mirror of
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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
90 lines
2.8 KiB
ReStructuredText
90 lines
2.8 KiB
ReStructuredText
===
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CRC
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===
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Overview
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========
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NuttX provides CRC (Cyclic Redundancy Check) implementations for CRC8, CRC16,
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and CRC32 in ``libs/libc/misc/``. Each family offers multiple polynomial
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variants, following a consistent naming convention.
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CRC32 Variants
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==============
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NuttX ships several CRC32 variants that differ in polynomial, init value,
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and final XOR. The two most commonly used are ``crc32()`` and ``crc32_ieee()``.
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crc32() — NuttX Native
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-----------------------
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The original NuttX CRC32 implementation using the standard ISO 3309 / ITU-T
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V.42 polynomial (0xEDB88320 reflected). It uses **init value 0** with
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**no final XOR**:
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.. code-block:: c
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uint32_t crc32(FAR const uint8_t *src, size_t len);
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uint32_t crc32part(FAR const uint8_t *src, size_t len, uint32_t crc32val);
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This is the default CRC32 used by internal NuttX subsystems (bbsram, sbram,
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etc.) and is maintained for backward compatibility.
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crc32_ieee() — Linux/zlib Compatible
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-------------------------------------
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An IEEE-standard CRC32 compatible with Linux ``crc32()`` (from zlib). It uses
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the same polynomial but with **init value 0xFFFFFFFF** and **final XOR
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0xFFFFFFFF**:
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.. code-block:: c
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uint32_t crc32_ieee(FAR const uint8_t *src, size_t len);
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uint32_t crc32_ieeepart(FAR const uint8_t *src, size_t len, uint32_t crc32val);
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Use this variant when interoperating with Linux systems, e.g., for CRC
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checksums in network protocols, file transfers, or UART communication layers.
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Comparison
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----------
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============== ============ ========== ==========
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Function Init Value Final XOR Polynomial
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============== ============ ========== ==========
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crc32() 0x00000000 None 0xEDB88320
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crc32_ieee() 0xFFFFFFFF 0xFFFFFFFF 0xEDB88320
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============== ============ ========== ==========
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Example: computing CRC32 of the string ``"123456789"``:
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- ``crc32("123456789", 9)`` returns ``0x2DFD2D88``
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- ``crc32_ieee("123456789", 9)`` returns ``0xCBF43926`` (matches Linux/zlib)
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Incremental Calculation
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-----------------------
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Both variants support incremental (chunked) CRC calculation via the ``_part``
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functions. For ``crc32_ieee``, the first call should pass ``0`` as the initial
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value:
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.. code-block:: c
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uint32_t crc = 0;
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crc = crc32_ieeepart(chunk1, len1, crc);
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crc = crc32_ieeepart(chunk2, len2, crc);
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/* crc now contains the final CRC of chunk1+chunk2 */
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Other CRC32 Variants
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--------------------
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NuttX also provides CRC32 variants using different polynomials:
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- ``crc32h04c11db7()`` / ``crc32h04c11db7_part()`` — Castagnoli (CRC-32C)
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- ``crc32hf4acfb13()`` / ``crc32hf4acfb13_part()`` — CRC-32Q
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CRC16 and CRC8
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==============
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CRC16 and CRC8 follow the same naming pattern with multiple variant functions.
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See ``include/nuttx/crc16.h`` and ``include/nuttx/crc8.h`` for available
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variants.
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