlock_irqrestore(&port->port_lock, flags);
+}
+
+static const struct tty_operations dbc_tty_ops = {
+	.install		= dbc_tty_install,
+	.open			= dbc_tty_open,
+	.close			= dbc_tty_close,
+	.write			= dbc_tty_write,
+	.put_char		= dbc_tty_put_char,
+	.flush_chars		= dbc_tty_flush_chars,
+	.write_room		= dbc_tty_write_room,
+	.chars_in_buffer	= dbc_tty_chars_in_buffer,
+	.unthrottle		= dbc_tty_unthrottle,
+};
+
+static struct tty_driver *dbc_tty_driver;
+
+int xhci_dbc_tty_register_driver(struct xhci_hcd *xhci)
+{
+	int			status;
+	struct xhci_dbc		*dbc = xhci->dbc;
+
+	dbc_tty_driver = tty_alloc_driver(1, TTY_DRIVER_REAL_RAW |
+					  TTY_DRIVER_DYNAMIC_DEV);
+	if (IS_ERR(dbc_tty_driver)) {
+		status = PTR_ERR(dbc_tty_driver);
+		dbc_tty_driver = NULL;
+		return status;
+	}
+
+	dbc_tty_driver->driver_name = "dbc_serial";
+	dbc_tty_driver->name = "ttyDBC";
+
+	dbc_tty_driver->type = TTY_DRIVER_TYPE_SERIAL;
+	dbc_tty_driver->subtype = SERIAL_TYPE_NORMAL;
+	dbc_tty_driver->init_termios = tty_std_termios;
+	dbc_tty_driver->init_termios.c_cflag =
+			B9600 | CS8 | CREAD | HUPCL | CLOCAL;
+	dbc_tty_driver->init_termios.c_ispeed = 9600;
+	dbc_tty_driver->init_termios.c_ospeed = 9600;
+	dbc_tty_driver->driver_state = &dbc->port;
+
+	tty_set_operations(dbc_tty_driver, &dbc_tty_ops);
+
+	status = tty_register_driver(dbc_tty_driver);
+	if (status) {
+		xhci_err(xhci,
+			 "can't register dbc tty driver, err %d\n", status);
+		put_tty_driver(dbc_tty_driver);
+		dbc_tty_driver = NULL;
+	}
+
+	return status;
+}
+
+void xhci_dbc_tty_unregister_driver(void)
+{
+	tty_unregister_driver(dbc_tty_driver);
+	put_tty_driver(dbc_tty_driver);
+	dbc_tty_driver = NULL;
+}
+
+static void dbc_rx_push(unsigned long _port)
+{
+	struct dbc_request	*req;
+	struct tty_struct	*tty;
+	bool			do_push = false;
+	bool			disconnect = false;
+	struct dbc_port		*port = (void *)_port;
+	struct list_head	*queue = &port->read_queue;
+
+	spin_lock_irq(&port->port_lock);
+	tty = port->port.tty;
+	while (!list_empty(queue)) {
+		req = list_first_entry(queue, struct dbc_request, list_pool);
+
+		if (tty && tty_throttled(tty))
+			break;
+
+		switch (req->status) {
+		case 0:
+			break;
+		case -ESHUTDOWN:
+			disconnect = true;
+			break;
+		default:
+			pr_warn("ttyDBC0: unexpected RX status %d\n",
+				req->status);
+			break;
+		}
+
+		if (req->actual) {
+			char		*packet = req->buf;
+			unsigned int	n, size = req->actual;
+			int		count;
+
+			n = port->n_read;
+			if (n) {
+				packet += n;
+				size -= n;
+			}
+
+			count = tty_insert_flip_string(&port->port, packet,
+						       size);
+			if (count)
+				do_push = true;
+			if (count != size) {
+				port->n_read += count;
+				break;
+			}
+			port->n_read = 0;
+		}
+
+		list_move(&req->list_pool, &port->read_pool);
+	}
+
+	if (do_push)
+		tty_flip_buffer_push(&port->port);
+
+	if (!list_empty(queue) && tty) {
+		if (!tty_throttled(tty)) {
+			if (do_push)
+				tasklet_schedule(&port->push);
+			else
+				pr_warn("ttyDBC0: RX not scheduled?\n");
+		}
+	}
+
+	if (!disconnect)
+		dbc_start_rx(port);
+
+	spin_unlock_irq(&port->port_lock);
+}
+
+static int dbc_port_activate(struct tty_port *_port, struct tty_struct *tty)
+{
+	struct dbc_port	*port = container_of(_port, struct dbc_port, port);
+
+	spin_lock_irq(&port->port_lock);
+	dbc_start_rx(port);
+	spin_unlock_irq(&port->port_lock);
+
+	return 0;
+}
+
+static const struct tty_port_operations dbc_port_ops = {
+	.activate =	dbc_port_activate,
+};
+
+static void
+xhci_dbc_tty_init_port(struct xhci_hcd *xhci, struct dbc_port *port)
+{
+	tty_port_init(&port->port);
+	spin_lock_init(&port->port_lock);
+	tasklet_init(&port->push, dbc_rx_push, (unsigned long)port);
+	INIT_LIST_HEAD(&port->read_pool);
+	INIT_LIST_HEAD(&port->read_queue);
+	INIT_LIST_HEAD(&port->write_pool);
+
+	port->in =		get_in_ep(xhci);
+	port->out =		get_out_ep(xhci);
+	port->port.ops =	&dbc_port_ops;
+	port->n_read =		0;
+}
+
+static void
+xhci_dbc_tty_exit_port(struct dbc_port *port)
+{
+	tasklet_kill(&port->push);
+	tty_port_destroy(&port->port);
+}
+
+int xhci_dbc_tty_register_device(struct xhci_hcd *xhci)
+{
+	int			ret;
+	struct device		*tty_dev;
+	struct xhci_dbc		*dbc = xhci->dbc;
+	struct dbc_port		*port = &dbc->port;
+
+	xhci_dbc_tty_init_port(xhci, port);
+	tty_dev = tty_port_register_device(&port->port,
+					   dbc_tty_driver, 0, NULL);
+	ret = IS_ERR_OR_NULL(tty_dev);
+	if (ret)
+		goto register_fail;
+
+	ret = kfifo_alloc(&port->write_fifo, DBC_WRITE_BUF_SIZE, GFP_KERNEL);
+	if (ret)
+		goto buf_alloc_fail;
+
+	ret = xhci_dbc_alloc_requests(port->in, &port->read_pool,
+				      dbc_read_complete);
+	if (ret)
+		goto request_fail;
+
+	ret = xhci_dbc_alloc_requests(port->out, &port->write_pool,
+				      dbc_write_complete);
+	if (ret)
+		goto request_fail;
+
+	port->registered = true;
+
+	return 0;
+
+request_fail:
+	xhci_dbc_free_requests(port->in, &port->read_pool);
+	xhci_dbc_free_requests(port->out, &port->write_pool);
+	kfifo_free(&port->write_fifo);
+
+buf_alloc_fail:
+	tty_unregister_device(dbc_tty_driver, 0);
+
+register_fail:
+	xhci_dbc_tty_exit_port(port);
+
+	xhci_err(xhci, "can't register tty port, err %d\n", ret);
+
+	return ret;
+}
+
+void xhci_dbc_tty_unregister_device(struct xhci_hcd *xhci)
+{
+	struct xhci_dbc		*dbc = xhci->dbc;
+	struct dbc_port		*port = &dbc->port;
+
+	tty_unregister_device(dbc_tty_driver, 0);
+	xhci_dbc_tty_exit_port(port);
+	port->registered = false;
+
+	kfifo_free(&port->write_fifo);
+	xhci_dbc_free_requests(get_out_ep(xhci), &port->read_pool);
+	xhci_dbc_free_requests(get_out_ep(xhci), &port->read_queue);
+	xhci_dbc_free_requests(get_in_ep(xhci), &port->write_pool);
+}
diff --git a/drivers/usb/host/xhci-trace.h b/drivers/usb/host/xhci-trace.h
index a3b57c781db1..67ff314a6f41 100644
--- a/drivers/usb/host/xhci-trace.h
+++ b/drivers/usb/host/xhci-trace.h
@@ -23,6 +23,7 @@
 
 #include <linux/tracepoint.h>
 #include "xhci.h"
+#include "xhci-dbgcap.h"
 
 #define XHCI_MSG_MAX	500
 
@@ -155,6 +156,21 @@ DEFINE_EVENT(xhci_log_trb, xhci_queue_trb,
 	TP_ARGS(ring, trb)
 );
 
+DEFINE_EVENT(xhci_log_trb, xhci_dbc_handle_event,
+	TP_PROTO(struct xhci_ring *ring, struct xhci_generic_trb *trb),
+	TP_ARGS(ring, trb)
+);
+
+DEFINE_EVENT(xhci_log_trb, xhci_dbc_handle_transfer,
+	TP_PROTO(struct xhci_ring *ring, struct xhci_generic_trb *trb),
+	TP_ARGS(ring, trb)
+);
+
+DEFINE_EVENT(xhci_log_trb, xhci_dbc_gadget_ep_queue,
+	TP_PROTO(struct xhci_ring *ring, struct xhci_generic_trb *trb),
+	TP_ARGS(ring, trb)
+);
+
 DECLARE_EVENT_CLASS(xhci_log_virt_dev,
 	TP_PROTO(struct xhci_virt_device *vdev),
 	TP_ARGS(vdev),
@@ -478,6 +494,49 @@ DEFINE_EVENT(xhci_log_portsc, xhci_handle_port_status,
 	     TP_ARGS(portnum, portsc)
 );
 
+DECLARE_EVENT_CLASS(xhci_dbc_log_request,
+	TP_PROTO(struct dbc_request *req),
+	TP_ARGS(req),
+	TP_STRUCT__entry(
+		__field(struct dbc_request *, req)
+		__field(bool, dir)
+		__field(unsigned int, actual)
+		__field(unsigned int, length)
+		__field(int, status)
+	),
+	TP_fast_assign(
+		__entry->req = req;
+		__entry->dir = req->direction;
+		__entry->actual = req->actual;
+		__entry->length = req->length;
+		__entry->status = req->status;
+	),
+	TP_printk("%s: req %p length %u/%u ==> %d",
+		__entry->dir ? "bulk-in" : "bulk-out",
+		__entry->req, __entry->actual,
+		__entry->length, __entry->status
+	)
+);
+
+DEFINE_EVENT(xhci_dbc_log_request, xhci_dbc_alloc_request,
+	TP_PROTO(struct dbc_request *req),
+	TP_ARGS(req)
+);
+
+DEFINE_EVENT(xhci_dbc_log_request, xhci_dbc_free_request,
+	TP_PROTO(struct dbc_request *req),
+	TP_ARGS(req)
+);
+
+DEFINE_EVENT(xhci_dbc_log_request, xhci_dbc_queue_request,
+	TP_PROTO(struct dbc_request *req),
+	TP_ARGS(req)
+);
+
+DEFINE_EVENT(xhci_dbc_log_request, xhci_dbc_giveback_request,
+	TP_PROTO(struct dbc_request *req),
+	TP_ARGS(req)
+);
 #endif /* __XHCI_TRACE_H */
 
 /* this part must be outside header guard */
diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
index f25b4ce31965..76bb0cb4aba5 100644
--- a/drivers/usb/host/xhci.c
+++ b/drivers/usb/host/xhci.c
@@ -21,6 +21,7 @@
 #include "xhci-trace.h"
 #include "xhci-mtk.h"
 #include "xhci-debugfs.h"
+#include "xhci-dbgcap.h"
 
 #define DRIVER_AUTHOR "Sarah Sharp"
 #define DRIVER_DESC "'eXtensible' Host Controller (xHC) Driver"
@@ -622,6 +623,8 @@ int xhci_run(struct usb_hcd *hcd)
 	xhci_dbg_trace(xhci, trace_xhci_dbg_init,
 			"Finished xhci_run for USB2 roothub");
 
+	xhci_dbc_init(xhci);
+
 	xhci_debugfs_init(xhci);
 
 	return 0;
@@ -654,6 +657,8 @@ static void xhci_stop(struct usb_hcd *hcd)
 
 	xhci_debugfs_exit(xhci);
 
+	xhci_dbc_exit(xhci);
+
 	spin_lock_irq(&xhci->lock);
 	xhci->xhc_state |= XHCI_STATE_HALTED;
 	xhci->cmd_ring_state = CMD_RING_STATE_STOPPED;
@@ -870,6 +875,8 @@ int xhci_suspend(struct xhci_hcd *xhci, bool do_wakeup)
 			xhci->shared_hcd->state != HC_STATE_SUSPENDED)
 		return -EINVAL;
 
+	xhci_dbc_suspend(xhci);
+
 	/* Clear root port wake on bits if wakeup not allowed. */
 	if (!do_wakeup)
 		xhci_disable_port_wake_on_bits(xhci);
@@ -1065,6 +1072,8 @@ int xhci_resume(struct xhci_hcd *xhci, bool hibernated)
 
 	spin_unlock_irq(&xhci->lock);
 
+	xhci_dbc_resume(xhci);
+
  done:
 	if (retval == 0) {
 		/* Resume root hubs only when have pending events. */
diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
index f1515500ed88..e613344f050a 100644
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -1856,6 +1856,7 @@ struct xhci_hcd {
 	struct dentry		*debugfs_slots;
 	struct list_head	regset_list;
 
+	void			*dbc;
 	/* platform-specific data -- must come last */
 	unsigned long		priv[0] __aligned(sizeof(s64));
 };