Linux Device Drivers, Second Edition

Drivers

Current Linux kernels support a huge number of devices. Device drivers account for half of the size of the source tree (actually two-thirds if you exclude architecture-specific code that you are not using). They account for almost 1500 C-language files and more than 800 headers.

The drivers directory itself doesn’t host any source file, only subdirectories (and, obviously, a makefile).

Structuring the huge amount of source code is not easy, and the developers haven’t followed any strict rules. The original division between drivers/char and drivers/block is inefficient nowadays, and more directories have been created according to several different requirements. Still, the most generic char and block drivers are found in drivers/char and drivers/block, so we’ll start by visiting those two.

drivers/char

The drivers/char directory is perhaps the most important in the drivers hierarchy, because it hosts a lot of driver-independent code.

The generic tty layer (as well as line disciplines, tty software drivers, and similar features) is implemented in this directory. console.c defines the linux terminal type (by implementing its specific escape sequences and keyboard encoding). vt.c defines the virtual consoles, including code for switching from one virtual console to another. Selection support (the cut-and-paste capability of the Linux text console) is implemented by selection.c; the default line discipline is implemented by n_tty.c.

There are other files that, ...