Device Nodes

Linux Device Nodes

Caveat: I used to have good Linux expertise a decade or so ago. But my current Linux knowledge is dated and rusty. I don't know anything about udev, SystemD, devtmpfs, sysfs, or any of that. So this is my simplified understanding…

Device files work quite a bit differently in Linux and NuttX. A device node in Unix/Linux only really contains the only type of the device and its major and minor device numbers, i.e., it just holds data. So creating the device node does not install or create the driver; it simply writes a tiny file containing some special data. Nothing happens until you try to open the device. If something in the operating system has not initialized and registered a driver for that type and major/minor numbers, then you fail to open the device.

So the device nodes and the device drivers are decoupled in Linux/Unix and there is a rendezvous that must occur later for the device node to actually refer to the device:

”…Linux maps the device special file passed in system calls (say to mount a file system on a block device) to the device's device driver using the major device number and a number of system tables, …The major number is actually the offset into the kernel's device driver table, which tells the kernel what kind of device it is (whether it is a hard disk or a serial terminal)…” http://www.linux-tutorial.info/modules.php?name=MContent&pageid=94

Normally, when you create a Linux file system, you also create all of the standard device nodes. But most of these do not map to real devices. If you try to access most of the devices under /dev in Linux, they will fail because the underlying driver that maps to that major/minor number has not been initialized.

NuttX Device Nodes

NuttX does not use major/minor device numbers and there are no device “system tables” to associate major/minor numbers to a driver implementation. NuttX simplifies this be removing the “man in the middle”: When you register the driver, you also create the device node. The device node IS the driver registry. This is a tremendous simplification and one of the things that makes NuttX usable in the constrained MCU environment.

In NuttX, device nodes are not really files at all. They are special entries in the NuttX root pseudo-filesystem (another simplification and another story. See NuttX Pseudo File System).

Usage Differences

And this leads to differences: Only devices drivers can create device nodes and the existence of the device node means that the device has been initialized, registered, and is ready for use (with the exception of some removable devices that may not actually be ready). You cannot create device nodes from applications.

You could argue that this simplification is a deviation from my Unix/Linux roadmap and would have to agree that you are right. But it is also the kind of enabling simplification that makes a tiny Unix-like operating system feasible on these lower end MCUs.

In Linux standard device drivers are initialized and registered as with NuttX. A (privileged) application can create a device node, but cannot initialize or register a device driver directly (as far as I know). I believe that if you want to instantiate an uninitialized, unregistered device driver you would have to install a kernel module containing the driver (which would probably also create the device nodes corresponding to the driver).

boardctl()

NuttX does support a sneak interface to support interactions with board-level OS logic. That sneak interface is boardctl(). That interface could potentially be used to force initialization of device drivers by application code. That discussion is to be provided.