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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
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ReStructuredText
==========================
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Inter-Thread Communication
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==========================
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How threads wait for each other and pass data. The mechanisms live under
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``sched/``: ``sched/mqueue/`` for message queues, ``sched/semaphore/`` for
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semaphores and mutexes, ``sched/signal/`` for signals and ``sched/event/``
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for events.
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Each of them is in three pieces, and knowing which is which saves a search.
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``sched/`` holds the internal machinery, named ``nxsem_``, ``nxmq_`` and so
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on. ``libs/libc/semaphore/`` and ``libs/libc/signal/`` hold the POSIX
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surface that calls into it -- ``sem_wait()``, ``sigemptyset()``. And
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``fs/mqueue/``, ``fs/semaphore/`` and ``fs/event/`` hold only the *named*
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variants, the ``open``, ``close`` and ``unlink`` of each: giving one of
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these a name makes it a file system object, which is why that part lives
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with the file system.
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Which one to reach for:
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.. list-table::
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:header-rows: 1
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:widths: 24 76
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* - Mechanism
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- Use it when
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* - Semaphore
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- Counting something: a resource with N instances, or one thread
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telling another that work is ready.
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* - Mutex
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- Protecting data. Unlike a semaphore it has an owner: only the thread
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that locked it may unlock it, and never from an interrupt handler.
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* - Message queue
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- Passing data, not just a signal. The queue decouples the sender from
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the receiver, up to the point where it fills: a sender that finds it
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full waits for room.
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* - Signal
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- Interrupting a thread that is doing something else, or reacting to an
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asynchronous event.
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* - Event
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- Waiting on a combination of conditions rather than a single one.
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An interrupt handler may deliver one, which a mutex cannot accept.
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The only one of the five you have to ask for: ``CONFIG_SCHED_EVENTS``
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is off by default, where message queues and signals are on and are
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removed by ``CONFIG_DISABLE_MQUEUE`` and
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``CONFIG_DISABLE_ALL_SIGNALS``.
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Priority inheritance is not one of the things that separates the two.
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``CONFIG_PRIORITY_INHERITANCE`` covers semaphores as well as mutexes, and
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when it is set both start out with inheritance enabled; a semaphore used for
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signalling rather than locking is the case where you turn it back off, with
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``sem_setprotocol(sem, SEM_PRIO_NONE)``.
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.. toctree::
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:maxdepth: 1
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mutex.rst
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events.rst
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signal_handlers.rst
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