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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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3.2 KiB
ReStructuredText
74 lines
3.2 KiB
ReStructuredText
====================
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CONFIG_NET_GUARDSIZE
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====================
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Global Option for All Drivers
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=============================
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``CONFIG_NET_GUARD_SIZE`` is global option. It is added to the allocated size of
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each driver packet buffer. Currently it is a very small value, defaulting to only
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two bytes. So it is not a memory hog and should be added to the packetsize for
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all drivers for commonality. But why?
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It should (eventually) be larger and common for all drivers. We need to look at
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how it is used today and how it might be used tomorrow. There is a probably a lot
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more involved than you might be initially considering.
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Packet Receipt
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==============
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For packet receipt, it is necessary for some hardware, but not for others. Often
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the hardware will DMA a 2 byte FCS at the end of the packet or possibly other
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hardware-specific info. But that is only part of the whole story.
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``CONFIG_NET_GUARDSIZE`` is not just for hardware packet receipt.
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Packet Transmission
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===================
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There are several issues for packet transmission. These are less well defined
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and need further study, but we need to keep all of the driver packet definitions
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in place until we understand how we are going to handle these things:
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* Memory Overrun Bugs
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There was in the past, a bug that caused write past the end of the buffer by
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a couple of bytes during TX message formatting. I don't know if that bug still
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exists, but the minimum, two-byte ``CONFIG_NET_GUARDSIZE`` was sufficient to
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eliminate the bug. That is why it has the name GUARD: Its primary purpose is
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to protect from overrunning the packet buffer and corrupting the following memory.
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I do no know if we have any such bugs today. Perhaps they still do?
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Perhaps they do not? Having such a guard is a good thing for reliability in
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any case.
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* Variable size IP/TCP headers
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There is a limitation in the way IP packets are formatted now. Basically they
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are formatted like this:
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#. When the packet is received a pointer to the location of the payload is
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set (d_appdata). This is an offset into the packet buffer For TCP, that
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accounts for the MAC/Ethernet header, the minimum IPv4/IPv6 header size,
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and the minimum TCP header size.
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#. The TCP payload is written at that location,
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#. The correctly sized IPv4/IPv6 headers and the correctly sized TCP header
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are added below the payload, and finally
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#. The MAC/Ethernet header as added.
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The start offset of the packet in the packet is no longer zero, but some
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variable offset into the packet buffer. That new start offset would have
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to be passed to driver in order to send the packet.
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The key to making this all work is:
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* Keep ``CONFIG_NET_GUARDSIZE`` in all driver buffers, and
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* Set the ``CONFIG_NET_GUARDSIZE`` to the maximum size of IPv4/IPv6 and TCP options
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(depending on which IP version is enabled and if TCP is enabled)
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* Extend the driver interface to accept data offset into the driver's packet buffer.
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* Variable MSS
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Closely related to this is the MSS which is the maximum size of the payload.
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Currently that is a constant because it assumes the minimum header lengths.
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It should be variable, depending on the actual header sizes.
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