[PATCH v6 4/4] cramfs: rehabilitate it
- Date: Thu, 12 Oct 2017 02:16:13 -0400
- From: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
- Subject: [PATCH v6 4/4] cramfs: rehabilitate it
Update documentation, pointer to latest tools, appoint myself as
maintainer. Given it's been unloved for so long, I don't expect anyone
Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxx>
Tested-by: Chris Brandt <chris.brandt@xxxxxxxxxxx>
Documentation/filesystems/cramfs.txt | 42 ++++++++++++++++++++++++++++++++++++
MAINTAINERS | 4 ++--
fs/cramfs/Kconfig | 9 +++++---
3 files changed, 50 insertions(+), 5 deletions(-)
diff --git a/Documentation/filesystems/cramfs.txt b/Documentation/filesystems/cramfs.txt
index 4006298f67..8e19a53d64 100644
@@ -45,6 +45,48 @@ you can just change the #define in mkcramfs.c, so long as you don't
mind the filesystem becoming unreadable to future kernels.
+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
+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.
+A version of mkcramfs that can take advantage of the latest capabilities
+described above can be found here:
diff --git a/MAINTAINERS b/MAINTAINERS
index 65b0c88d5e..cd621c5f52 100644
@@ -3676,8 +3676,8 @@ F: drivers/cpuidle/*
-S: Orphan / Obsolete
+M: Nicolas Pitre <nico@xxxxxxxxxx>
diff --git a/fs/cramfs/Kconfig b/fs/cramfs/Kconfig
index ef86b06bc0..f937082f32 100644
@@ -1,5 +1,5 @@
- tristate "Compressed ROM file system support (cramfs) (OBSOLETE)"
+ tristate "Compressed ROM file system support (cramfs)"
Saying Y here includes support for CramFs (Compressed ROM File
@@ -15,8 +15,11 @@ config CRAMFS
cramfs. Note that the root file system (the one containing the
directory /) cannot be compiled as a module.
- This filesystem is obsoleted by SquashFS, which is much better
- in terms of performance and features.
+ This filesystem is limited in capabilities and performance on
+ purpose to remain small and low on RAM usage. It is most suitable
+ for small embedded systems. If you have ample RAM to spare, you may
+ consider a more capable compressed filesystem such as SquashFS
+ which is much better in terms of performance and features.
If unsure, say N.