Jim's Depository

The following modules need to be added to the Debian init ramdisk to support root on a 9p disk:

./kernel/fs/9p/9p.ko
./kernel/net/9p/9pnet.ko
./kernel/net/9p/9pnet_rdma.ko
./kernel/net/9p/9pnet_virtio.ko

To add these modules, you will extract an init ramdisk (which is just a compressed cpio archive), add the files, and run a depmod -a in it. Then re-archive it.

The commands I used to make mine are:

#!/bin/sh

set -e

mkdir /tmp/9p-ramdisk
cd /tmp/9p-ramdisk
zcat /boot/initrd.img-3.16.0-4-amd64 | cpio -id

mkdir lib/modules/3.16.0-4-amd64/kernel/fs/9p
mkdir lib/modules/3.16.0-4-amd64/kernel/net/9p

cp /lib/modules/3.16.0-4-amd64/kernel/fs/9p/9p.ko lib/modules/3.16.0-4-amd64/kernel/fs/9p/9p.ko 
cp /lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet.ko lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet.ko 
cp /lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet_rdma.ko lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet_rdma.ko 
cp /lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet_virtio.ko lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet_virtio.ko 

depmod -b /tmp/9p-ramdisk

find . | cpio -o -H newc | gzip > /boot/initrd.img-3.16.0-4-amd64-9p

Update: This works, but seems to be a bad idea. When I try to do aptitude update it fails with lines like this…

E: Unable to determine file size for fd 7 - fstat (2: No such file or directory)
E: Problem opening /var/lib/apt/lists/ftp.debian.org_debian_dists_jessie_contrib_binary-amd64_Packages
E: The package lists or status file could not be parsed or opened.
E: Unable to determine file size for fd 8 - fstat (2: No such file or directory)
E: Problem opening /var/lib/apt/lists/ftp.debian.org_debian_dists_jessie_contrib_binary-amd64_Packages
E: The package lists or status file could not be parsed or opened.

… which seems to say that 9p has a problem with some operation aptitude needs.

Back to disk images for me.

If you have an old HP plotter, you will find that the Mac OS X drivers available from HP’s support site can not be installed because they are in an old VISE .hqx format installer.

If you go and get a generic PPD for the printer, and stick it gzipped in /Library/Printers/PPDs/Contents/Resources then you will be able to select it as a printer type when you add the printer.

Attached is the one I found and used. It needs more work, I am getting postscript errors at the end of prints which can use a lot of paper. But it is working.

Update: It looks like you can disable postscript error printing with…

defaults write com.apple.print DoPostScriptErrorHandler NO

…and re-enable it with…

defaults delete com.apple.print DoPostScriptErrorHandler

Update2: The defaults thing does not appear to work.

Update3: Using HP JetDirect as the protocol instead of LPR seems to have fixed the PostScript error problem.

I’d just like to mention that when you use QEMU from a Macintosh running OS X, using the regular Terminal.app terminal, the terminal type will be listed as xterm-256. This will make your monitor console be black on black no mater what you do, except that making the background transparent will make the text somewhat visible.

The “solution” is to change your terminal type to regular old xterm. I’m doing something likes this on my QEMU commands…

$ TERM=xterm qemu-system-i386 ...the-rest-of-my-arguments…

I made a video of my cat chasing the shadow of her tail. It got posted to a social media site. I have contributed to videos of kittens.

If you find yourself making ajax requests for HTML pages, then using jquery to massage them or extract data before displaying it, you should be aware that you may be attempting to load the <img> resources from the page. This can be a performance killer or information leak.

Consider…

function parseSomeDudesPage( pageText) {
    var page = $(pageText);   // <<<<------   This is the problem
    ... do a bunch of stuff with page...
}

...
   $.get('http://somedudeswebsite.com/blah/blah.html', parseSomeDudesPage);
...

That line marked as the problem will parse the text into DOM nodes which will be in your main document. The browser may decide to start fetching the images contained in pageText, (Safari 8.0.2 does). Depending how you fetched the document you could be getting images, broken links, or security domain violations for pulling http: resources into your https: page.

I think the solution is to keep the nodes out of your main document by creating a new one. Reading this stackoverflow comment and not caring about old browsers in my application I went with…

function parseSomeDudesPage( pageText) {
    var page = (new DOMParser).parseFromString( pageText, 'text/html');
    ... do a bunch of stuff with page which is a new document and doesn't trigger <img> loading.
}

Since you have a new document, you won’t be able to move nodes from page to your main document, but I tend to create nodes anyway, so I don’t miss the ability.

A quick warning: When you upgrade your Debian installation to Jessie, you may find the grub2 can no longer install itself and you are left with an unbootable system. (Been there, done that, managed to save myself.)

Preconditions:

• Your disks have been MBR partitioned so far in the past that there isn’t a megabyte hiding between the MBR and your first partition.

• Your /boot is sitting on something which requires more than a tiny amount of code to access. In my case ext3 on an LVM partition.

When grub2 tries to update you will get messages about core.img being too big. At this point you may be screwed.

Am I safe?

If you are MBR partitioned, then check where your first partition is. If it starts at block 2048, you have plenty of space. If it is 63, you are in trouble.

If you are GUID partitioned, make sure you have a 1MB partition for boot loader.

How I recovered:

Since I was using LVM I plowed all the data off of my /dev/sda spindle using pvmove (many times), that involved de-mirroring things since I only had three spindles in the machine. Then I could repartition the drive. I went to GUID partitioning, where you explicitly make a boot loader partition for the use of grub2.

Then I could create an LVM physical volume on the remaining space of /dev/sda and shuffle all my data back into mirrors. The process was more complicated since I went through and reformatted each of the drives in case one blows and the BIOS chooses another, but all told it is possible to perform without a reboot.

I have a web site which displays a remote camera view. I like to update the <img> once a minute or so without forcing a page reload and all of its flicker. I’ve done many horrible things over the years, mostly related to adding a query parameter to the end of the URL and accepting that I will trash the cache and perform redundant downloads when the image is not changing.

The <img> tag is 21 years old, surely if it can drink in the USA it has had a .refresh() method added. brief check of specifications: and no.

stackoverflow is littered with questions and solutions about how to reload an <img>, most of them involve cache busting serial numbers in the query parameters.

Notable exceptions:

• Some answers recommend using a hash tag serial, since this should leave the URL to the server unchanged. This works in some browsers, but doesn’t cause a reload in others. Sometimes there is a combination of HTTP Cache-control headers that will cause a reload, sometimes not. Too touchy to use.

• You can use an <iframe> containing just the URI of your image to force a reload using an actual, supported mechanism! When that completes you can bludgeon your <img> tag enough to use the new version. See Method #4. I have dispensed with replacing the <img> element and just set the src to null and back. This keeps Safari and Chrome from flickering in January of 2015.

The <iframe> method is my new favorite. It requires Javascript, but I’m not sure how you would be deciding to refresh without Javascript, so that isn’t a loss. From Safari, it does cause two requests for the resource, but the second will be a 304 if you are using any sane caching mechanisms, and the first will only be a 200 if it really changed. (We are in 2015 and I don’t worry so much about the extra request since the servers are all SPDY and its not like its a new TCP connection or blocking anything else.)

Note to W3C: Add .reload() method which tells an <img> to reconsider the caching information and make a new request if required. I suppose a force boolean argument wouldn’t be too much. But nothing more complicated.

After too many years of femtoblogger succumbing to PHP language drift, I’ve been idly reimplementing it in Go.

I’ve been through several iterations wherein I tried to force sophisticated abstractions onto Go, with rather poor success. I finally realized that Go deliberately eschews abstraction. I gave up abstracting and just wrote out the code in as many minor variations as required to do the job, and it was a much more pleasant experience.

In the end I’m looking at about 2,000 lines of Go (not counting imports I didn’t write) which has most of the functionality of the 3,000 lines of PHP. So, within a power of two, they appear to be a wash.

femtoblogger has gained some important ground:

1. It is statically type checked. Typos and errors won’t be lurking around to explode at run time.

2. I have an executable which will continue to run until I break it. No more will the web site disintegrate when I update PHP on the server because the language drifted or a Debian packager changed a setting.

3. I feel better about its security. PHP always made me slightly nervous.

4. I’ve ditched WYSIWYG editing for markdown. The HTML folk have had plenty of time to make editing work, it’s not my problem if they can’t get their act together without thousands of lines of Javascript repair code.

So here’s to another 7 years of femtoblogger. Who knows what I’ll rewrite it in when 2021 comes around.

I purchased a pcduino v3 for a project. I specifically selected it for its Allwinner A20 processor with its dual Cortex A7 cores and SATA interface.

The vendor image I downloaded had a linux 3.4.79+ kernel which failed to work reliably with the SATA port and would freeze the computer with no debug output randomly, say every 10 minutes or so under heavy load. As is typical with ARM SoC units, the kernel source is a bit of a mess. In this case you have to get the kernel from Allwinner’s back ported android kernel, sunxi. There is some sunxi 7 support in mainline linux 3.17. Do not be deceived, it is not enough to work. I built 3.4.103++ from sunxi, which is a bit of a trial to get configured, there are a fair number of dependencies that you will only find when the compile breaks.

The 3.4.103++ kernel is working reliably for me with SATA, though my X display has broken. I’ll look into that later.

If there is a bootable SD card in the computer, it will boot from there. If not, it will fall back to its onboard NAND. I’ve elected to not touch the NAND and use it as my recovery method. Creating a bootable SD card requires a non-obvious trick. There is boot loader code which must be at block 8 and block 20 of the card. This means you cannot use a GUID partitioned card. The GUID partition table is in blocks 2 through 33.

You will need to build the these boot loaders, a stern googling for u-boot-sunxi-with-spl.bin will find all you need to know about it.

In partition 1 you will need to copy in the uEnv.txt, script.bin, and a uImage file. The uEnv.txt tells u-boot where to find your kernel and how to load it. The uImage is your kernel.

Some other things I learned along the way:

1. Linux no longer supports root=LABEL=MYDISK on the kernel command line. It looks like it supports partition UUIDs, but without a GUID labeled disk that isn’t going anywhere. There is a NT label fallback, but I couldn’t get it to work. I’ve had to resort to direct device names and the nondeterminism that brings in the face of a drive failure.

2. There is no meaningful documentation for the A20 chip except for a bunch of register names.

3. If there is a CPU temperature monitor in the A20, no one knows how to use it. There is one in the AXP209 PMU which may get exposed in /sys/ if you are clever or fortunate. (I added a heatsink and fan while fighting the system hang. It didn’t help, so CPU temperature is probably not an issue. Still, I’m leaving the heatsink on.)

4. The power connection to the board is unfortunate. They use a micro USB for power and ask for a 2 amp power source. The warning sign here is that micro USB connectors are only spec’d for 1.8 amps. Depending which of my 6 foot USB cables I use, I either get about 4.8V at 500mA, or 4.6V at 500mA. That is an uncomfortable voltage loss. I’d really have appreciated a couple of through holes where I could solder on a real header of some sort, especially since they suggest this board is suitable for 24x7 continuous use in devices. The schematic is large and disjoint, but I don’t think there is a good spot to pick this up.

5. You can monitor your incoming voltage and current use with:

   cat /sys/devices/platform/sunxi-i2c.0/i2c-0/0-0034/axp20-supplyer.28/power_supply/ac/voltage_now
   cat /sys/devices/platform/sunxi-i2c.0/i2c-0/0-0034/axp20-supplyer.28/power_supply/ac/current_now
   

6. For an idea of computer speed, a kernel compile using both cores takes about 50 minutes of wall clock time. My Core i7, (4 cores + hyper threading) does it in under 2 minutes. If you plan to do kernel work, cross compile.

7. You don’t need that initramfs that is in the default kernel config. If you are trying to install Debian it will even mess you up. Leave it out when you build the kernel.

8. You can get STL files for a pcduino v3 case. It is a little tight around the micro USB power connector and accessing the microSD card is about hopeless.

9. There are mounting holes, but be careful, there isn’t much clearance around them. It would be easy to make contact between a screw head and a component. I made tiny o-rings by slicing some insulation from #8 AWG wire to stand the screw head high enough to not touch components. Nylon screws with tiny heads would be a great idea.

I was unhappy with my options for a template system on a current project, so I have created a new one which may be useful to you.

The template language is inspired by Terrence Parr’s Enforcing Strict Model-View Separation in Template Engines in which he argues that Turing complete template languages are a mistake. Logic is best left to the model and controller, with the template engine acting as a view and simply converting from the model to the desired representation.

Having used View0 for a fair bit of source code generation, I have to agree.

View0 syntax leans toward meaningful words rather than cryptic symbols for the sake of non-programmers who might need to edit templates. There are only five directives.

You can find View0 and its manual at https://bitbucket.org/jimstudt/view0.