diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile
index b444f2e..bec0665 100644
--- a/Documentation/DocBook/Makefile
+++ b/Documentation/DocBook/Makefile
@@ -14,7 +14,8 @@
 	    genericirq.xml s390-drivers.xml uio-howto.xml scsi.xml \
 	    80211.xml debugobjects.xml sh.xml regulator.xml \
 	    alsa-driver-api.xml writing-an-alsa-driver.xml \
-	    tracepoint.xml drm.xml media_api.xml w1.xml
+	    tracepoint.xml drm.xml media_api.xml w1.xml \
+	    writing_musb_glue_layer.xml
 
 include Documentation/DocBook/media/Makefile
 
diff --git a/Documentation/DocBook/writing_musb_glue_layer.tmpl b/Documentation/DocBook/writing_musb_glue_layer.tmpl
new file mode 100644
index 0000000..837eca7
--- /dev/null
+++ b/Documentation/DocBook/writing_musb_glue_layer.tmpl
@@ -0,0 +1,873 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="Writing-MUSB-Glue-Layer">
+ <bookinfo>
+  <title>Writing an MUSB Glue Layer</title>
+
+  <authorgroup>
+   <author>
+    <firstname>Apelete</firstname>
+    <surname>Seketeli</surname>
+    <affiliation>
+     <address>
+      <email>apelete at seketeli.net</email>
+     </address>
+    </affiliation>
+   </author>
+  </authorgroup>
+
+  <copyright>
+   <year>2014</year>
+   <holder>Apelete Seketeli</holder>
+  </copyright>
+
+  <legalnotice>
+   <para>
+     This documentation is free software; you can redistribute it
+     and/or modify it under the terms of the GNU General Public
+     License as published by the Free Software Foundation; either
+     version 2 of the License, or (at your option) any later version.
+   </para>
+
+   <para>
+     This documentation is distributed in the hope that it will be
+     useful, but WITHOUT ANY WARRANTY; without even the implied
+     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+     See the GNU General Public License for more details.
+   </para>
+
+   <para>
+     You should have received a copy of the GNU General Public License
+     along with this documentation; if not, write to the Free Software
+     Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
+     02111-1307 USA
+   </para>
+
+   <para>
+     For more details see the file COPYING in the Linux kernel source
+     tree.
+   </para>
+  </legalnotice>
+ </bookinfo>
+
+<toc></toc>
+
+  <chapter id="introduction">
+    <title>Introduction</title>
+    <para>
+      The Linux MUSB subsystem is part of the larger Linux USB
+      subsystem. It provides support for embedded USB Device Controllers
+      (UDC) that do not use Universal Host Controller Interface (UHCI)
+      or Open Host Controller Interface (OHCI).
+    </para>
+    <para>
+      Instead, these embedded UDC rely on the USB On-the-Go (OTG)
+      specification which they implement at least partially. The silicon
+      reference design used in most cases is the Multipoint USB
+      Highspeed Dual-Role Controller (MUSB HDRC) found in the Mentor
+      Graphics Inventra™ design.
+    </para>
+    <para>
+      As a self-taught exercise I have written an MUSB glue layer for
+      the Ingenic JZ4740 SoC, modelled after the many MUSB glue layers
+      in the kernel source tree. This layer can be found at
+      drivers/usb/musb/jz4740.c. In this documentation I will walk
+      through the basics of the jz4740.c glue layer, explaining the
+      different pieces and what needs to be done in order to write your
+      own device glue layer.
+    </para>
+  </chapter>
+
+  <chapter id="linux-musb-basics">
+    <title>Linux MUSB Basics</title>
+    <para>
+      To get started on the topic, please read USB On-the-Go Basics (see
+      Resources) which provides an introduction of USB OTG operation at
+      the hardware level. A couple of wiki pages by Texas Instruments
+      and Analog Devices also provide an overview of the Linux kernel
+      MUSB configuration, albeit focused on some specific devices
+      provided by these companies. Finally, getting acquainted with the
+      USB specification at USB home page may come in handy, with
+      practical instance provided through the Writing USB Device Drivers
+      documentation (again, see Resources).
+    </para>
+    <para>
+      Linux USB stack is a layered architecture in which the MUSB
+      controller hardware sits at the lowest. The MUSB controller driver
+      abstract the MUSB controller hardware to the Linux USB stack.
+    </para>
+    <programlisting>
+      ------------------------
+      |                      | &lt;------- drivers/usb/gadget
+      | Linux USB Core Stack | &lt;------- drivers/usb/host
+      |                      | &lt;------- drivers/usb/core
+      ------------------------
+                 ⬍
+     --------------------------
+     |                        | &lt;------ drivers/usb/musb/musb_gadget.c
+     | MUSB Controller driver | &lt;------ drivers/usb/musb/musb_host.c
+     |                        | &lt;------ drivers/usb/musb/musb_core.c
+     --------------------------
+                 ⬍
+  ---------------------------------
+  | MUSB Platform Specific Driver |
+  |                               | &lt;-- drivers/usb/musb/jz4740.c
+  |       aka &quot;Glue Layer&quot;        |
+  ---------------------------------
+                 ⬍
+  ---------------------------------
+  |   MUSB Controller Hardware    |
+  ---------------------------------
+    </programlisting>
+    <para>
+      As outlined above, the glue layer is actually the platform
+      specific code sitting in between the controller driver and the
+      controller hardware.
+    </para>
+    <para>
+      Just like a Linux USB driver needs to register itself with the
+      Linux USB subsystem, the MUSB glue layer needs first to register
+      itself with the MUSB controller driver. This will allow the
+      controller driver to know about which device the glue layer
+      supports and which functions to call when a supported device is
+      detected or released; remember we are talking about an embedded
+      controller chip here, so no insertion or removal at run-time.
+    </para>
+    <para>
+      All of this information is passed to the MUSB controller driver
+      through a platform_driver structure defined in the glue layer as:
+    </para>
+    <programlisting linenumbering="numbered">
+static struct platform_driver jz4740_driver = {
+	.probe		= jz4740_probe,
+	.remove		= jz4740_remove,
+	.driver		= {
+		.name	= "musb-jz4740",
+	},
+};
+    </programlisting>
+    <para>
+      The probe and remove function pointers are called when a matching
+      device is detected and, respectively, released. The name string
+      describes the device supported by this glue layer. In the current
+      case it matches a platform_device structure declared in
+      arch/mips/jz4740/platform.c. Note that we are not using device
+      tree bindings here.
+    </para>
+    <para>
+      In order to register itself to the controller driver, the glue
+      layer goes through a few steps, basically allocating the
+      controller hardware resources and initialising a couple of
+      circuits. To do so, it needs to keep track of the information used
+      throughout these steps. This is done by defining a private
+      jz4740_glue structure:
+    </para>
+    <programlisting linenumbering="numbered">
+struct jz4740_glue {
+	struct device           *dev;
+	struct platform_device  *musb;
+	struct clk		*clk;
+};
+    </programlisting>
+    <para>
+      The dev and musb members are both device structure variables. The
+      first one holds generic information about the device, since it's
+      the basic device structure, and the latter holds information more
+      closely related to the subsystem the device is registered to. The
+      clk variable keeps information related to the device clock
+      operation.
+    </para>
+    <para>
+      Let's go through the steps of the probe function that leads the
+      glue layer to register itself to the controller driver.
+    </para>
+    <para>
+      N.B.: For the sake of readability each function will be split in
+      logical parts, each part being shown as if it was independent from
+      the others.
+    </para>
+    <programlisting linenumbering="numbered">
+static int jz4740_probe(struct platform_device *pdev)
+{
+	struct platform_device		*musb;
+	struct jz4740_glue		*glue;
+	struct clk                      *clk;
+	int				ret;
+
+	glue = devm_kzalloc(&amp;pdev->dev, sizeof(*glue), GFP_KERNEL);
+	if (!glue)
+		return -ENOMEM;
+
+	musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO);
+	if (!musb) {
+		dev_err(&amp;pdev->dev, "failed to allocate musb device\n");
+		return -ENOMEM;
+	}
+
+	clk = devm_clk_get(&amp;pdev->dev, "udc");
+	if (IS_ERR(clk)) {
+		dev_err(&amp;pdev->dev, "failed to get clock\n");
+		ret = PTR_ERR(clk);
+		goto err_platform_device_put;
+	}
+
+	ret = clk_prepare_enable(clk);
+	if (ret) {
+		dev_err(&amp;pdev->dev, "failed to enable clock\n");
+		goto err_platform_device_put;
+	}
+
+	musb->dev.parent		= &amp;pdev->dev;
+
+	glue->dev			= &amp;pdev->dev;
+	glue->musb			= musb;
+	glue->clk			= clk;
+
+	return 0;
+
+err_platform_device_put:
+	platform_device_put(musb);
+	return ret;
+}
+    </programlisting>
+    <para>
+      The first few lines of the probe function allocate and assign the
+      glue, musb and clk variables. The GFP_KERNEL flag (line 8) allows
+      the allocation process to sleep and wait for memory, thus being
+      usable in a blocking situation. The PLATFORM_DEVID_AUTO flag (line
+      12) allows automatic allocation and management of device IDs in
+      order to avoid device namespace collisions with explicit IDs. With
+      devm_clk_get() (line 18) the glue layer allocates the clock -- the
+      <literal>devm_</literal> prefix indicates that clk_get() is
+      managed: it automatically frees the allocated clock resource data
+      when the device is released -- and enable it.
+    </para>
+    <para>
+      Then comes the registration steps:
+    </para>
+    <programlisting linenumbering="numbered">
+static int jz4740_probe(struct platform_device *pdev)
+{
+	struct musb_hdrc_platform_data	*pdata = &amp;jz4740_musb_platform_data;
+
+	pdata->platform_ops		= &amp;jz4740_musb_ops;
+
+	platform_set_drvdata(pdev, glue);
+
+	ret = platform_device_add_resources(musb, pdev->resource,
+					    pdev->num_resources);
+	if (ret) {
+		dev_err(&amp;pdev->dev, "failed to add resources\n");
+		goto err_clk_disable;
+	}
+
+	ret = platform_device_add_data(musb, pdata, sizeof(*pdata));
+	if (ret) {
+		dev_err(&amp;pdev->dev, "failed to add platform_data\n");
+		goto err_clk_disable;
+	}
+
+	return 0;
+
+err_clk_disable:
+	clk_disable_unprepare(clk);
+err_platform_device_put:
+	platform_device_put(musb);
+	return ret;
+}
+    </programlisting>
+    <para>
+      The first step is to pass the device data privately held by the
+      glue layer on to the controller driver through
+      platform_set_drvdata() (line 7). Next is passing on the device
+      resources information, also privately held at that point, through
+      platform_device_add_resources() (line 9).
+    </para>
+    <para>
+      Finally comes passing on the platform specific data to the
+      controller driver (line 16). Platform data will be discussed in
+      <link linkend="device-platform-data">Chapter 4</link>, but here
+      we are looking at the platform_ops function pointer (line 5) in
+      musb_hdrc_platform_data structure (line 3).  This function
+      pointer allows the MUSB controller driver to know which function
+      to call for device operation:
+    </para>
+    <programlisting linenumbering="numbered">
+static const struct musb_platform_ops jz4740_musb_ops = {
+	.init		= jz4740_musb_init,
+	.exit		= jz4740_musb_exit,
+};
+    </programlisting>
+    <para>
+      Here we have the minimal case where only init and exit functions
+      are called by the controller driver when needed. Fact is the
+      JZ4740 MUSB controller is a basic controller, lacking some
+      features found in other controllers, otherwise we may also have
+      pointers to a few other functions like a power management function
+      or a function to switch between OTG and non-OTG modes, for
+      instance.
+    </para>
+    <para>
+      At that point of the registration process, the controller driver
+      actually calls the init function:
+    </para>
+    <programlisting linenumbering="numbered">
+static int jz4740_musb_init(struct musb *musb)
+{
+	musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
+	if (!musb->xceiv) {
+		pr_err("HS UDC: no transceiver configured\n");
+		return -ENODEV;
+	}
+
+	/* Silicon does not implement ConfigData register.
+	 * Set dyn_fifo to avoid reading EP config from hardware.
+	 */
+	musb->dyn_fifo = true;
+
+	musb->isr = jz4740_musb_interrupt;
+
+	return 0;
+}
+    </programlisting>
+    <para>
+      The goal of jz4740_musb_init() is to get hold of the transceiver
+      driver data of the MUSB controller hardware and pass it on to the
+      MUSB controller driver, as usual. The transceiver is the circuitry
+      inside the controller hardware responsible for sending/receiving
+      the USB data. Since it is an implementation of the physical layer
+      of the OSI model, the transceiver is also referred to as PHY.
+    </para>
+    <para>
+      Getting hold of the MUSB PHY driver data is done with
+      usb_get_phy() which returns a pointer to the structure
+      containing the driver instance data. The next couple of
+      instructions (line 12 and 14) are used as a quirk and to setup
+      IRQ handling respectively. Quirks and IRQ handling will be
+      discussed later in <link linkend="device-quirks">Chapter
+      5</link> and <link linkend="handling-irqs">Chapter 3</link>.
+    </para>
+    <programlisting linenumbering="numbered">
+static int jz4740_musb_exit(struct musb *musb)
+{
+	usb_put_phy(musb->xceiv);
+
+	return 0;
+}
+    </programlisting>
+    <para>
+      Acting as the counterpart of init, the exit function releases the
+      MUSB PHY driver when the controller hardware itself is about to be
+      released.
+    </para>
+    <para>
+      Again, note that init and exit are fairly simple in this case due
+      to the basic set of features of the JZ4740 controller hardware.
+      When writing an musb glue layer for a more complex controller
+      hardware, you might need to take care of more processing in those
+      two functions.
+    </para>
+    <para>
+      Returning from the init function, the MUSB controller driver jumps
+      back into the probe function:
+    </para>
+    <programlisting linenumbering="numbered">
+static int jz4740_probe(struct platform_device *pdev)
+{
+	ret = platform_device_add(musb);
+	if (ret) {
+		dev_err(&amp;pdev->dev, "failed to register musb device\n");
+		goto err_clk_disable;
+	}
+
+	return 0;
+
+err_clk_disable:
+	clk_disable_unprepare(clk);
+err_platform_device_put:
+	platform_device_put(musb);
+	return ret;
+}
+    </programlisting>
+    <para>
+      This is the last part of the device registration process where the
+      glue layer adds the controller hardware device to Linux kernel
+      device hierarchy: at this stage, all known information about the
+      device is passed on to the Linux USB core stack.
+    </para>
+    <programlisting linenumbering="numbered">
+static int jz4740_remove(struct platform_device *pdev)
+{
+	struct jz4740_glue	*glue = platform_get_drvdata(pdev);
+
+	platform_device_unregister(glue->musb);
+	clk_disable_unprepare(glue->clk);
+
+	return 0;
+}
+    </programlisting>
+    <para>
+      Acting as the counterpart of probe, the remove function unregister
+      the MUSB controller hardware (line 5) and disable the clock (line
+      6), allowing it to be gated.
+    </para>
+  </chapter>
+
+  <chapter id="handling-irqs">
+    <title>Handling IRQs</title>
+    <para>
+      Additionally to the MUSB controller hardware basic setup and
+      registration, the glue layer is also responsible for handling the
+      IRQs:
+    </para>
+    <programlisting linenumbering="numbered">
+static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
+{
+	unsigned long   flags;
+	irqreturn_t     retval = IRQ_NONE;
+	struct musb     *musb = __hci;
+
+	spin_lock_irqsave(&amp;musb->lock, flags);
+
+	musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
+	musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
+	musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
+
+	/*
+	 * The controller is gadget only, the state of the host mode IRQ bits is
+	 * undefined. Mask them to make sure that the musb driver core will
+	 * never see them set
+	 */
+	musb->int_usb &amp;= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
+	    MUSB_INTR_RESET | MUSB_INTR_SOF;
+
+	if (musb->int_usb || musb->int_tx || musb->int_rx)
+		retval = musb_interrupt(musb);
+
+	spin_unlock_irqrestore(&amp;musb->lock, flags);
+
+	return retval;
+}
+    </programlisting>
+    <para>
+      Here the glue layer mostly has to read the relevant hardware
+      registers and pass their values on to the controller driver which
+      will handle the actual event that triggered the IRQ.
+    </para>
+    <para>
+      The interrupt handler critical section is protected by the
+      spin_lock_irqsave() and counterpart spin_unlock_irqrestore()
+      functions (line 7 and 24 respectively), which prevent the
+      interrupt handler code to be run by two different threads at the
+      same time.
+    </para>
+    <para>
+      Then the relevant interrupt registers are read (line 9 to 11):
+    </para>
+    <itemizedlist>
+      <listitem>
+        <para>
+          MUSB_INTRUSB: indicates which USB interrupts are currently
+          active,
+        </para>
+      </listitem>
+      <listitem>
+        <para>
+          MUSB_INTRTX: indicates which of the interrupts for TX
+          endpoints are currently active,
+        </para>
+      </listitem>
+      <listitem>
+        <para>
+          MUSB_INTRRX: indicates which of the interrupts for TX
+          endpoints are currently active.
+        </para>
+      </listitem>
+    </itemizedlist>
+    <para>
+      Note that musb_readb() is used to read 8-bit registers at most,
+      while musb_readw() allows us to read at most 16-bit registers.
+      There are other functions that can be used depending on the size
+      of your device registers. See musb_io.h for more information.
+    </para>
+    <para>
+      Instruction on line 18 is another quirk specific to the JZ4740
+      USB device controller, which will be discussed later in <link
+      linkend="device-quirks">Chapter 5</link>.
+    </para>
+    <para>
+      The glue layer still needs to register the IRQ handler though.
+      Remember the instruction on line 14 of the init function:
+    </para>
+    <programlisting linenumbering="numbered">
+static int jz4740_musb_init(struct musb *musb)
+{
+	musb->isr = jz4740_musb_interrupt;
+
+	return 0;
+}
+    </programlisting>
+    <para>
+      This instruction sets a pointer to the glue layer IRQ handler
+      function, in order for the controller hardware to call the handler
+      back when an IRQ comes from the controller hardware. The interrupt
+      handler is now implemented and registered.
+    </para>
+  </chapter>
+
+  <chapter id="device-platform-data">
+    <title>Device Platform Data</title>
+    <para>
+      In order to write an MUSB glue layer, you need to have some data
+      describing the hardware capabilities of your controller hardware,
+      which is called the platform data.
+    </para>
+    <para>
+      Platform data is specific to your hardware, though it may cover a
+      broad range of devices, and is generally found somewhere in the
+      arch/ directory, depending on your device architecture.
+    </para>
+    <para>
+      For instance, platform data for the JZ4740 SoC is found in
+      arch/mips/jz4740/platform.c. In the platform.c file each device of
+      the JZ4740 SoC is described through a set of structures.
+    </para>
+    <para>
+      Here is the part of arch/mips/jz4740/platform.c that covers the
+      USB Device Controller (UDC):
+    </para>
+    <programlisting linenumbering="numbered">
+/* USB Device Controller */
+struct platform_device jz4740_udc_xceiv_device = {
+	.name = "usb_phy_gen_xceiv",
+	.id   = 0,
+};
+
+static struct resource jz4740_udc_resources[] = {
+	[0] = {
+		.start = JZ4740_UDC_BASE_ADDR,
+		.end   = JZ4740_UDC_BASE_ADDR + 0x10000 - 1,
+		.flags = IORESOURCE_MEM,
+	},
+	[1] = {
+		.start = JZ4740_IRQ_UDC,
+		.end   = JZ4740_IRQ_UDC,
+		.flags = IORESOURCE_IRQ,
+		.name  = "mc",
+	},
+};
+
+struct platform_device jz4740_udc_device = {
+	.name = "musb-jz4740",
+	.id   = -1,
+	.dev  = {
+		.dma_mask          = &amp;jz4740_udc_device.dev.coherent_dma_mask,
+		.coherent_dma_mask = DMA_BIT_MASK(32),
+	},
+	.num_resources = ARRAY_SIZE(jz4740_udc_resources),
+	.resource      = jz4740_udc_resources,
+};
+    </programlisting>
+    <para>
+      The jz4740_udc_xceiv_device platform device structure (line 2)
+      describes the UDC transceiver with a name and id number.
+    </para>
+    <para>
+      At the time of this writing, note that
+      &quot;usb_phy_gen_xceiv&quot; is the specific name to be used for
+      all transceivers that are either built-in with reference USB IP or
+      autonomous and doesn't require any PHY programming. You will need
+      to set CONFIG_NOP_USB_XCEIV=y in the kernel configuration to make
+      use of the corresponding transceiver driver. The id field could be
+      set to -1 (equivalent to PLATFORM_DEVID_NONE), -2 (equivalent to
+      PLATFORM_DEVID_AUTO) or start with 0 for the first device of this
+      kind if we want a specific id number.
+    </para>
+    <para>
+      The jz4740_udc_resources resource structure (line 7) defines the
+      UDC registers base addresses.
+    </para>
+    <para>
+      The first array (line 9 to 11) defines the UDC registers base
+      memory addresses: start points to the first register memory
+      address, end points to the last register memory address and the
+      flags member defines the type of resource we are dealing with. So
+      IORESOURCE_MEM is used to define the registers memory addresses.
+      The second array (line 14 to 17) defines the UDC IRQ registers
+      addresses. Since there is only one IRQ register available for the
+      JZ4740 UDC, start and end point at the same address. The
+      IORESOURCE_IRQ flag tells that we are dealing with IRQ resources,
+      and the name &quot;mc&quot; is in fact hard-coded in the MUSB core
+      in order for the controller driver to retrieve this IRQ resource
+      by querying it by its name.
+    </para>
+    <para>
+      Finally, the jz4740_udc_device platform device structure (line 21)
+      describes the UDC itself.
+    </para>
+    <para>
+      The &quot;musb-jz4740&quot; name (line 22) defines the MUSB
+      driver that is used for this device; remember this is in fact
+      the name that we used in the jz4740_driver platform driver
+      structure in <link linkend="linux-musb-basics">Chapter
+      2</link>. The id field (line 23) is set to -1 (equivalent to
+      PLATFORM_DEVID_NONE) since we do not need an id for the device:
+      the MUSB controller driver was already set to allocate an
+      automatic id in <link linkend="linux-musb-basics">Chapter
+      2</link>. In the dev field we care for DMA related information
+      here. The dma_mask field (line 25) defines the width of the DMA
+      mask that is going to be used, and coherent_dma_mask (line 26)
+      has the same purpose but for the alloc_coherent DMA mappings: in
+      both cases we are using a 32 bits mask. Then the resource field
+      (line 29) is simply a pointer to the resource structure defined
+      before, while the num_resources field (line 28) keeps track of
+      the number of arrays defined in the resource structure (in this
+      case there were two resource arrays defined before).
+    </para>
+    <para>
+      With this quick overview of the UDC platform data at the arch/
+      level now done, let's get back to the MUSB glue layer specific
+      platform data in drivers/usb/musb/jz4740.c:
+    </para>
+    <programlisting linenumbering="numbered">
+static struct musb_hdrc_config jz4740_musb_config = {
+	/* Silicon does not implement USB OTG. */
+	.multipoint = 0,
+	/* Max EPs scanned, driver will decide which EP can be used. */
+	.num_eps    = 4,
+	/* RAMbits needed to configure EPs from table */
+	.ram_bits   = 9,
+	.fifo_cfg = jz4740_musb_fifo_cfg,
+	.fifo_cfg_size = ARRAY_SIZE(jz4740_musb_fifo_cfg),
+};
+
+static struct musb_hdrc_platform_data jz4740_musb_platform_data = {
+	.mode   = MUSB_PERIPHERAL,
+	.config = &amp;jz4740_musb_config,
+};
+    </programlisting>
+    <para>
+      First the glue layer configures some aspects of the controller
+      driver operation related to the controller hardware specifics.
+      This is done through the jz4740_musb_config musb_hdrc_config
+      structure.
+    </para>
+    <para>
+      Defining the OTG capability of the controller hardware, the
+      multipoint member (line 3) is set to 0 (equivalent to false)
+      since the JZ4740 UDC is not OTG compatible. Then num_eps (line
+      5) defines the number of USB endpoints of the controller
+      hardware, including endpoint 0: here we have 3 endpoints +
+      endpoint 0. Next is ram_bits (line 7) which is the width of the
+      RAM address bus for the MUSB controller hardware. This
+      information is needed when the controller driver cannot
+      automatically configure endpoints by reading the relevant
+      controller hardware registers. This issue will be discussed when
+      we get to device quirks in <link linkend="device-quirks">Chapter
+      5</link>. Last two fields (line 8 and 9) are also about device
+      quirks: fifo_cfg points to the USB endpoints configuration table
+      and fifo_cfg_size keeps track of the size of the number of
+      entries in that configuration table. More on that later in <link
+      linkend="device-quirks">Chapter 5</link>.
+    </para>
+    <para>
+      Then this configuration is embedded inside
+      jz4740_musb_platform_data musb_hdrc_platform_data structure (line
+      11): config is a pointer to the configuration structure itself,
+      and mode tells the controller driver if the controller hardware
+      may be used as MUSB_HOST only, MUSB_PERIPHERAL only or MUSB_OTG
+      which is a dual mode.
+    </para>
+    <para>
+      Remember that jz4740_musb_platform_data is then used to convey
+      platform data information as we have seen in the probe function
+      in <link linkend="linux-musb-basics">Chapter 2</link>
+    </para>
+  </chapter>
+
+  <chapter id="device-quirks">
+    <title>Device Quirks</title>
+    <para>
+      Completing the platform data specific to your device, you may also
+      need to write some code in the glue layer to work around some
+      device specific limitations. These quirks may be due to some
+      hardware bugs, or simply be the result of an incomplete
+      implementation of the USB On-the-Go specification.
+    </para>
+    <para>
+      The JZ4740 UDC exhibits such quirks, some of which we will discuss
+      here for the sake of insight even though these might not be found
+      in the controller hardware you are working on.
+    </para>
+    <para>
+      Let's get back to the init function first:
+    </para>
+    <programlisting linenumbering="numbered">
+static int jz4740_musb_init(struct musb *musb)
+{
+	musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
+	if (!musb->xceiv) {
+		pr_err("HS UDC: no transceiver configured\n");
+		return -ENODEV;
+	}
+
+	/* Silicon does not implement ConfigData register.
+	 * Set dyn_fifo to avoid reading EP config from hardware.
+	 */
+	musb->dyn_fifo = true;
+
+	musb->isr = jz4740_musb_interrupt;
+
+	return 0;
+}
+    </programlisting>
+    <para>
+      Instruction on line 12 helps the MUSB controller driver to work
+      around the fact that the controller hardware is missing registers
+      that are used for USB endpoints configuration.
+    </para>
+    <para>
+      Without these registers, the controller driver is unable to read
+      the endpoints configuration from the hardware, so we use line 12
+      instruction to bypass reading the configuration from silicon, and
+      rely on a hard-coded table that describes the endpoints
+      configuration instead:
+    </para>
+    <programlisting linenumbering="numbered">
+static struct musb_fifo_cfg jz4740_musb_fifo_cfg[] = {
+{ .hw_ep_num = 1, .style = FIFO_TX, .maxpacket = 512, },
+{ .hw_ep_num = 1, .style = FIFO_RX, .maxpacket = 512, },
+{ .hw_ep_num = 2, .style = FIFO_TX, .maxpacket = 64, },
+};
+    </programlisting>
+    <para>
+      Looking at the configuration table above, we see that each
+      endpoints is described by three fields: hw_ep_num is the endpoint
+      number, style is its direction (either FIFO_TX for the controller
+      driver to send packets in the controller hardware, or FIFO_RX to
+      receive packets from hardware), and maxpacket defines the maximum
+      size of each data packet that can be transmitted over that
+      endpoint. Reading from the table, the controller driver knows that
+      endpoint 1 can be used to send and receive USB data packets of 512
+      bytes at once (this is in fact a bulk in/out endpoint), and
+      endpoint 2 can be used to send data packets of 64 bytes at once
+      (this is in fact an interrupt endpoint).
+    </para>
+    <para>
+      Note that there is no information about endpoint 0 here: that one
+      is implemented by default in every silicon design, with a
+      predefined configuration according to the USB specification. For
+      more examples of endpoint configuration tables, see musb_core.c.
+    </para>
+    <para>
+      Let's now get back to the interrupt handler function:
+    </para>
+    <programlisting linenumbering="numbered">
+static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
+{
+	unsigned long   flags;
+	irqreturn_t     retval = IRQ_NONE;
+	struct musb     *musb = __hci;
+
+	spin_lock_irqsave(&amp;musb->lock, flags);
+
+	musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
+	musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
+	musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
+
+	/*
+	 * The controller is gadget only, the state of the host mode IRQ bits is
+	 * undefined. Mask them to make sure that the musb driver core will
+	 * never see them set
+	 */
+	musb->int_usb &amp;= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
+	    MUSB_INTR_RESET | MUSB_INTR_SOF;
+
+	if (musb->int_usb || musb->int_tx || musb->int_rx)
+		retval = musb_interrupt(musb);
+
+	spin_unlock_irqrestore(&amp;musb->lock, flags);
+
+	return retval;
+}
+    </programlisting>
+    <para>
+      Instruction on line 18 above is a way for the controller driver to
+      work around the fact that some interrupt bits used for USB host
+      mode operation are missing in the MUSB_INTRUSB register, thus left
+      in an undefined hardware state, since this MUSB controller
+      hardware is used in peripheral mode only. As a consequence, the
+      glue layer masks these missing bits out to avoid parasite
+      interrupts by doing a logical AND operation between the value read
+      from MUSB_INTRUSB and the bits that are actually implemented in
+      the register.
+    </para>
+    <para>
+      These are only a couple of the quirks found in the JZ4740 USB
+      device controller. Some others were directly addressed in the MUSB
+      core since the fixes were generic enough to provide a better
+      handling of the issues for others controller hardware eventually.
+    </para>
+  </chapter>
+
+  <chapter id="conclusion">
+    <title>Conclusion</title>
+    <para>
+      Writing a Linux MUSB glue layer should be a more accessible task,
+      as this documentation tries to show the ins and outs of this
+      exercise.
+    </para>
+    <para>
+      The JZ4740 USB device controller being fairly simple, I hope its
+      glue layer serves as a good example for the curious mind. Used
+      with the current MUSB glue layers, this documentation should
+      provide enough guidance to get started; should anything gets out
+      of hand, the linux-usb mailing list archive is another helpful
+      resource to browse through.
+    </para>
+  </chapter>
+
+  <chapter id="acknowledgements">
+    <title>Acknowledgements</title>
+    <para>
+      Many thanks to Lars-Peter Clausen and Maarten ter Huurne for
+      answering my questions while I was writing the JZ4740 glue layer
+      and for helping me out getting the code in good shape.
+    </para>
+    <para>
+      I would also like to thank the Qi-Hardware community at large for
+      its cheerful guidance and support.
+    </para>
+  </chapter>
+
+  <chapter id="resources">
+    <title>Resources</title>
+    <para>
+      USB Home Page:
+      <ulink url="http://www.usb.org">http://www.usb.org</ulink>
+    </para>
+    <para>
+      linux-usb Mailing List Archives:
+      <ulink url="http://marc.info/?l=linux-usb">http://marc.info/?l=linux-usb</ulink>
+    </para>
+    <para>
+      USB On-the-Go Basics:
+      <ulink url="http://www.maximintegrated.com/app-notes/index.mvp/id/1822">http://www.maximintegrated.com/app-notes/index.mvp/id/1822</ulink>
+    </para>
+    <para>
+      Writing USB Device Drivers:
+      <ulink url="https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html">https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html</ulink>
+    </para>
+    <para>
+      Texas Instruments USB Configuration Wiki Page:
+      <ulink url="http://processors.wiki.ti.com/index.php/Usbgeneralpage">http://processors.wiki.ti.com/index.php/Usbgeneralpage</ulink>
+    </para>
+    <para>
+      Analog Devices Blackfin MUSB Configuration:
+      <ulink url="http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb">http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb</ulink>
+    </para>
+  </chapter>
+
+</book>