Documentation/admin-guide: blockdev/ramdisk: remove use of "rdev"

	1) Overview

	2) Kernel Command Line Parameters

	3) Using "rdev -r"

	3) Using "rdev"

	4) An Example of Creating a Compressed RAM Disk

	rd_size

		See ramdisk_size.

3) Using "rdev -r"

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3) Using "rdev"

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The usage of the word (two bytes) that "rdev -r" sets in the kernel image is

as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up

to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit

14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a

prompt/wait sequence is to be given before trying to read the RAM disk. Since

the RAM disk dynamically grows as data is being written into it, a size field

is not required. Bits 11 to 13 are not currently used and may as well be zero.

These numbers are no magical secrets, as seen below::

"rdev" is an obsolete, deprecated, antiquated utility that could be used

to set the boot device in a Linux kernel image.

  ./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK     0x07FF

  ./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG          0x8000

  ./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG            0x4000

Instead of using rdev, just place the boot device information on the

kernel command line and pass it to the kernel from the bootloader.

Consider a typical two floppy disk setup, where you will have the

kernel on disk one, and have already put a RAM disk image onto disk #2.

You can also pass arguments to the kernel by setting FDARGS in

arch/x86/boot/Makefile and specify in initrd image by setting FDINITRD in

arch/x86/boot/Makefile.

Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk

starts at an offset of 0 kB from the beginning of the floppy.

The command line equivalent is: "ramdisk_start=0"

Some of the kernel command line boot options that may apply here are::

You want bit 14 as one, indicating that a RAM disk is to be loaded.

The command line equivalent is: "load_ramdisk=1"

You want bit 15 as one, indicating that you want a prompt/keypress

sequence so that you have a chance to switch floppy disks.

The command line equivalent is: "prompt_ramdisk=1"

Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.

So to create disk one of the set, you would do::

	/usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0

	/usr/src/linux# rdev /dev/fd0 /dev/fd0

	/usr/src/linux# rdev -r /dev/fd0 49152

  ramdisk_start=N

  ramdisk_size=M

If you make a boot disk that has LILO, then for the above, you would use::

	append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"

Since the default start = 0 and the default prompt = 1, you could use::

	append = "load_ramdisk=1"

	append = "ramdisk_start=N ramdisk_size=M"

4) An Example of Creating a Compressed RAM Disk

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	dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400

g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.

   For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would

   have 2^15 + 2^14 + 400 = 49552::

	rdev /dev/fd0 /dev/fd0

	rdev -r /dev/fd0 49552

g) Make sure that you have already specified the boot information in

   FDARGS and FDINITRD or that you use a bootloader to pass kernel

   command line boot options to the kernel.

That is it. You now have your boot/root compressed RAM disk floppy. Some

users may wish to combine steps (d) and (f) by using a pipe.

Changelog:

----------

SEPT-2020 :

                Removed usage of "rdev"

10-22-04 :

		Updated to reflect changes in command line options, remove

		obsolete references, general cleanup.

		James Nelson (james4765@gmail.com)

12-95 :

		Original Document

	memblock=debug	[KNL] Enable memblock debug messages.

	load_ramdisk=	[RAM] List of ramdisks to load from floppy

			See Documentation/admin-guide/blockdev/ramdisk.rst.

	load_ramdisk=	[RAM] [Deprecated]

	lockd.nlm_grace_period=P  [NFS] Assign grace period.

			Format: <integer>

			Param: <number> - step/bucket size as a power of 2 for

				statistical time based profiling.

	prompt_ramdisk=	[RAM] List of RAM disks to prompt for floppy disk

			before loading.

			See Documentation/admin-guide/blockdev/ramdisk.rst.

	prompt_ramdisk=	[RAM] [Deprecated]

	prot_virt=	[S390] enable hosting protected virtual machines

			isolated from the hypervisor (if hardware supports

	ramdisk_size=	[RAM] Sizes of RAM disks in kilobytes

			See Documentation/admin-guide/blockdev/ramdisk.rst.

	ramdisk_start=	[RAM] RAM disk image start address

	random.trust_cpu={on,off}

			[KNL] Enable or disable trusting the use of the

			CPU's random number generator (if available) to