USB stands for Universal Serial Bus and is basically an external interface standard that enables communication between the computer and various other peripherals. Some of the most commonly used USB devices today are keyboards, mice, pen drives, digital cameras, external CD & DVD writers, printers etc.

There are currently two versions of USB in use, i.e. USB 1.1 and USB 2.0. Since USB has always been backward compatible with its previous versions, USB 2.0 is backwards compatible with USB 1.1. The latest USB devices are typically USB 2.0 compatible. USB 2.0 supports a maximum data transmission speed of 480 Mbps or 60 MBps and this is the major difference between the two standards. Another advantage with USB is that the devices are all hot-pluggable, which means that you do not have to restart your system in order for you to be able to use these devices.

Before we go onto the exact configuration options in the kernel, it would be apt to look at USB in a little more detail. If you're in a hurry or want to skip this section, please go to Kernel Configuration.

A USB system has a host controller, hubs, a root hub amongst others and can support up to 127 USB devices including the hubs. The host controller is nothing but the hardware interface between the USB device and the operating system. There are a couple of HCI (Host Controller Interface) in use today and they are the OHCI (Open HCI) by Compaq, UHCI (Universal HCI) and EHCI (Enhanced HCI), both from Intel. The OHCI/UHCI are the two industry standard USB 1.1 interfaces whereas the EHCI is for USB 2.0.

The hardware vendor provides an interface for the programmer that allows the system to interact with the hardware and this is called the HCD or Host Controller Device. It is through this HCD that the device interacts with the system software. The following diagram should make things easier to comprehend.

(Software consists of other components as well like the device driver, but
for the sake of simplicity, they are not shown)

    + ----  Hardware  ----   + ----  Software  ---- +
    |                        |                      |
    | [USB Dev] -+-> {EHCI} -+--->  ( EHCD )        |
    |            |           |                      |  User
    |            `-> {UHCI} -+--->  ( UHCD )        |
    |                        |                      |
    + ----  Hardware  ----   + ----  Software  ---- +

A USB device can either use a custom driver or use one already present in the system and this is based on the concept of a device class. This means that if a device belongs to a certain class, then other devices belonging to the same class can make use of the same device driver. Some of these classes are the USB HID (Human Interface Devices) class which covers input devices like keyboards and mice, the USB Mass Storage devices class which covers devices like pen drives, digital cameras, audio players etc and the USB CDC (Communication Devices Class) which essentially covers USB modems and similar devices.

It is very simple to find out whether your machine has USB 2.0 support or not. We make use of the lspci command for this purpose.

(In system that is USB 1.1 compliant, note the UHCI only)

# lspci -v | grep USB
0000:00:04.2 USB Controller: Intel Corp. 82371AB/EB/MB PIIX4 USB (rev 01) (prog-if 00 [UHCI])

(A system that is USB 2.0 compliant, note the EHCI and UHCI)

00:1d.0 USB Controller: Intel Corp. 82801DB USB (Hub #1) (rev 01) (prog-if 00 [UHCI])
00:1d.1 USB Controller: Intel Corp. 82801DB USB (Hub #2) (rev 01) (prog-if 00 [UHCI])
00:1d.2 USB Controller: Intel Corp. 82801DB USB (Hub #3) (rev 01) (prog-if 00 [UHCI])
00:1d.7 USB Controller: Intel Corp. 82801DB USB EHCI Controller (rev 01) (prog-if 20 [EHCI])

So using the lspci command, we can find out if the system supports USB 2.0. This is useful as we will be enabling the corresponding options in the kernel.

First emerge the kernel sources of your choice. Here we'll use the gentoo-sources. For more information on the various kernel sources available on Portage, please look up the Gentoo Linux Kernel Guide.

# emerge gentoo-sources

Now, lets get on with the task of configuring the kernel.

# cd /usr/src/linux
# make menuconfig

Now we will look at some of the options we will have to enable in the 2.6 kernel to ensure proper USB support for our devices. If you are using a 2.4 kernel, please proceed with Config options for the 2.4.x kernel.

Device Drivers  --->
  SCSI device support  --->

(Although SCSI will be enabled automatically when selecting USB Mass Storage,
we need to enable disk support.)
---   SCSI support type (disk, tape, CD-ROM)
<*>   SCSI disk support

(Then move back a level and go into USB support)
USB support  --->

(This is the root hub and is required for USB support.
If you'd like to compile this as a module, it will be called usbcore.)
<*> Support for Host-side USB

(Enable this option to see your USB devices in /proc/bus/usb.
This is recommended.)
 [*]   USB device filesystem

(Select at least one of the HCDs. If you are unsure, picking all is fine.)
--- USB Host Controller Drivers
<*> EHCI HCD (USB 2.0) support
< > OHCI HCD support
<*> UHCI HCD (most Intel and VIA) support

(Moving a little further down, we come to CDC and mass storage.)
< > USB Modem (CDC ACM) support
<*> USB Printer support
<*> USB Mass Storage support

(Then the HID bits.
You have to select HID input support if you have a USB keyboard,
mouse, joystick or any other USB input device)
--- USB Input Devices
<*> USB Human Interface Device (full HID) support
 [*]   HID input layer support

(If you have a USB Network Card like the RTL8150, you'll need this)
USB Network Adapters  --->
    <*> USB RTL8150 based ethernet device support (EXPERIMENTAL)

(If you have a serial to USB converter like the Prolific 2303, you'll need this)
USB Serial Converter support  --->
    <*> USB Serial Converter support
    <*> USB Prolific 2303 Single Port Serial Driver (NEW)

Now that your options are set, you can (re)compile the kernel and USB support should be functional once you reboot into the new kernel. You can now proceed to Seeing USB at work and see if everything is working as it should.

We will look at the options the we will have to enable in the 2.4 kernel to ensure proper USB support for our devices. If you are using a 2.6 kernel, please look at Config options for the 2.6.x kernel.

(This immediate config is only for those who have USB input devices.
Input core support is needed by USB HID later.)
Input core support  --->
   <*> Input core support
   < >   Keyboard support
   < >   Mouse support
   < >   Event interface support

USB support  --->

(This is the root hub and is required for USB support.
If you'd like to compile this as a module, it will be called usbcore.o)
<*> Support for USB

(Enable this option to see your USB devices in /proc/bus/usb.
This is recommended.)
 [*]   Preliminary USB device filesystem

(Select at least one of the HCDs. If you are unsure, picking all is fine.)
--- USB Host Controller Drivers
<*>   UHCI Alternate Driver (JE) support
< >   OHCI (Compaq, iMacs, OPTi, SiS, ALi, ...) support

(This is the device section. Select only what you need.)
--- USB Device Class drivers
< >   USB Audio support
<*>   USB Mass Storage support
< >   USB Modem (CDC ACM) support
<*>   USB Printer support

(Followed by the HID section. This is needed if you have an USB based input device.)
--- USB Human Interface Devices (HID)
<*>   USB Human Interface Device (full HID) support
 [*]     HID input layer support

(If you have a serial to USB converter like the Prolific 2303, you'll need this)
USB Serial Converter support  --->
    <*> USB Serial Converter support
    <*> USB Prolific 2303 Single Port Serial Driver (NEW)

Now that the options are set, you can (re)compile the kernel and USB support should be functional once you reboot into the new kernel.

The time has finally come to play with those USB devices :) So let's get started. In this chapter, we'll see how the system responds to various USB devices. We'll start by plugging in a USB 512 MB Memory Stick/Pen Drive. You could use some other similar mass storage device. We will primarily use dmesg to see what is happening and how the system responds to the device.

(Plug in Memory Stick into available USB port and then..)
# dmesg | less

(The device is picked up as a USB 1.1 and allocated an address.
Also says what HCD it is using.)
usb 1-1: new full speed USB device using uhci_hcd and address 2

(SCSI emulation automatically kicks in)
scsi0 : SCSI emulation for USB Mass Storage devices
usb-storage: device found at 2

(Now the device information including model number is retrieved)
usb-storage: waiting for device to settle before scanning
  Vendor: JetFlash  Model: TS512MJF2A        Rev: 1.00
  Type:   Direct-Access                      ANSI SCSI revision: 02
SCSI device sda: 1003600 512-byte hdwr sectors (514 MB)

(The write-protect sense is EXPERIMENTAL code in the later kernels)
sda: Write Protect is off
sda: Mode Sense: 0b 00 00 08
sda: assuming drive cache: write through
SCSI device sda: 1003600 512-byte hdwr sectors (514 MB)
/dev/scsi/host0/bus0/target0/lun0: p1
Attached scsi removable disk sda at scsi0, channel 0, id 0, lun 0
Attached scsi generic sg0 at scsi0, channel 0, id 0, lun 0,  type 0
usb-storage: device scan complete
(At this point, the device is generally accessible by mounting /dev/sda1)

(When the device is disconnected, the system acknowledges the same)
usb 1-1: USB disconnect, address 2

Once the device is connected and mounted, you can access it like a normal hard disk. Usual operations like cp, mv, rm, etc work fine. You could also create a filesystem on the USB stick/format it.

# mount /dev/sda1 /mnt/usb
# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda8             9.4G  7.5G  1.9G  80% /
/dev/hda9              11G  8.1G  2.4G  78% /usr
none                  189M     0  189M   0% /dev/shm
/dev/sda1             490M   34M  457M   7% /mnt/usb

How would a USB mouse show up in case you had one? It will show up as an HID device.

# dmesg | grep USB
drivers/usb/input/hid-core.c: v2.0:USB HID core driver
usb 1-1: new low speed USB device using address 2
input: USB HID v1.10 Mouse [Logitech USB-PS/2 Optical Mouse] on usb-0000:00:07.2-1

Another nifty command you can use to see the status of your USB ports is lsusb. This is part of sys-apps/usbutils and will be covered in the next chapter.

So far we've seen how much support exists on the kernel/system side for USB on Linux. Now we'll take a peek into what kind of support is provided by Gentoo for USB in the userspace.

One of the most useful tools around is lsusb. This lists all the usb devices connected to the system. Installing it is a breeze.

# emerge usbutils

Once installed, you can just run lsusb to get simple info on the USB devices attached to the machine.

# lsusb
(This is the 512 MB Memory Stick from Transcend)
Bus 001 Device 003: ID 0c76:0005 JMTek, LLC. USBdisk
(This is the Optical Mouse)
Bus 001 Device 002: ID 046d:c00e Logitech, Inc. Optical Mouse
(This is the root hub)
Bus 001 Device 001: ID 0000:0000

If you are one of those types who love to see lots of information, you have the option of running lsusb -v. Try that and see the amount of info it gives out. Another good option is that lsusb dumps the current physical USB hierarchy as a tree and thus makes it easier to understand the exact device map. The command is lsusb -t. For example,

# lsusb -t
Bus#  1
`-Dev#   1 Vendor 0x0000 Product 0x0000
  |-Dev#   2 Vendor 0x046d Product 0xc00e
  `-Dev#   3 Vendor 0x0c76 Product 0x0005

You can easily correlate the outputs of lsusb and lsusb -t, which helps debugging as well as understanding how USB works.

Gentoo uses two packages, sys-apps/hotplug and sys-apps/coldplug to do some magic with hot-pluggable devices. Just like any other magic trick, there is a simple logic behind this one too. We shall now see what that is, and in the process hopefully we will be able to understand these twins better.

Firmware can be defined as the software on a piece of hardware that is loaded and executed but can't be modified easily. Many devices have firmware in them to ensure that they work properly and often firmware may contain code that is critical to ensure that the hardware performs as expected. Firmware is present in a wide variety of computer devices ranging from ROM chips to state of the art USB and PCMCIA cards. When a device is plugged in, the firmware (which may, in some cases, be the driver as well) is read and loaded onto memory after which the device can be used by the system.

To cut the long story short, Gentoo uses sys-apps/hotplug to handle the firmware side of things in hot-pluggable devices. sys-apps/hotplug will use the required firmware to make that device usable. The firmware should be put in the /lib/firmware directory and is picked up from there. Getting it is simple; the usual emerge will do.

# emerge hotplug

Now the obvious question would be, what is coldplug and why is it needed? sys-apps/coldplug does what hotplug does, but it does it for hot-pluggable devices that are already connected at boot time. A good example of this would be a USB Network card. Earlier, hotplug was the package responsible for handling both, but then it was split into hotplug and coldplug, with coldplug doing all the work. Install it if you have hot-pluggable devices that you need activated on boot up.

# emerge coldplug
(And you can add it to the boot runlevel)
# rc-update add coldplug boot
 * coldplug added to runlevel boot
 * rc-update complete.

A good number of online documents helped me during the development of this document and there are some that are highly technical but truly interesting. I thought they all deserve some credit, so here we go!

• The Official USB Website
• The USB FAQ
• Compaq's OHCI Standard
• Intel's UHCI Standard
• Intel's EHCI Standard

• Gentoo Linux LiveUSB HOWTO