Plugable

Emacs macro to ease the pain of checkpatchitis

Posted on 19. Feb, 2010 by Bernie Thompson in udlfb

In order to submit patches to the Linux kernel — as I’ve been doing lately to help improve Linux’s DisplayLink driver udlfb — the changed code has to pass a script called checkpatch.pl, which flags violations of the Linux kernel style guidelines.

Style wars (e.g. “tabs vs. spaces”) are a never-ending source of tension on projects, so I actually appreciate automation like checkpatch.pl to just lay down the law, and be done with it.

But .. having to sift through endless warnings when you run checkpatch.pl late in the development process is frustrating. And there’s no way most occasional kernel patchers will remember all the rules (especially if you strongly disagree with some of them — which I do). It’s much better if your editor can just warn you when you step off the golden path of enlightenment. I use emacs, so I looked around for solutions, but they were only incomplete, or not real-time (e.g. checkpatch.pl has an –emacs option to run in the compile window of emacs).

So here’s something a little less incomplete that I’ve added to my .emacs file to catch the majority of checkpatch.pl errors in real-time (by highlighting the offending code in red). Works only on recent emacs versions. Hopefully it’ll be useful to others pounding away on their patches.

;; *** BEGIN highlight checkpatch.pl warnings and errors ***
(add-hook 'c-mode-common-hook
    (lambda ()
      ;; this sets defaults to match many checkpatch.pl guidelines
      (c-set-style "linux")))
;; but doesn't warn us about violations these regexp catch common ones
(custom-set-faces
    '(my-warning-face ((((class color)) (:background "red"))) t))
(add-hook 'font-lock-mode-hook
    (function
        (lambda ()
            (setq font-lock-keywords
                (append font-lock-keywords
		    '(("^.\\{81\\}" (0 'my-warning-face t)))
                    '(("\\/\\/.*" (0 'my-warning-face t)))
                    '((";[_A-Za-z0-9]+" (0 'my-warning-face t)))
                    '((",[_A-Za-z0-9]+" (0 'my-warning-face t)))
                    '(("return[[:blank:]]*(.+);" (0 'my-warning-face t)))
                    '(("([_A-Za-z0-9]+[\\*]+)" (0 'my-warning-face t)))
                    '(("[[:blank:]]+\\)"(0 'my-warning-face t)))
		    '(("^[[:blank:]]+{[[:blank:]]*$" (0 'my-warning-face t)))
		    '(("[_A-Za-z0-9]+\\*[[:blank:]]" (0 'my-warning-face t)))
		)))))
;; exercise to reader - move regexps into c hook
;; *** END highlight checkpatch.pl warnings and errors ***

Any enhancements or corrections are welcome.

9 USB displays illuminate energy security

Posted on 26. Jan, 2010 by Bernie Thompson in udlfb

Here’s a great application for USB displays.

Hal Glenn from 2G Engineering has created an information display (on energy security and alternative methods of energy generation) with 9 USB touch screens, all running off a single Mac Mini running Ubuntu 9.10. And all using the available open source DisplayLink drivers and scripts and info at displaylink.org.

Doing this without USB displays would

• Require a big desktop box to support several PCIe graphics cards
• Would have triple the cords – Hal’s setup runs a single USB cable to each display. With VGA, you’d still need the USB cable (for touch function), but then would also need VGA and power to each!
• Would consume much more power – which would be kind of embarrassing for an energy security display, wouldn’t it?

Hal’s setup builds on and extends some of the USB terminal scripts demoed during this talk at Linux Plumbers Conference 2009.

There are several reasons why this demo is easiest on Linux, one of which is by default, DisplayLink devices are limited to 6 displays on Windows and Mac. The Linux drivers have no limit, so you can connect as many displays as you like — keeping in mind you’re sharing a 480Mbs bus (that itself has a 127 device limit). But it is enough for the apps Hal is running on those nine 800×480 touchscreens.

It will be interesting to see how many screens people get up to for various applications.

Linux USB multiseat audio support

Posted on 20. Jan, 2010 by Bernie Thompson in udlfb

Here’s how to add audio support on top of the previous instructions for getting USB multiseat running on Linux, with a Plugable UD-160-A type device.

Add the following line to the bottom of the /lib/udev/rules.d/50-usbseat.rules file created per the previous instructions.

KERNEL=="control*", SUBSYSTEM=="sound", SUBSYSTEMS=="usb", PROGRAM="/bin/cat /sys/%p/../../../../../devnum", SYMLINK+="usbseat/%c/sound"

Then create a new /etc/X11/Xsession.d/50usbseat file which will be run at Xsession create time, with the following contents

oldIFS=$IFS
IFS=:
set $DISPLAY
IFS=.
set $2
SEAT_ID=$1
LN=`ls -al /dev/usbseat/$SEAT_ID/sound`
IFS=C
set $LN
CARD_ID=$2
export ALSA_CARD=$2
export ALSA_PCM_CARD=$2
IFS=$oldIFS

Each of the users who might need access to the USB devices needs to be added to the ‘audio’ group. On Ubuntu 9.04, this can be done with these commands to backup and then modify the groups (replace MY_USERNAME, of course) …

sudo cp /etc/group /etc/group_backup
sudo chmod a-wx /etc/group_backup
sudo adduser MY_USERNAME audio

See Ubuntu Sound TroubleShooting for details on that step.

Now, as you connect UD-160-A terminals, a new X instance and GDM login will pop up as before, but also each of them will have /dev/usbseat/%SEAT_ID%/sound linking to their sound device, and the ALSA_CARD environment variable for all processes off of that X session, set to the matching sound card ID. For apps which support ALSA/Pulse (like most browsers, flash, etc.), audio will now come out the correct terminal — all in a completely plug-and-play fashion.

If you’re wondering what the strange IFS stuff is in the above script, it’s bash’s built-in Internal Field Separator variable, which is an easy way to split strings without having to launch a separate sed or awk process.

Note, as before, these instructions are specific to and tested on an older version of Ubuntu: 9.04, and may need to be ported to other distros until the distros themselves integrate these scripts.

Thanks to Alexander Todorov’s earlier work on multiseat sound support, which demonstrated how to match the USB audio devices in udev, and which ALSA_ environment variables to set. Alexander reported some problems reliably matching the audio devices, but with these scripts (with limited testing so far), things are working as expected.