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f6ecf80ebb
The documentation grew one page at a time, so the tree follows the
history of who wrote what and not the shape of NuttX. Scheduling is
spread over three places, a driver page can sit above the subsystem
that owns it, and the front page lists everything at the same level.
That is a lot to face when all you want to know is where the scheduler
lives.
This change files every page under the code it describes. It is a move,
not a rewrite: outside the ten pages named below, every page keeps the
text that is already in master, and no page's text is deleted.
What it does:
* Groups the table of contents into nine chapters.
* Moves the OS subsystems under os/: scheduling, memory, drivers,
filesystem, networking, IPC, interrupts, libs, time.
* Renames the platform pages to the names the source tree uses, and
derives their tags from the tree instead of by hand.
* Splits guides/ by subject.
* Adds Documentation/redirects.py, with a rule for every page that left
its old path, so old URLs keep working. The redirect page also carries
a link's #anchor across to the new page.
Ten pages have text that is new or rewritten. Nine of them are the
landing page of a chapter, which has to exist for the new structure:
index the front page
os/index OS Design
os/scheduling/index Scheduling
os/interrupts/index Interrupts
os/ipc/index IPC
os/time/index Time and timers
about/index About
developing/index Developing NuttX
ReleaseNotes/index Release notes
The tenth is os/libs/libbuiltin, the only page here with technical
content: libs/libbuiltin/ had no page at all. Five SVG diagrams come
with these pages, hand-written XML with no editor metadata.
Nothing outside Documentation/ is touched.
How it was checked:
* Sphinx builds with -W: no warnings, and no document left outside a
toctree.
* A script, offered in the PR, proves the narrow claim this rests on.
For every page outside the ten named above it erases what a move
touches -- link target, path, tag line, toctree block, table border --
from the whole old text and the whole new text, and requires the two
to be byte for byte identical. It also requires every sentence of a
deleted page to turn up somewhere, and every page that left its old
path to have a redirect, from a URL that existed, to where its content
went. It exits non-zero and names the page if any of that is not true,
and it tests added pages too, so forgetting to declare one cannot make
it pass.
* An independent audit checked 133 factual claims on these ten pages
against the tree, one shell command per claim: 130 confirmed, 1
refuted and fixed here, 2 not checkable.
* tools/checkpatch.sh is clean over the range.
The diff is large because moving a page changes every link that points
to it. Most of it is pure renames, and board pages that gained one tag
line.
Assisted-by: Claude:claude-opus-5
94 lines
3.9 KiB
ReStructuredText
94 lines
3.9 KiB
ReStructuredText
.. _device-nodes:
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============
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Device Nodes
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============
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Linux Device Nodes
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==================
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I used to have good Linux expertise a decade or so ago.
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But my current Linux knowledge is dated and rusty.
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I don't know anything about udev, SystemD, devtmpfs, sysfs, or any of that.
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So this is my simplified understanding.
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Device files work quite a bit differently in Linux and NuttX.
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A device node in Unix/Linux only really contains the only type of the device
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and its major and minor device numbers, i.e., it just holds data.
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So creating the device node does not install or create the driver;
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it simply writes a tiny file containing some special data.
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Nothing happens until you try to open the device.
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If something in the operating system has not initialized and registered
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a driver for that type and major/minor numbers,
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then you fail to open the device.
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So the device nodes and the device drivers are decoupled in Linux/Unix
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and there is a rendezvous that must occur later for the device node
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to actually refer to the device.
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"(..) Linux maps the device special file passed in system calls
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(say to mount a file system on a block device) to the device's
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device driver using the major device number and a number of system
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tables, ...The major number is actually the offset into the
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kernel's device driver table, which tells the kernel what
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kind of device it is (whether it is a hard disk or a serial
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terminal) (..)"
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-- Source:
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www.linux-tutorial.info/modules.php?name=MContent&pageid=94.
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Normally, when you create a Linux file system, you also create all of the
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standard device nodes. But most of these do not map to real devices.
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If you try to access most of the devices under ``/dev`` in Linux, they will
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fail because the underlying driver that maps to that major/minor number
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has not been initialized.
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NuttX Device Nodes
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==================
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NuttX does not use major/minor device numbers and there are no device
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"system tables" to associate major/minor numbers to a driver implementation.
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NuttX simplifies this be removing the "man in the middle": When you register
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the driver, you also create the device node.
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.. important:: The device node IS the driver registry.
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This is a tremendous simplification and one of the things that
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makes NuttX usable in the constrained MCU environment.
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In NuttX, device nodes are not really files at all.
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They are special entries in the NuttX root pseudo-filesystem.
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See :ref:`NuttX Pseudo File System <nuttx-pseudofs>` for more details.
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Usage Differences
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=================
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.. important:: Only devices drivers can create device nodes and the existence
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of the device node means that the device has been initialized,
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registered, and is ready for use (with the exception of some
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removable devices that may not actually be ready).
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You cannot create device nodes from applications!
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You could argue that this simplification is a deviation from my Unix/Linux
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roadmap and would have to agree that you are right.
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But it is also the kind of enabling simplification that makes a tiny
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Unix-like operating system feasible on these lower end MCUs.
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In Linux standard device drivers are initialized and registered as with NuttX.
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A (privileged) application can create a device node, but cannot initialize
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or register a device driver directly (as far as I know).
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I believe that if you want to instantiate an uninitialized, unregistered
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device driver you would have to install a kernel module containing
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the driver (which would probably also create the device nodes corresponding
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to the driver).
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boardctl()
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==========
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NuttX does support a sneak interface to support interactions with board-level
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OS logic. That sneak interface is ``boardctl()`` (see :ref:`board-ioctl` and
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:ref:`nuttx-initialization-sequence` for more details).
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That interface could potentially be used to force initialization of device
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drivers by application code. That discussion is to be provided.
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