xref: /kernel/linux/linux-6.6/Documentation/driver-api/usb/writing_musb_glue_layer.rst

=========================
Writing a MUSB Glue Layer
=========================

:Author: Apelete Seketeli

Introduction
============

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).

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.

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.

.. _musb-basics:

Linux MUSB Basics
=================

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).

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::

	  ------------------------
	  |                      | <------- drivers/usb/gadget
	  | Linux USB Core Stack | <------- drivers/usb/host
	  |                      | <------- drivers/usb/core
	  ------------------------
		     ⬍
	 --------------------------
	 |                        | <------ drivers/usb/musb/musb_gadget.c
	 | MUSB Controller driver | <------ drivers/usb/musb/musb_host.c
	 |                        | <------ drivers/usb/musb/musb_core.c
	 --------------------------
		     ⬍
      ---------------------------------
      | MUSB Platform Specific Driver |
      |                               | <-- drivers/usb/musb/jz4740.c
      |       aka "Glue Layer"        |
      ---------------------------------
		     ⬍
      ---------------------------------
      |   MUSB Controller Hardware    |
      ---------------------------------

As outlined above, the glue layer is actually the platform specific code
sitting in between the controller driver and the controller hardware.

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.

All of this information is passed to the MUSB controller driver through
a :c:type:`platform_driver` structure defined in the glue layer as::

    static struct platform_driver jz4740_driver = {
	.probe      = jz4740_probe,
	.remove     = jz4740_remove,
	.driver     = {
	    .name   = "musb-jz4740",
	},
    };

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.

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::

    struct jz4740_glue {
	struct device           *dev;
	struct platform_device  *musb;
	struct clk      *clk;
    };


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.

Let's go through the steps of the probe function that leads the glue
layer to register itself to the controller driver.

.. note::

   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.

.. code-block:: c
    :emphasize-lines: 8,12,18

    static int jz4740_probe(struct platform_device *pdev)
    {
	struct platform_device      *musb;
	struct jz4740_glue      *glue;
	struct clk                      *clk;
	int             ret;

	glue = devm_kzalloc(&pdev->dev, sizeof(*glue), GFP_KERNEL);
	if (!glue)
	    return -ENOMEM;

	musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO);
	if (!musb) {
	    dev_err(&pdev->dev, "failed to allocate musb device\n");
	    return -ENOMEM;
	}

	clk = devm_clk_get(&pdev->dev, "udc");
	if (IS_ERR(clk)) {
	    dev_err(&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(&pdev->dev, "failed to enable clock\n");
	    goto err_platform_device_put;
	}

	musb->dev.parent        = &pdev->dev;

	glue->dev           = &pdev->dev;
	glue->musb          = musb;
	glue->clk           = clk;

	return 0;

    err_platform_device_put:
	platform_device_put(musb);
	return ret;
    }

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
locking 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 :c:func:`devm_clk_get`
(line 18) the glue layer allocates the clock -- the ``devm_`` prefix
indicates that :c:func:`clk_get` is managed: it automatically frees the
allocated clock resource data when the device is released -- and enable
it.



Then comes the registration steps:

.. code-block:: c
    :emphasize-lines: 3,5,7,9,16

    static int jz4740_probe(struct platform_device *pdev)
    {
	struct musb_hdrc_platform_data  *pdata = &jz4740_musb_platform_data;

	pdata->platform_ops     = &jz4740_musb_ops;

	platform_set_drvdata(pdev, glue);

	ret = platform_device_add_resources(musb, pdev->resource,
			    pdev->num_resources);
	if (ret) {
	    dev_err(&pdev->dev, "failed to add resources\n");
	    goto err_clk_disable;
	}

	ret = platform_device_add_data(musb, pdata, sizeof(*pdata));
	if (ret) {
	    dev_err(&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;
    }

The first step is to pass the device data privately held by the glue
layer on to the controller driver through :c:func:`platform_set_drvdata`
(line 7). Next is passing on the device resources information, also privately
held at that point, through :c:func:`platform_device_add_resources` (line 9).

Finally comes passing on the platform specific data to the controller
driver (line 16). Platform data will be discussed in
:ref:`musb-dev-platform-data`, 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::

    static const struct musb_platform_ops jz4740_musb_ops = {
	.init       = jz4740_musb_init,
	.exit       = jz4740_musb_exit,
    };

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.

At that point of the registration process, the controller driver
actually calls the init function:

   .. code-block:: c
    :emphasize-lines: 12,14

    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;
    }

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.

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 :ref:`musb-dev-quirks` and
:ref:`musb-handling-irqs`\ ::

    static int jz4740_musb_exit(struct musb *musb)
    {
	usb_put_phy(musb->xceiv);

	return 0;
    }

Acting as the counterpart of init, the exit function releases the MUSB
PHY driver when the controller hardware itself is about to be released.

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.

Returning from the init function, the MUSB controller driver jumps back
into the probe function::

    static int jz4740_probe(struct platform_device *pdev)
    {
	ret = platform_device_add(musb);
	if (ret) {
	    dev_err(&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;
    }

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:

   .. code-block:: c
    :emphasize-lines: 5,6

    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;
    }

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.

.. _musb-handling-irqs:

Handling IRQs
=============

Additionally to the MUSB controller hardware basic setup and
registration, the glue layer is also responsible for handling the IRQs:

   .. code-block:: c
    :emphasize-lines: 7,9-11,14,24

    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(&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 &= 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(&musb->lock, flags);

	return retval;
    }

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.

The interrupt handler critical section is protected by the
:c:func:`spin_lock_irqsave` and counterpart :c:func:`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.

Then the relevant interrupt registers are read (line 9 to 11):

-  ``MUSB_INTRUSB``: indicates which USB interrupts are currently active,

-  ``MUSB_INTRTX``: indicates which of the interrupts for TX endpoints are
   currently active,

-  ``MUSB_INTRRX``: indicates which of the interrupts for TX endpoints are
   currently active.

Note that :c:func:`musb_readb` is used to read 8-bit registers at most, while
:c:func:`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.

Instruction on line 18 is another quirk specific to the JZ4740 USB
device controller, which will be discussed later in :ref:`musb-dev-quirks`.

The glue layer still needs to register the IRQ handler though. Remember
the instruction on line 14 of the init function::

    static int jz4740_musb_init(struct musb *musb)
    {
	musb->isr = jz4740_musb_interrupt;

	return 0;
    }

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.

.. _musb-dev-platform-data:

Device Platform Data
====================

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.

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.

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.

Here is the part of ``arch/mips/jz4740/platform.c`` that covers the USB
Device Controller (UDC):

   .. code-block:: c
    :emphasize-lines: 2,7,14-17,21,22,25,26,28,29

    /* 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          = &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,
    };

The ``jz4740_udc_xceiv_device`` platform device structure (line 2)
describes the UDC transceiver with a name and id number.

At the time of this writing, note that ``usb_phy_gen_xceiv`` 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.

The ``jz4740_udc_resources`` resource structure (line 7) defines the UDC
registers base addresses.

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 ``mc`` 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.

Finally, the ``jz4740_udc_device`` platform device structure (line 21)
describes the UDC itself.

The ``musb-jz4740`` 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 :ref:`musb-basics`.
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 :ref:`musb-basics`. 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).

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``:

   .. code-block:: c
    :emphasize-lines: 3,5,7-9,11

    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 = &jz4740_musb_config,
    };

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`` :c:type:`musb_hdrc_config` structure.

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
:ref:`musb-dev-quirks`. 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
:ref:`musb-dev-quirks`.

Then this configuration is embedded inside ``jz4740_musb_platform_data``
:c:type:`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.

Remember that ``jz4740_musb_platform_data`` is then used to convey
platform data information as we have seen in the probe function in
:ref:`musb-basics`.

.. _musb-dev-quirks:

Device Quirks
=============

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.

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.

Let's get back to the init function first:

   .. code-block:: c
    :emphasize-lines: 12

    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;
    }

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.

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::

    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, },
    };

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).

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``.

Let's now get back to the interrupt handler function:

   .. code-block:: c
    :emphasize-lines: 18-19

    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(&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 &= 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(&musb->lock, flags);

	return retval;
    }

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.

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.

Conclusion
==========

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.

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.

Acknowledgements
================

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.

I would also like to thank the Qi-Hardware community at large for its
cheerful guidance and support.

Resources
=========

USB Home Page: https://www.usb.org

linux-usb Mailing List Archives: https://marc.info/?l=linux-usb

USB On-the-Go Basics:
https://www.maximintegrated.com/app-notes/index.mvp/id/1822

:ref:`Writing USB Device Drivers <writing-usb-driver>`

Texas Instruments USB Configuration Wiki Page:
http://processors.wiki.ti.com/index.php/Usbgeneralpage