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docs: filesystems: convert romfs.txt to ReST

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- Add a SPDX header;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d2cc83e7cd6de63c793ccd3f2588ea40f7f1e764.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mauro Carvalho Chehab 2020-02-17 17:12:22 +01:00 committed by Jonathan Corbet
parent 56e6d5c0eb
commit 6db0a480aa
2 changed files with 26 additions and 17 deletions
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1
Documentation/filesystems/index.rst
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@ -85,5 +85,6 @@ Documentation for filesystem implementations.
qnx6
ramfs-rootfs-initramfs
relay
romfs
virtiofs
vfat

42
Documentation/filesystems/romfs.txt → Documentation/filesystems/romfs.rst
View File

@ -1,4 +1,8 @@
ROMFS - ROM FILE SYSTEM
.. SPDX-License-Identifier: GPL-2.0
=======================
ROMFS - ROM File System
=======================
This is a quite dumb, read only filesystem, mainly for initial RAM
disks of installation disks. It has grown up by the need of having
@ -51,9 +55,9 @@ the 16 byte padding for the name and the contents, also 16+14+15 = 45
bytes. This is quite rare however, since most file names are longer
than 3 bytes, and shorter than 15 bytes.
The layout of the filesystem is the following:
The layout of the filesystem is the following::
offset content
offset content
+---+---+---+---+
0 | - | r | o | m | \
@ -84,9 +88,9 @@ the source. This algorithm was chosen because although it's not quite
reliable, it does not require any tables, and it is very simple.
The following bytes are now part of the file system; each file header
must begin on a 16 byte boundary.
must begin on a 16 byte boundary::
offset content
offset content
+---+---+---+---+
0 | next filehdr|X| The offset of the next file header
@ -114,7 +118,9 @@ file is user and group 0, this should never be a problem for the
intended use. The mapping of the 8 possible values to file types is
the following:
== =============== ============================================
mapping spec.info means
== =============== ============================================
0 hard link link destination [file header]
1 directory first file's header
2 regular file unused, must be zero [MBZ]
@ -123,6 +129,7 @@ the following:
5 char device - " -
6 socket unused, MBZ
7 fifo unused, MBZ
== =============== ============================================
Note that hard links are specifically marked in this filesystem, but
they will behave as you can expect (i.e. share the inode number).
@ -158,24 +165,24 @@ to romfs-subscribe@shadow.banki.hu, the content is irrelevant.
Pending issues:
- Permissions and owner information are pretty essential features of a
Un*x like system, but romfs does not provide the full possibilities.
I have never found this limiting, but others might.
Un*x like system, but romfs does not provide the full possibilities.
I have never found this limiting, but others might.
- The file system is read only, so it can be very small, but in case
one would want to write _anything_ to a file system, he still needs
a writable file system, thus negating the size advantages. Possible
solutions: implement write access as a compile-time option, or a new,
similarly small writable filesystem for RAM disks.
one would want to write _anything_ to a file system, he still needs
a writable file system, thus negating the size advantages. Possible
solutions: implement write access as a compile-time option, or a new,
similarly small writable filesystem for RAM disks.
- Since the files are only required to have alignment on a 16 byte
boundary, it is currently possibly suboptimal to read or execute files
from the filesystem. It might be resolved by reordering file data to
have most of it (i.e. except the start and the end) laying at "natural"
boundaries, thus it would be possible to directly map a big portion of
the file contents to the mm subsystem.
boundary, it is currently possibly suboptimal to read or execute files
from the filesystem. It might be resolved by reordering file data to
have most of it (i.e. except the start and the end) laying at "natural"
boundaries, thus it would be possible to directly map a big portion of
the file contents to the mm subsystem.
- Compression might be an useful feature, but memory is quite a
limiting factor in my eyes.
limiting factor in my eyes.
- Where it is used?
@ -183,4 +190,5 @@ limiting factor in my eyes.
Have fun,
Janos Farkas <chexum@shadow.banki.hu>