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Linux 2.6.26.5 embedded. I need manually mount the root file system from busybox shell (initramfs). Because this Linux kernel not use devtmpfs, I have to create the basic device nodes manually, use mknod (/dev/null, /dev/zero, dev/mtdblock {0-10}, ttys). I'm not sure about correct format for /dev/mtdblock and ttys. Should I create nodes for both /dev/mtdblock and /dev/mtd? The device creates 11 MTD partitions on nand0. Where to find the major and minor numbers assigned for a devices? The same question for ttys: what is correct mknod command for 5 /dev/tty and 4 tty - are they in different places?

mknod -m 666 tty c 4 0
mknod -m 666 /dev/tty c 5 0

Edit: partitions, devices and filesystems

# cat /proc/partitions
major minor  #blocks  name

  31     0        384 mtdblock0
  31     1        128 mtdblock1
  31     2      20352 mtdblock2
  31     3       7168 mtdblock3
  31     4      18816 mtdblock4
  31     5       2048 mtdblock5
  31     6       1024 mtdblock6
  31     7        512 mtdblock7
  31     8        128 mtdblock8
  31     9        512 mtdblock9
  31    10        512 mtdblock10
# cat /proc/devices
Character devices:
  1 mem
  2 pty
  3 ttyp
  4 /dev/vc/0
  4 tty
  4 ttyS
  5 /dev/tty
  5 /dev/console
  5 /dev/ptmx
  7 vcs
 10 misc
 13 input
 89 i2c
 90 mtd
108 ppp
128 ptm
136 pts
153 spi
204 ttyJ
254 cordless

Block devices:
  1 ramdisk
 31 mtdblock
 93 nftl
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  • 2
    I'm voting to keep this question open as questions about official documentation are explicitly on-topic.
    – Anthony Geoghegan
    Dec 4, 2017 at 12:36
  • mknod /dev/mtdblock0 b 31 0 will create the mtdblock0 for you. Tricky question: what do you think, what could mean the "major" and "minor" columns in /proc/partitions? :-)
    – peterh
    Dec 5, 2017 at 21:20
  • @peterh yes, I know that the "major" and "minor" numbers is specified in columns in /proc/partitions. What's not clear: I have 11 partitions, but I want access only root (mtd4), so would be the mtd4/mtdblock4 device node pair sufficient here? Or I have to create all device nodes mtdx and mtdblockx for each partition?(even if I want accees only root)
    – Lexx Luxx
    Dec 6, 2017 at 14:04
  • @triwo Then ask this in a new question. You've got your answer for your problem, now you can have your next question.
    – peterh
    Dec 6, 2017 at 14:06

2 Answers 2

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Device files are essentially a table in the kernel. A C array.

More exactly, a two-level tree structure of C arrays: the upper level for the major numbers and the lower for the minors.

Drivers (and the kernel core), can register driver handlers for them. A driver uses always a major number. For example, the software raid ("md") users the block major 9.

You can get the list of the currently registered block- and char devices in the /proc/devices files.

The final answer is in the kernel source, there are "(un)register_block_device" or similar calls of the kernel core. Find them, grep for them, and so you can get a full list.

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  • Should be created nodes for both mtdblockX and mtdX devices? Also, not clear about TTY devices.
    – Lexx Luxx
    Dec 4, 2017 at 13:00
  • @triwo tty is only a character device, in its functionality is it essentially a socket, extended with some terminal-controlling ioctl()-s. There is a "tty driver" in the kernel, it registers the ttys with a register_char_device(4) (or similar, check the source) call. From that point, the open/close/read/write/ioctl and similar calls to character device files with major number 4 will be implemented by the tty driver. The tty driver handles the character console of the Linux kernel. There is also a pts driver, its major number is 136, it is mainly for the daemons, like sshd.
    – peterh
    Dec 4, 2017 at 13:08
  • @triwo From the ttys I wrote more here. | mtd & mtdblock: I don't know the details. But: It is quite possible that a driver registers multiple block major numbers. Typically they mean different functionality: for example, in kernels until ca. 2.2, /dev/sr0 (block major 11) meaned an scsi cdrom, while /dev/sg0(block major 21) was a generic scsi device. The scsi cdrom driver used both. I think some similar could happen also in the mtd / mtdblock line.
    – peterh
    Dec 4, 2017 at 13:14
  • @triwo The kernel core doesn't know anything about the path/filename of the device files, it see only block/char and major/minor numbers. The device files below /dev are normally created by the userspace tool named udev, but from the view of the kernel, it is not a requirement. The kernel can see a table of major numbers and that which driver belongs to them.
    – peterh
    Dec 4, 2017 at 13:19
  • I added mtd & mtdblock commands to the topic. Which version for mtd chars should I use?
    – Lexx Luxx
    Dec 5, 2017 at 11:12
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From devices.txt in the kernel documentation.

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