USB Device Bus (Demo)
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Video Transcription
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>> Hey Cybrarians.
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>> Welcome back to the Linux+ course here at Cybrary,
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>> I'm your instructor Rob Goelz.
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>> In today's lesson,
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>> we're going to be talking about the USB Device Bus.
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So upon completion of this lesson,
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you're going to be able to understand
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how the Universal Serial Bus,
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aka USB, works with Linux.
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We're going to be able to explain how
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USB device connections are handled and
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use the "lsusb" command to list
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devices on the system when we get into our demo.
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USB supports multiple devices on a single serial bus,
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it can theoretically support anywhere
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from 96 to 255 devices,
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but that is limited by the number of
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controllers, power, and bandwidth.
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I've never seen a system that can take 255 USB devices,
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but I guess it's theoretically possible.
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Now USB is a high-speed interface and
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it's gone faster with each and every release.
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It started out with 12 megabits per second on USB 1.0,
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and now we're up to 800 megabits per second on USB 3.0.
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USB actually refers to the bus, the bus type,
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while the port refers to
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the connection into the bus or we plug in.
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A thumb drive is a USB device form factor,
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and we can see some examples of that on the right.
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A flash drive is
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a USB storage device that uses flash memory.
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Now, in order for USB device to
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be recognized by a Linux system,
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this system has to have kernel modules installed that
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recognize the USB controller
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that's installed in the system, right?
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Where the system plugs into,
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there's actually a controller on the system,
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and then we also have kernel modules
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installed that recognize the device itself.
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What is getting plugged into the system.
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The connection process for USB
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has the following rough steps.
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Basically, the kernel module is going to detect
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the USB device and it's going to
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create a uevent in this system files system, sys.
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From here, we remember udev detects as
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uevents and configures the device per the rules
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that has this udev rules and it creates an entry
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for the device in the /dev directory.
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Some Linux distros may have udev rules to
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non-USB devices automatically enforced/media,
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but this is not always the case.
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Remember this, the "lsusb" command is
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used to list USB devices on the system.
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Now we can see USB devices in a tree or
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hierarchical view format by using the command "lsusb -t".
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But we can also use "lsusb -v" for evictor,
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which displays additional output by
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using the verbose flag, v for verbose.
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Let's have a look at this command with some demo time.
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Okay, so here we are in
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our demo environment and
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let's just play around with lsusb.
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If Ralph the [inaudible] was, let's just go
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ahead and type lsusb,
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we see that's pretty boring output.
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There's only two devices on this VM, I'm sorry.
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If we want to get a more verbose output,
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maybe a little bit more detailed,
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we could do an lsusb -v "Enter".
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There we go. Alright.
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That's a little too much information, right?
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So let's try doing
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just an lsusb -t. Let's
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see this in the tree or hierarchical view.
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All right, and then we can see how
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these devices are actually laid out,
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so the human interface driver is actually coming
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off of the root hub or the USB controller,
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probably more likely, right?
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And if we wanted to, we could
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combine the two options together,
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lsusb -tv, and we get more of like a Goldilocks zone,
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somewhere in the middle, which is,
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too much information and not enough.
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Now we can see that tree output
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and we can also see a little bit more of
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verbose information about all of the USB devices.
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With that, we've reached the end of this lesson.
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In this lesson, we covered how USB works in Linux,
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we talked about the USB connection
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processing requirements,
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and then also we used the "lsusb" command
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to list USB devices in our demo.
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Thanks so much for being here
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>> and I look forward to seeing you in the next lesson.
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