[PATCH v2 12/12] usb: doc: add document for xHCI DbC driver

From: Lu Baolu
Date: Tue Nov 03 2015 - 03:15:16 EST

Add Documentation/usb/xhci-dbc.txt. This document includes
development status and user guide for USB3 debug port.

Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Documentation/usb/xhci-dbc.txt | 325 +++++++++++++++++++++++++++++++++++++++++
drivers/usb/early/xhci-dbc.c | 3 +
3 files changed, 329 insertions(+)
create mode 100644 Documentation/usb/xhci-dbc.txt

diff --git a/Documentation/usb/xhci-dbc.txt b/Documentation/usb/xhci-dbc.txt
new file mode 100644
index 0000000..441b40c
--- /dev/null
+++ b/Documentation/usb/xhci-dbc.txt
@@ -0,0 +1,325 @@
+ xHCI debug capability driver
+ Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
+ Last-updated: September 2015
+ Contents:
+ ---------
+ * What is xHCI DbC?
+ * Debug topologies
+ * Debug stacks
+ * Port Multiplexing
+ * Hardware initialization
+ * External reset
+ * Port reset
+ * Interrupt/DMA/Memory during early boot
+ * Endpoint STALL
+ * Debug device information
+ * How to use DbC early printk?
+ * Limitations
+ What is xHCI DbC?
+ -----------------
+The xHCI Debugging Capability defined in section 7.6 of xHCI spec 1.1
+provides an optional functionality that enables low-level system debug
+over USB. It provides a means of connecting two systems where one system
+is a Debug Host and the other a Debug Target (System Under Test). The
+Debug Capability provides an interface that is completely independent
+of the xHCI interface. A Debug Target enumerates as a USB debug device
+to the Debug Host, allowing a Debug Host to access a Debug Target through
+the standard USB software stack.
+ Debug topologies
+ ----------------
+Multiple Debug Targets may be attached to a single Debug Host. Debug
+Targets may be connected to any downstream facing port below a Debug
+Host (i.e. anywhere in the fabric, root port or external hub puts).
+A Debug Target may only connect to a Debug Host through a Root Hub port
+of the target. That means connection of a Debug Target to a Debug Host
+through the ports of an external hub is not supported.
+Below is a typical connection between Debug Host and Debug target. Two
+Debug targets are connected to a single Debug host.
+ ________________ ________________
+ | Debug Host | | Debug Target |
+ |________________| |________________|
+ |xHC without DbC | | xHC with DbC |
+ |or DbC disabled | | enabled |
+ |________________| |________________|
+ |P1| |p2| |P1| |p2|
+ |__| |__| |__| |__|
+ | | |
+ | |_________________________|
+ |_
+ |
+ _______|________ ________________
+ | HUB | | Debug Target |
+ |________________| |________________|
+ | Superspeed hub | | xHC with DbC |
+ | | | enabled |
+ |________________| |________________|
+ |P1| |p2| |P1| |p2|
+ |__| |__| |__| |__|
+ | |
+ |_________________________|
+ Debug stacks
+ ------------
+Below is a software stack diagram of both Debug Host and Debug Target.
+ ________________ ________________
+ | Debug Host | | Debug Target |
+ |________________| |________________|
+ | debug App | | |
+ | | | system debug |
+ | usb_debug | | hooks |
+ | | | |
+ | usbcore | | |
+ | | |debug capability|
+ | xhci_hcd | | driver |
+ |________________| |________________|
+ |xHC without DbC | | xHC with DbC |
+ |or DbC disabled | | enabled |
+ |________________| |________________|
+ |P1| |p2| |P1| |p2|
+ |__| |__| |__| |__|
+ | |
+ |_________________________|
+ Port Multiplexing
+ -----------------
+A debug port is always multiplexed with the first xHCI root hub port.
+Whenever debug capability is supported and enabled, and the first root
+hub port is detected to be connected to a downstream super-speed port
+of a Debug Host, the root hub port is assigned to the debug capability
+and operating in an upstream facing mode. Otherwise, all root hub ports
+act as normal downstream facing ports. When the root port is assigned
+to debug capability, it appears through the xHCI as a fully functional
+root hub port that never sees a device attach.
+ Hardware initialization
+ -----------------------
+xHCI debug capability is initialized during early boot. "early_param"
+micro provides one option. It also provides a global function so that
+early code can call it to initialize the hardware.
+int __init early_xdbc_init(char *s)
+early_param("string", early_xdbc_init)
+Keep in mind that things such as interrupt, system memory, DMA memory,
+PCI configure space access, etc., are all different from a normal device
+Software probes the debug capability and all its memory and register
+interfaces by walking through the xHCI extended capability List. Debug
+capability is implemented with the capability ID of 10.
+After a xHCI debug capability interface was detected, software could
+initialize the hardware so that debug host can enumerate it as a debug
+device. The USB debug device provided by xHC debug capacity experiences
+three modes: disabled mode, enumeration mode and run mode. In each mode,
+software and xHC itself should complete required steps to move it to the
+next mode. Below diagram describes the evolution of each mode.
+ __________ ___________ ___________
+ | | | | | |
+ | | | | | |
+ | Disabled | DbC enable|Enumeration| DbC run | Run |
+ | mode |---------->| mode |---------->| mode |
+ | |flag 0 to 1| |flag 0 to 1| |
+ | | | | | |
+ | | | | | |
+ |__________| |___________| |___________|
+To change DbC mode from disabled to enumeration, software should
+1) allocate and initialize all DbC memory data structures. All data
+structures required by a DbC are defined in 7.6 of the spec 1.1;
+2) initialize the registers. All registers required to be initialized
+in disabled mode are defined in of the spec 1.1. With data
+structure and registers initialized, software can Set the debug
+capability enable (DCE) bit to 1 in the Debug Capability Control
+Register (DCCTRL) and DbC enters enumeration mode as soon as this
+bit is set.
+DbC hardware is responsible to change DbC mode from enumerate to run.
+As soon as DbC enters enumeration mode, DbC appears as a normal USB
+device which can be enumerated by debug host. Hence, DbC hardware
+should support a default control endpoint, which responds to standard
+After DbC has been configure by debug host, it enters run mode, it's
+the working mode.
+ External reset
+ --------------
+External reset means DbC reset caused by something outside of DbC hardware
+and software. The external reset sources depends on the DbC System Bus
+Reset(SBR) bit in status register.
+If SBR reads 1, the reset source includes,
+1) Assertion of chip hardware reset;
+2) System bus reset (e.g. the assertion of PCI RST#);
+3) Transition from the PCI PM D3hot to D0.
+If SBR reads 0, the reset source includes,
+1) Assertion of chip hardware reset;
+2) Assertion of host controller reset;
+3) Light host controller Reset.
+Resetting the DbC shall clear Debug Capability Enable(DCE) bit to 0.
+Software can determing the reset event during runtime by checking the
+DCE bit. The debug device enters disabled mode after reset. DbC driver
+needs to follow above steps to re-initialize the hardware and bring
+the debug device to run state.
+ Port reset
+ ----------
+Debug port resets itself when it detects reset signal from the debug
+host. Software can determine the port reset event by reading the DbC
+port control and staus registers.
+The debug device enters enumeration state as soon as the reset signal
+completes. DbC driver should follow above steps to bring the debug
+device into running mode.
+ Interrupt/DMA/Memory during early boot
+ --------------------------------------
+The driver code needs to take special care during early boot, especially
+when it comes to memory allocation, interrupt, DMA, device MMIO and PCI
+configuration space. This section defines the interfaces used in DbC driver
+for the prior aspects.
+All events generated by DbC are put in the event ring, software will
+periodically poll the Event Ring Not Empty bit in the Debug Capability
+Status Register (DCST) to check pending events. To do this, DbC driver
+should 1)poll the event ring after a transfer trb queued and wait until
+transfer completes, or 2) start a thread to do the periodically poll.
+A segment of fixed virtual address is reserved for MMIO access purpose.
+Debug capability driver will map the MMIO physical address (exposed in
+PCI BAR) with this fixed virtual address segment.
+Debug Capability needs contiguous memory for DMA purpose. The driver
+reserves DMA memory by stating arrays of PAGE_SIZE and ask the compiler
+to align the arrays to PAGE_SIZE.
+Base on the following reasons, the driver uses an outb to port 0x80 as
+an I/O delay: timer subsystem might not be ready yet when DbC starts
+to initialize; DbC driver is independent of OS as possible as it can.
+ Endpoint STALL
+ --------------
+Endpoint STALL happens when data buffer error, parameter Error,
+TRB error, vendor defined error, or undefined error is detected.
+A transfer event will be generated to notify software.
+To clear the STALL situation, debug host will send a ClearFeature
+(ENDPOINT_HALT) request. DbC will clear the halt transfer ring
+flag, clear any internal endpoint state, and move the TR dequeue
+pointer to the next TRB in transfer ring.
+DbC does not support Soft Retry. Driver must check and determine
+whether to retry the failed transfer.
+ Debug device information
+ ------------------------
+When debug target boots with xHCI debug capability enabled, it appears
+to debug host as a debug device. The debug device is built using one
+interface which declares two bulk endpoints: an IN and an OUT.
+class code: 0xdc (diagnostic device, assigned by USB-IF)
+subclass code: 0x02 (debug device, assigned by USB-IF)
+Manufacturer: "Linux"
+Product: "Remote GDB"
+Serial: "0001"
+The USB device ID:
+idVendor: 0x1d6b (Linux Foundation)
+idProduct: 0x0004
+ How to use DbC early printk?
+ ----------------------------
+Before using any kernel functionalities based on DbC, users need to check
+whether debug port is supported by the xHCI host in the system.
+On a machine which supports USB3 debug port, a file named "debug_port_state"
+will be created under /sys/bus/pci/drivers/xhci_hcd/<pci_bus_name>/. Reading
+this file will show the state (disabled, enabled or configured) of the debug
+port. On a machine that doesn't support USB3 debug port, this file doesn't
+On debug target system, user needs to enable the following kernel
+config option:
+Users also need to add below kernel parameter:
+ "earlyprintk=xdbc"
+If there are multiple xHCI controllers in the system, user can append
+a host contoller index to this kernel parameter. The index is started
+from 0.
+On debug host side, user needs to make sure usb_debug module is included.
+On some platforms, such as Intel, you need to disable auto-pm of usb
+subsystem on debug host when you are debugging with DbC.
+#echo on | tee /sys/bus/usb/devices/*/power/control
+Before starting the debug target, user needs to connect the debug port
+on debug target with a root port or port of external hub on the debug
+host. The cable used to connect these two ports should be a USB 3.0
+super-speed A-to-A debugging cable.
+During early boot of debug target, DbC hardware gets initialized. Debug
+host should be able to enuerate debug target as a debug device. Debug
+host will bind the debug device with usb_debug driver module and create
+a tty file for serial communication application.
+After tty file being created, user needs to open serial communication
+application, such as minicom. After configuring minicom to open the
+tty file created above, user should be able to see below welcome message
+in minicom:
+Press Y to continue...
+After 'Y' key is pressed, debug target will go ahead with booting and
+all early printk messages should be routed to the minicom on debug
+ Limitations
+ -----------
+Early printk through DbC has been verified to work with Intel Sunrise Point
+chip with below known issues:
+1. DbC debug device doesn't support suspend/resume. Users need to disable
+auto-pm of the host controller on debug host.
+2. Early prink "keep" option doesn't support by current phase.
+3. After several restarts of debug target, debug host might fail to read
+the device descriptor of debug device. Users need to restart the debug host.
index 585a369..eb48df1 100644
@@ -11046,6 +11046,7 @@ L: linux-usb@xxxxxxxxxxxxxxx
S: Supported
F: drivers/usb/early/xhci-dbc.c
F: include/linux/usb/xhci-dbc.h
+F: Documentation/usb/xhci-dbc.txt

L: linux-wireless@xxxxxxxxxxxxxxx
diff --git a/drivers/usb/early/xhci-dbc.c b/drivers/usb/early/xhci-dbc.c
index b75c523..676fb41 100644
--- a/drivers/usb/early/xhci-dbc.c
+++ b/drivers/usb/early/xhci-dbc.c
@@ -6,6 +6,9 @@
* Author: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
* Some code shared with EHCI debug port and xHCI driver.
+ * Please read Documentation/usb/xhci-dbc.txt before you start to develop
+ * or use code in this file.
+ *
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.

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