Arch Linux

dav7

Member

From: Australia

Registered: 2008-02-08

Posts: 674

Detecting when a USB device is plugged in/removed

So I was poking around recently and decided to configure a PC I had laying around as a 3rd display. So I grab the display and swivel it around so I can measure its cable as part of a placement experiment. And notice a USB port I'd either never seen before or saw, disregarded and forgot about.

"This screen has had a hub in it this whole time?!...", I thought, and poked around the front of it for ports. None turn up. The only info I've been able to find out about it is that it's a port for a mouse, of all things, so one can control the OSD via point/click. Crazy and/or unlikely, but whatever. It has ID 0471:0003 for anyone curious; Google doesn't turn much up for it, before you check. If you were to actually investigate what it does, you'd likely be better off starting with the display itself - it's a CRT, a Philips 107B (the back says it's specifically model 107B10).

Why I'm interested in the port now is because a suspicion of mine is correct: the... whatever-it-is is powered via the screen, not via USB, so when I physically turn the screen on, the device switches on too, and Linux picks the new device up. Aaand when I switch the display off, the USB thingy switches off too, so Linux reports it as removed.

My question is this: how can I detect these events without eating any more than a few % of CPU time and/or running a multibillion-layer framework like HAL? The PC the display is connected to (both USB- and VGA-wise) is a 450MHz P3, so it'd be very easy to send CPU usage skyrocketing if I don't use efficient code, and I don't see the point in unnecessarily straining it.

I'm prepared to go as far as writing a small kernel module, if that helps, isn't more than 150 SLOC, and is heavily documented.

-dav7

Last edited by dav7 (2008-11-26 15:16:16)

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Windows was made for looking at success from a distance through a wall of oversimplicity. Linux removes the wall, so you can just walk up to success and make it your own.
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Zariel

Member

Registered: 2008-10-07

Posts: 446

Re: Detecting when a USB device is plugged in/removed

cant you use udev for that?

dav7

Member

From: Australia

Registered: 2008-02-08

Posts: 674

Re: Detecting when a USB device is plugged in/removed

Okay, it does run udev. I'll look into configuring it for this. Thanks.

(I'll mark this solved when I post the solution I find.)

-dav7

Last edited by dav7 (2008-11-27 00:02:17)

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Windows was made for looking at success from a distance through a wall of oversimplicity. Linux removes the wall, so you can just walk up to success and make it your own.
--
Reinventing the wheel is fun. You get to redefine pi.

dav7

Member

From: Australia

Registered: 2008-02-08

Posts: 674

Re: Detecting when a USB device is plugged in/removed

Well, udev is extremely slow (10-15 second wait before it responds).

Are there any faster ways I could manage this?

-dav7

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Windows was made for looking at success from a distance through a wall of oversimplicity. Linux removes the wall, so you can just walk up to success and make it your own.
--
Reinventing the wheel is fun. You get to redefine pi.

Daenyth

Forum Fellow

From: Boston, MA

Registered: 2008-02-24

Posts: 1,244

Re: Detecting when a USB device is plugged in/removed

udev is pretty much "the" way to do that sort of thing. Maybe ask on LKML?

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dav7

Member

From: Australia

Registered: 2008-02-08

Posts: 674

Re: Detecting when a USB device is plugged in/removed

Are there any web interfaces for LKML? I dislike mailinglists; the last one I subscribed to randomly stopped sending messages to me after about an hour. I might try usenet; there are some good-looking groups on there I think.

Also, I'm sure there has to be a way to do this outside of udev. I mean, if udev can be made to work without using like 30% CPU (which I'd imagine would be what would happen if it polled every 1 second, which is what I'd want it to do), I'll use it, but what about thinking outside the box a little?

I mean, my first thought was to write a kernel module that responded to dmesg at the kernel level; dmesg doesn't update all that often, and I could run through all the lines I recieve and look for, for example, my PCI ID and a signature telling me if it was added or removed. I just have no idea how to do that.

-dav7

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Windows was made for looking at success from a distance through a wall of oversimplicity. Linux removes the wall, so you can just walk up to success and make it your own.
--
Reinventing the wheel is fun. You get to redefine pi.

robmaloy

Member

From: Germany

Registered: 2008-05-14

Posts: 263

Re: Detecting when a USB device is plugged in/removed

whats the difference from using the screens usb port and a usb port on your machine?

450MHz shouldnt have problems with udev, the only way you could (maybe) improve speed i to compile linux with all drivers you need directly to the kernel instead of modules.

if you compare time/effort you're way better off with udev

Last edited by robmaloy (2008-11-27 17:58:09)

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dav7

Member

From: Australia

Registered: 2008-02-08

Posts: 674

Re: Detecting when a USB device is plugged in/removed

Uh, hi there. Quoting my original post:

"This screen has had a hub in it this whole time?!...", I thought, and poked around the front of it for ports. None turn up. The only info I've been able to find out about it is that it's a port for a mouse, of all things, so one can control the OSD via point/click.

But thanks for the advice.

And I don't think udev was specifically designed for this... I would think udev is more suited toward drives, printers, etc. Although I could well be wrong.

-dav7

Last edited by dav7 (2008-11-28 13:44:50)

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Windows was made for looking at success from a distance through a wall of oversimplicity. Linux removes the wall, so you can just walk up to success and make it your own.
--
Reinventing the wheel is fun. You get to redefine pi.

test1000

Member

Registered: 2005-04-03

Posts: 834

Re: Detecting when a USB device is plugged in/removed

try running HAL and see if it's really like "running a multibillion-layer framework like HAL" (maybe you don't even have to run the full hal(!))

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dav7

Member

From: Australia

Registered: 2008-02-08

Posts: 674

Re: Detecting when a USB device is plugged in/removed

Well that started dbus as well and now the system seems more eager to sit on 2.0% CPU usage instead of 1.3%

-dav7

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Windows was made for looking at success from a distance through a wall of oversimplicity. Linux removes the wall, so you can just walk up to success and make it your own.
--
Reinventing the wheel is fun. You get to redefine pi.

iphitus

Forum Fellow

From: Melbourne, Australia

Registered: 2004-10-09

Posts: 4,927

Re: Detecting when a USB device is plugged in/removed

Use HAL. You far overstate it's size.

Otherwise... as said earlier, use udev. It's already detecting the events, so you just need to configure a script to perform whatever tasks you need.

Writing a kernel module for this task is silly, and something that really belongs on dailywtf.

dav7

Member

From: Australia

Registered: 2008-02-08

Posts: 674

Re: Detecting when a USB device is plugged in/removed

Okay, I'm willing to use HAL and/or udev, I just need to make it "instant".

I set up a rule when I started out on this that echoed some text to a tty to say that it was working, and I just reran it now. There's a 3-4 second delay between my screen switching on and the text appearing in the console.

That might not sound like a lot, but if dmesg can has instant updates, why can't I too?

-dav7

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Windows was made for looking at success from a distance through a wall of oversimplicity. Linux removes the wall, so you can just walk up to success and make it your own.
--
Reinventing the wheel is fun. You get to redefine pi.