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L4Re: Textual Debugging Output on the Framebuffer

I have actually been in the process of drafting another article about writing device drivers to run within the L4 Runtime Environment (L4Re) on top of the Fiasco.OC microkernel, this being for the Ben NanoNote and Letux 400 notebook computers. That article started to trail behind a lot of the work being done, and there are a few loose ends to be tied up before I can finish it.

Meanwhile, on the way towards some kind of achievement with L4Re, confounded somewhat by the sometimes impenetrable APIs, I managed to eventually get something working that I had thought would have been one of the first things to demonstrate. When initially perusing the range of software in the “pkg” directory within the L4Re distribution, I saw a package called “fbterminal” providing a terminal program that shows itself on the framebuffer (or display).

I imagined being able to launch this on top of the graphical user interface multiplexer, Mag, and then have the “hello” program provide some output to this terminal. I even imagined having the terminal accept input from the keyboard, but we aren’t quite at that point, and that is where my other article comes in. Of course, I initially had no idea how to achieve this, and there needed to be a lot of work put in just to get back to this particular point of entry.

Now, however, the act of launching fbterminal and have it work is fairly straightforward. A few additional packages are required, but the framebuffer works satisfactorily as far as the other components are concerned, and the result will be a blank region of the screen with the terminal showing precisely nothing. Obviously, we want it to show something in order to confirm that it is working. I had to get used to seeing this blank terminal for a while.

The intended companion to fbterminal for testing purposes is the hello program which merely writes output to what might be described as a logging destination. This particular output channel is usually the serial console for the device, which meant that when porting the system to the Ben and the Letux, the hello program was of no use to me. But now, with a framebuffer to show things on, and with a terminal that might be able to accept output from other things, it becomes interesting to see if the hello program can be persuaded to send its output elsewhere.

It was useful to investigate how the output from the hello program actually makes its way to its destination. Since it uses standard C library functions, there has to be a mechanism for those functions to use. As far as I know, these would typically involve various system calls concerning files and streams. A perusal of the sources dredged up an interesting symbol called “__rtld_l4re_env_posix_vfs_ops”. Further investigation led me to the L4Re virtual filesystem (Vfs) functionality and the following interesting files:

  • pkg/l4re-core/l4re_vfs/include/vfs.h
  • pkg/l4re-core/l4re_vfs/include/impl/vfs_impl.h

And these in turn led me to the virtual console (Vcon) functionality:

  • pkg/l4re-core/l4re_vfs/include/impl/vcon_stream.h
  • pkg/l4re-core/l4re_vfs/include/impl/vcon_stream_impl.h

It seems that standard output from the hello program goes via the standard C calls and Vfs functions and is packaged up and sent using the Vcon mechanisms to the logging destination, which is typically provided by the root task, Moe. Given that fbterminal understands the Vcon protocol and acts as a console server, there appeared to be some potential in looking at Vcon mechanisms more closely. It seemed that fbterminal might be able to take the place of Moe.

Indeed, the documentation offers some clues. In the description of the init process, Ned, a mention is made of a program loader configuration parameter called “log_fab” that indicates an object that can create a suitable logging destination. When starting a program, the program loader creates such an object using “log_fab” and presents it to the new program as a capability (or object reference).

However, this is not quite what we want because we don’t need anything else to be created: we already have fbterminal ready for us to use. I suppose something could be conjured up to act as a factory and provide a fbterminal instance, and maybe this is not too arduous in the Lua-based configuration environment used by Ned, but I wanted a more direct solution.

Contemplating this, I went and rediscovered the definitions used by Ned to support its configuration scripting (found in pkg/l4re-core/ned/server/src/ned.lua). Here, the workings of the “log_fab” mechanism can be found and studied. But what I started to favour was a way of just indicating a capability to the hello program and not have the loader create something else. This required a simple edit to one of the functions:

function App_env:log()
  Class.check(self, App_env);
  if self.loader.log_fab == nil or self.loader.log_fab.create == nil then
    error ("Starting an application without valid log factory", 4);
  return self.loader.log_fab:create(Proto.Log, table.unpack(self.log_args));

Here, we want to ignore “log_fab” and just have our existing capability used instead. So, I introduced another clause to the if statement:

  if self.log_cap then
    return self.log_cap
  elseif self.loader.log_fab == nil or self.loader.log_fab.create == nil then
    error ("Starting an application without valid log factory", 4);

Now, if we specify “log_cap” when starting a program, it should want to direct logging messages to the referenced object instead. So, with this available to us, it becomes possible to adjust the way the hello program is started. First of all, we define the way fbterminal is set up and started:

local term = l:new_channel();

    caps = {
      fb = mag_caps.svc:create(L4.Proto.Goos, "g=320x230+0+0", "barheight=10"),
      term = term:svr(),

Since fbterminal needs to “export” its console abilities using a capability called “term”, this needs to be indicated in the “caps” table. (It doesn’t matter what the local variable for the channel is called.) So, the hello program is defined accordingly:

    log_cap = term,

Here, we make use of “log_cap” and allow the output to be directed to the terminal that has already been started. And the result is this:

fbterminal on the Ben NanoNote showing the hello program's output

fbterminal on the Ben NanoNote showing the hello program's output

And at long last, it becomes possible to see what programs are printing out to the log!

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