architecture/code-update/flash-layout.md

4 code update application requires to be supported out-of-the-box, which includes
7 [code-update.md](code-update.md) for details about code update interfaces.
13 For system initialization and bootstrap, [Das U-Boot][] was selected as the
36 For storage of the root filesystem, a read-only volume was selected. This allows
38 data files, to be stored in a read-only filesystem image. Replacing read-only
50 read-only filesystem content. This applies to systems with limited attached
56 for the BMC can be stored in the U-Boot environment, and an init script can
70 #### JFFS2 on MTD partition
72 The majority of the filesystem is stored in a read-only squashfs in an MTD
73 partition using the block emulation driver (mtdblock). A second MTD partition is
74 mounted read-write using the JFFS2 filesystem. This read-write filesystem is
80 scripts (init, shutdown, and update) that locate the MTD partitions by name.
81 These scripts are installed by [obmc-phosphor-initfs][].
83 In code update mode, the squashfs image and white-listed files from the
84 read-write filesystem are copied into RAM by the initramfs and used to assemble
86 images at runtime. An orderly shutdown writes remaining images to like-named raw
87 MTD partitions and white listed files to the writable overlay filesystem.
92 based around the AST2400 and AST2500 system-on-chip controllers from Aspeed
93 Technology. These SOCs support 1 and 2 GB of DDR RAM, while the attached flash
94 storage is typically in the 10s of MB, so staging the filesystem to RAM is not
97 #### UBI on MTD partition
99 The majority of the filesystem is stored in a read-only squashfs in a static UBI
103 installed by the [preinit-mounts][] package before `systemd` is started.
104 Selecting UBI allows the writes to the read-write overlay to be distributed over
105 the full UBI area instead of just the read-write MTD partition.
107 The environment for Das U-boot continues to be stored at fixed sectors in the
108 flash. The Das U-boot environment contains enough MTD partition definition to
113 This option is enabled via the `obmc-ubi-fs` OpenBMC distro feature. Used in the
119 flash and the U-Boot environment selects which kernel to use.
136 [das u-boot]: https://www.denx.de/wiki/U-Boot
138   https://github.com/openbmc/docs/blob/master/architecture/openbmc-systemd.md
142 [obmc-phosphor-initfs]:
143 …tps://github.com/openbmc/openbmc/blob/master/meta-phosphor/recipes-phosphor/initrdscripts/obmc-pho…
144 [preinit-mounts]:
145   https://github.com/openbmc/openbmc/tree/master/meta-phosphor/recipes-phosphor/preinit-mounts
147   https://github.com/openbmc/docs/blob/master/architecture/code-update/emmc-storage-design.md