linux-next/Documentation/DocBook/writing_musb_glue_layer.tmpl

<?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>