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| author | Dapeng Mi <dapeng1.mi@linux.intel.com> | 2026-09-17 09:52:33 +0800 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2026-09-22 11:37:46 +0200 |
| commit | 0102c8c7fdfc4bd700971daff2b94470f4eafae4 (patch) | |
| tree | 531306c2299f9a269f0a986e29f17ba972180104 /Documentation/filesystems/cramfs.rst | |
| download | linux-stable-0102c8c7fdfc4bd700971daff2b94470f4eafae4.tar.gz linux-stable-0102c8c7fdfc4bd700971daff2b94470f4eafae4.zip | |
perf/x86/intel: Rename DMR offcore_rsp attribute to offmodule_rspgrafted
DMR introduces Offmodule Response events in place of the legacy
Offcore Response events, but it still exposes the inherited
offcore_rsp PMU attribute for programming the corresponding MSR data.
Rename the DMR PMU attribute to offmodule_rsp so the sysfs interface
matches the underlying event name and avoids user & tooling confusion.
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/20260917015234.981153-12-dapeng1.mi@linux.intel.com
Diffstat (limited to 'Documentation/filesystems/cramfs.rst')
| -rw-r--r-- | Documentation/filesystems/cramfs.rst | 129 |
1 files changed, 129 insertions, 0 deletions
diff --git a/Documentation/filesystems/cramfs.rst b/Documentation/filesystems/cramfs.rst new file mode 100644 index 000000000..221c0bf91 --- /dev/null +++ b/Documentation/filesystems/cramfs.rst @@ -0,0 +1,129 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=========================================== +Cramfs - cram a filesystem onto a small ROM +=========================================== + +cramfs is designed to be simple and small, and to compress things well. + +It uses the zlib routines to compress a file one page at a time, and +allows random page access. The meta-data is not compressed, but is +expressed in a very terse representation to make it use much less +diskspace than traditional filesystems. + +You can't write to a cramfs filesystem (making it compressible and +compact also makes it _very_ hard to update on-the-fly), so you have to +create the disk image with the "mkcramfs" utility. + + +Usage Notes +----------- + +File sizes are limited to less than 16MB. + +Maximum filesystem size is a little over 256MB. (The last file on the +filesystem is allowed to extend past 256MB.) + +Only the low 8 bits of gid are stored. The current version of +mkcramfs simply truncates to 8 bits, which is a potential security +issue. + +Hard links are not preserved. mkcramfs deduplicates files with +identical content, but two names for the same on-disk inode in the +source tree become two separate (content-shared) entries in the +image, and cramfs always reports a link count of 1. + +Cramfs directories have no ``.`` or ``..`` entries. Directories (like +every other file on cramfs) always have a link count of 1. (There's +no need to use -noleaf in ``find``, btw.) + +No timestamps are stored in a cramfs, so these default to the epoch +(1970 GMT). Recently-accessed files may have updated timestamps, but +the update lasts only as long as the inode is cached in memory, after +which the timestamp reverts to 1970, i.e. moves backwards in time. + +The on-disk layout is host-endian: the kernel does not swab, and +refuses to mount an image whose endianness does not match the CPU. +For cross-builds, mkcramfs -B / -L forces the output endianness so +that a host of one endianness can produce an image for a target of +the other. + +The on-disk block size is fixed at 4096 bytes. On systems with a +larger PAGE_SIZE you can change the #define in mkcramfs.c, with the +caveat that the resulting image will only be readable on kernels +configured for the same PAGE_SIZE. + + +Memory Mapped cramfs image +-------------------------- + +The CRAMFS_MTD Kconfig option adds support for loading data directly from +a physical linear memory range (usually non volatile memory like Flash) +instead of going through the block device layer. This saves some memory +since no intermediate buffering is necessary to hold the data before +decompressing. + +And when data blocks are kept uncompressed and properly aligned, they will +automatically be mapped directly into user space whenever possible providing +eXecute-In-Place (XIP) from ROM of read-only segments. Data segments mapped +read-write (hence they have to be copied to RAM) may still be compressed in +the cramfs image in the same file along with non compressed read-only +segments. Both MMU and no-MMU systems are supported. This is particularly +handy for tiny embedded systems with very tight memory constraints. + +The location of the cramfs image in memory is system dependent. You must +know the proper physical address where the cramfs image is located and +configure an MTD device for it. Also, that MTD device must be supported +by a map driver that implements the "point" method. Examples of such +MTD drivers are cfi_cmdset_0001 (Intel/Sharp CFI flash) or physmap +(Flash device in physical memory map). MTD partitions based on such devices +are fine too. Then that device should be specified with the "mtd:" prefix +as the mount device argument. For example, to mount the MTD device named +"fs_partition" on the /mnt directory:: + + $ mount -t cramfs mtd:fs_partition /mnt + +To boot a kernel with this as root filesystem, suffice to specify +something like "root=mtd:fs_partition" on the kernel command line. + + +Tools +----- + +A version of mkcramfs that can take advantage of the latest capabilities +described above can be found here: + +https://github.com/npitre/cramfs-tools + + +For /usr/share/magic +-------------------- + +===== ======================= ======================= +0 ulelong 0x28cd3d45 Linux cramfs offset 0 +>4 ulelong x size %d +>8 ulelong x flags 0x%x +>12 ulelong x future 0x%x +>16 string >\0 signature "%.16s" +>32 ulelong x fsid.crc 0x%x +>36 ulelong x fsid.edition %d +>40 ulelong x fsid.blocks %d +>44 ulelong x fsid.files %d +>48 string >\0 name "%.16s" +512 ulelong 0x28cd3d45 Linux cramfs offset 512 +>516 ulelong x size %d +>520 ulelong x flags 0x%x +>524 ulelong x future 0x%x +>528 string >\0 signature "%.16s" +>544 ulelong x fsid.crc 0x%x +>548 ulelong x fsid.edition %d +>552 ulelong x fsid.blocks %d +>556 ulelong x fsid.files %d +>560 string >\0 name "%.16s" +===== ======================= ======================= + + +Hacker Notes +------------ + +See fs/cramfs/README for filesystem layout and implementation notes. |
