Me and a RaspberryPi: Don’t Open That I/O Port

The article’s title is a play on the phrase “don’t open that door”, and makes more sense in Italian as we use the same word for ‘door’ and ‘port’…

So you left your hero (me) working on setting up a Raspberry Pi with at least a partial base of cross-compilation. The whole thing worked to a decent extent, but it wasn’t really as feasible as I hoped. Too many things, including Python, cannot cross-compile without further tricks, and the time it takes to figure out how to cross-compile them, tend to be more than that needed to just wait for it to build on the board itself. I guess this is why there is that little interest in getting cross-compilation supported.

But after getting a decent root, or stage4 as you prefer to call it, I needed to get a kernel to boot the device. This wasn’t easy.; there is no official configuration file published — what they tell you is, if you want to build a new custom kernel, to zcat /proc/config.gz from Raspian. I didn’t want to use Raspian, so I looked further. The next step is to check out the defconfig settings that the kernel repository includes, a few, different of them exist.

You’d expect them to be actually thought out to enable exactly what the RaspberryPi provides, and nothing more or less. Some leeway can be expected for things like network options, but at least the “cutdown” version should not include all of IrDA, Amateur Radio, Wireless, Bluetooth, USB network, PPP, … After disabling a bunch of options, since the system I need to run will have very few devices connected – in particular, only the Davis Vantage Pro station, maybe a printer – I built the kernel and copied it over the SD card. It booted, it crashed. Kernel panicked right away, due to a pointer dereference.

After some rebuild-copy-test cycles I was able to find out what the problem is. It’s a problem that is not unique to the RPi actually, as I found the same trace from an OMAP3 user reporting it somewhere else. The trick was disabling the (default-enabled) in-kernel debugger – which I couldn’t access anyway, as I don’t have an USB keyboard at hand right now – so that it would print the full trace of the error .That pointed at the l4_init function, which is the initialization of the Lightning 4 gameport controller — an old style, MIDI game port.

My hunch is that this expansion card is an old-style ISA card, since it does not rely on PCI structures to probe for the device — I cannot confirm it because googling for “lightning 4” only comes up with images of iPad and accessories. What it does, is simply poking at the 0x201 address, and the moment when it does, you get a bad dereference from the kernel exactly at that address. I’ve sent a (broken, unfortunately) patch to the LKML to see if there is an easy way to solve this.

To be honest and clear, if you just take a defconfig and build it exactly as-is, you won’t be hitting that problem. The problem happens to me because in this kernel, like in almost every other one I built, I do one particular thing: I disable modules so that a single, statically build kernel. This in turn means that all the drivers are initialized when you start the kernel, and the moment when the L4 driver is started, it crashes the kernel. Possibly it’s not the only one.

This is most likely not strictly limited to the RaspberryPi but it doesn’t help that there is no working minimal configuration – mine is, by the way, available here – and I’m pretty sure there are other similar situations even when the arch is x86… I guess it’s just a matter of reporting them when you encounter them.

Me and a RaspberryPi: Cross-linking

You can probably guess that my target is not building packages for an embedded device on the device itself. I usually have a good luck with cross-compilation, but I usually also use very nasty hacks to get it to work. This time around I’m trying to use as few hacks as humanly possible. And linking is becoming trouble.

So first of all you need to get your cross compiler, but that’s easy:

# crossdev armv6j-hardfloat-linux-gnueabi

And everything will be taken care of for you. The whole toolchain, prefixed with armv6j-hardfloat-linux-gnueabi- will then be available, including a suitable armv6j-hardfloat-linux-gnueabi-pkg-config so that you don’t have to deal with it manually.

Now you got two options on how you want to proceed: you may only need a single configuration for all your cross-compiled targets, or you may want to customize the configuration. The former case is the easiest: a root will be set up by crossdev in /usr/armv6j-hardfloat-linux-gnueabi so you can just configure it like a chroot and just armv6j-hardfloat-linux-gnueabi-emerge --root=$path to build the packages.

But I considered that a bad hack so I wanted to do something different: I wanted to have a self-contained root and configuration. The newest GCC theoretically allows this in a very simple fashion: you just need to have already the basic components (glibc, for once) in the root, and then you can use the --with-sysroot flag to switch out of the crossdev-installed headers and libraries. Unfortunately, while the compiler behaves perfectly with this switch, the same can’t be said of the link editor.

Indeed, while even ld supports the --with-sysroot parameter, it will ignore it, making it impossible to find the libraries that are not installed in /usr/armv6j-hardfloat-linux-gnueabi. The classical solution to this is to use -L$ROOT/usr/lib -L$ROOT/lib so that the link editor is forced to search those paths as well — unfortunately this can cause problems due to the presence of .la files, and even more so due to the presence of ldscripts in /usr/lib.

You might remember a quite older post of mine that discussed the names of shared libraries. The unversioned name link is used by the link editor to find which library to link to when you ask for -lfoo. For most libraries this alias is provided by a symlink, but for a number of reasons (which honestly are not that clear to me) for libraries that are installed in /lib, an ldscript is created. This ldscript will provide a non-root-relative path to the shared object, so for instance $ROOT/usr/lib/ will point to /lib/ and that’s not going to fly very well unless sysroot gets respected, but luckily for us, this does actually work.

What it seems like, is that at least the BFD link editor coming from binutils 2.23 has trouble with the implementation of --sysroot for search paths only (it works fine when expanding the paths for the ldscripts) — what about the gold link editor that should be the shiniest out there? Well, it looks like --sysroot there, while technically supported, is implemented in an even worse way: it does not use it when expanding the paths in the ldscripts, which is a 2009 fix from Gentoo on the BFD link editor side.

At the end, my current solution involves setting this in the make.conf in the root:

CFLAGS="-O2 -pipe -march=armv6j -mfpu=vfp -mfloat-abi=hard --sysroot=/home/flame/rasppy/root"
LDFLAGS="-Wl,-O1,--as-needed --sysroot=/home/flame/rasppy/root -Wl,--sysroot=/home/flame/rasppy/root -L=/usr/lib"


And then use this command as emerge:

% sudo armv6j-hardfloat-linux-gnueabi-emerge --root=/home/flame/rasppy/root --config-root=/home/flame/rasppy/root

And it seems to work decently enough. Some packages, of course, fail to cross-compile at all, but that’s a story for a different time.

Me and a RaspberryPi: Introduction

People who follow me on Google+ might have noticed last night that I ordered a RaspberryPi board. This might sound strange, but the reason is simple: I needed a very small, very low power computer to set up a friend of mine with, for a project that we’ve decided to work on together, but let’s put this in order.

My friend owns a Davis Vantage Pro2 weather station, and he publishes on his website the data coming from it — up to now he’s been doing that with the software that comes with the station itself, which runs on Windows, and in particular on his laptop. So no update if he’s not at home.

So, what has this to do with a RaspberryPi board? Well, the station connects to a PC via an USB cable, connected in turn to an USB-to-serial adapter, which means that there is no low-level protocol to reverse engineer, and not only that, but Davis publishes the protocol specifications as well as a number of other documentation and SDKs for Windows and Macintosh.

It is strange that Davis does not publish anything for Linux themselves, but I found an old project and a newer one that seems to do exactly what my friend needs — the latter in particular does exactly what we need, which means that my task in all this is to set up a Gentoo Linux install (cross-compiled, of course) to run on that Pi and have wview to actually work on Gentoo — it requires some packaging, and more likely than not, some fixing.

Thankfully, I don’t have to start from scratch; Elias pointed out that we have a good page on the wiki with the instructions, even though they do not include cross-compilation pointers and other things like that that could be extremely useful. I’ll probably extend from there with whatever I’ll find useful beside the general cross compilation. Depending on whether I’ll need non-cross-compilable software I might end up experimenting with qemu-user for ARM chroots..

At any rate, this post serves as an introduction of what you might end up reading in this blog in the future, which might or might not end up on Planet Gentoo depending on what the topic of the single post is.