Linux 1.0 kernel source reading - 18: Character devices, part 2

Now, the terminals end of the characters devices. I've already read serial.c, which means I've read a terminal driver already without being properly aware. pty.c is the first one I read deliberately. Yes, it looks like a couple of connected terminals.

Then, onto keyboard code. defkemp.c, and diacr.h look like dull scan code conversion tables, so meh. kd.h is complicated - it's not just keyboard stuff but also includes video modes etc. kbd_kern.h covers LEDs, locks and modes for the keyboard. keyboard.c handles the keyboard itself - mostly how to handle incoming key presses (unsurprisingly), with a horrlbe mix of table-driven code, switch statements and ifs. Messy but not mysterious. It even uses an octal constant.

vt.h is short and obscure. vt_kern.h is short and obscure. vt.c is the ioctls for the terminal.

Then, onto console.c, a big and interesting file to implement the console on a text mode display. It starts off with a giant structure and then macros to make it convenient to access those fields. Scrolling functionality is a pile of assembly, which I'm sure is efficient, but I'm not going to try to decode inline gcc assembly.

This first part of the file is actually pleasantly readable video buffer wrangling. Then there's a 300 line function to interpret the stream of characters written to the console, then other console functionality before finally doing some font-setting stuff. In the end, much more straight-forward than I expected for a 2kloc file.

tty_ioctl.c is what I'm used to for this kind of file by now. Spoiler: It's a great long switch statement.

Finally, tty_io.c. Ugh, so tedious. I'm impressed Linus wrote all this as a student. Writing schedulers and VM and stuff is fun, but this is deeply boring and very practical, and... it's done. It's needed to make a basic usable Unix clone, and he did it. I really don't want to read it, and I'm mostly skimming.

There are some comments about "be really careful modifying this code, don't modify unless you understand it, as it's full of race conditions". Hint: Don't do this. If there are risks of races, explain them explicitly or even better remove the risk. Don't write an obscure comment, feel proud about how clever you and your code are, and wander off.

One function in particular I found interesting: tty_write_data covers the same issue I saw in the SCSI stack with "How do I call callbacks if I don't want to overflow the stack?". In this case, it's implemented with a bottom-half handler, which I guess is following the same pattern of "queue it up until the current work unit is complete". In other words, we convert the usual LIFO execution model for a FIFO model, from stack to queue.

Anyway, I declare character devices both boring and done. 17 thousand lines of network stack to go and I'm done with the Linux kernel!

Posted 2018-09-09.