Jeff Epler's blog

1 September 2015, 14:39 UTC

DragonBoard™ 410c with rt-preempt failure

I was excited to obtain a DragonBoard, an affordable 64-bit ARM single-board computer, but unfortunately it hasn't lived up to my hopes.

First of all, it shipped with one terrible problem, making it useless for headless development: The onboard wireless doesn't receive multicast traffic, including ARP requests! And their kernel doesn't provide many modular drivers, so adding any old USB wireless dongle doesn't work until you rebuild the kernel. As of this writing, there is no useful workaround.

And that's when I hit the second show-stopper problem. I planned to build kernels for this thing anyway, because the goal is to run LinuxCNC on it with rt-preempt realtime. So I stuck in a large capacity micro-sd card, obtained the kernel source, built it, and rebooted with my new kernel.

(incidentally, testing kernels is a PITA. You have two choices: one, flash the one and only boot area with your new and untested kernel and hope for the best; two, tether to a PC (losing USB keyboard and mouse on the dragonboard!) and use fastboot every time you boot. ugh)

.. but the new kernel just didn't work. It scrolled kernel messages, but got stuck partway through booting. dmesg implicated the wireless card. This one at least has a workaround: cherry-pick a certain commit from their kernel git. As I understand it, this bug is triggered by using the compiler on the dragonboard, and not encountered with the linaro cross-compiler running on a traditional x86 desktop.

Anyway, having found out about this I was briefly celebratory. I booted with my own kernel and had a working USB dongle. Unfortunately, that celebration was short-lived. The USB dongle hasn't been terribly reliable either, particularly under high CPU load.

I soldiered on and patched the linaro 4.0 kernel with the 4.0-rt5 patchset. There were a few minor conflicts which I believe I resolved correctly. Unfortunately, the LinuxCNC latency-test readily encounters latencies above 15ms (15000us). LinuxCNC really would like sub-100us max latencies.

I briefly enabled tracers, and have traces that seem to implicate a variety of subsystems: networking, usb, and video *all* figure. More experienced developers than I will have to be the ones to sort out RT on 64-bit ARM.


5 August 2015, 13:13 UTC

Proven Delights (and what are proofs anyway)

I won a copy of the book Hacker's Delight from John Regehr for my entry in his nibble sort contest earlier this year.

In that thread I had heard about the CBMC Bounded Model Checking software, and it gave me the idea to combine the two: a project to take implementations of the algorithms from Hacker's Delight and prove the algorithms' properties with CBMC.

I have a modest start on github, which I am calling "Proven Delights":

read more…

24 June 2014, 12:35 UTC

Better pasting for irssi in X

While it's otherwise an excellent irc client, I've been frustrated by pasting in irssi. There are two main problems:

read more…

2 December 2013, 19:30 UTC

mailpie moves to github

My e-mail full text search software mailpie is now hosted at github:


23 November 2013, 18:19 UTC

SNTP from Python: getting server's esimate of time quality

Anders Wallin recently discussed generating graphs of server time error using a timestamp from the Date: header of an http request.

NTP servers themselves have estimates of time error, and it's possible to get the local server's idea of the error by sending SNTP packets to the local machine. You can also see the last time this NTP server successfully communicated with a peer or upstream, and see whether the server considers itself desynchronized by checking the stratum and leap second fields.

Based on my tinkering, the root_dispersion field or the derived LAMBDA value is the easiest to look at for estimating how bad the NTP time is. Between communications with a peer, root_dispersion and LAMBA increase at a rate of 15µs/s, ntp's arbitrary estimate of the accuracy of the local undisciplined clock. (YMMV and you can appaerntly tweak CLOCK_PHI in your NTP configuration file)

The attached program is a basic Python SNTP client which by default polls the local server every 5 seconds and prints some information. License: GPLv2+

Typical output:

12:25:58  0.06233  228.95875 ST-SYNCH LI-SYNCH
12:26:03  0.06241  233.96404 ST-SYNCH LI-SYNCH
12:26:08  0.06247  238.96745 ST-SYNCH LI-SYNCH
12:26:13  0.06255  243.97141 ST-SYNCH LI-SYNCH
12:26:18  0.06262  248.97673 ST-SYNCH LI-SYNCH
12:26:23  0.06270  253.98213 ST-SYNCH LI-SYNCH
12:26:28  0.06277  258.98746 ST-SYNCH LI-SYNCH

Files currently attached to this page:



30 September 2013, 14:19 UTC

GNU 30th—Celebrating 30 years

The GNU project and the Free Software movement are two of the most important computer-related things in my life. At $DAY_JOB we depend critically on many pieces of GNU software (such as gcc). This blog is hosted on a system with a GPL kernel and a GNU userspace, and the blog software itself is covered by the GPL license. My everyday computer is a GNU/Linux system, and my cellphone and tablet have GPL-licensed major components, such as the operating system kernel.

I personally have contributed thousands of unpaid hours developing GPL-licensed software, which is my way of repaying everyone else who has developed Free software.

On the other hand, we haven't come as far as one might wish. My everyday computer doesn't function as well without proprietary drivers, and many devices that should be general purpose computers don't ship in a mode that allows their owners to install the software of their choice. Careful choices are still necessary when buying hardware, particularly if you insist on an operating system that consists only of Free software.

Here's hoping that the next 30 years bring more gains for Free software and the GNU system. Either way, we'll still be here.


19 September 2013, 1:34 UTC

Debugging realtime components in UBC3

Here's how to run the debugger on realtime code in the "unified-build-candidate3" branch from This assumes you are using a run-in-place tree with a userspace realtime model, and that you understand the basics of gdb.

read more…

16 July 2013, 16:32 UTC

Rotary Delta Kinematics

Together with Chris Radek, I've been working on a visualization for his Rotary Delta Kinematics. Here's a picture of the work, which he hopes to push to soon. Of course the plot is of the AXIS splash screen.


6 March 2013, 13:59 UTC

Adventures in localization

19 February 2013, 22:42 UTC

Online Anagrammer with AJAX

6 February 2013, 14:59 UTC

The growth of Unicode over time

14 January 2013, 13:42 UTC

Hash saturation calculator

4 January 2013, 20:37 UTC

Screen-scrape ting device usage

10 December 2012, 20:21 UTC

New hardware

1 December 2012, 16:52 UTC

OpenID Delegation is easy!

All older entries
Website Copyright © 2004-2024 Jeff Epler