17. Boot

Warren Gay¹

(1)

St. Catharine’s, Ontario, Canada

When the power is first applied to the Raspberry Pi, or it has been reset, a boot sequence is initiated. It is actually the Pi’s GPU that brings up the ARM CPU. Originally, the way that the Raspberry Pi was designed, it had to be booted from firmware found on the SD card. RISC code for the GPU is provided by the Raspberry Pi Foundation in the file bootcode.bin. After the second-stage boot loader was executed, it was possible to load other operating systems or boot loaders, such as U-Boot.

Due to public interest, changes have been introduced along the way to allow a direct boot from USB. This chapter covers the process of booting in its various configuratons.

Booting ARM Linux

The modern boot procedure consists of the following sequence of events:

1.
At power-up (or reset), the ARM CPU is offline, but the GPU is powered up.
2.
A small RISC core in the GPU begins to execute the OTP (one time programmable) ROM code (first-stage boot loader).
3.
By default, it determines booting from the following priority list:
1. a.
  SD card boot
2. b.
  USB device boot
3. c.
  GPIO boot mode

GPIO boot mode is described here:

https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/bootflow.md

There are several warnings described there including:

OTP settings cannot be undone.
Do not try to use program_gpio_bootmode unless your firmware is dated Oct. 20, 2017 or later.

The program_gpio_bootmode is perhaps more applicable to the compute module than the regular Pi.

The boot process continues, according to the media type (SD card, USB, or GPIO):

4.
The GPU initializes the SD card/USB/GPIO hardware.
5.
The GPU looks at the FAT (file allocation table) partition(s) in the SD card or USB media. The search continues with FAT partitions until the file bootcode.bin is found. The sequence changes if the OTP settings are modified. Normally:
1. a.
  The SD card is checked first, which takes up to 5 seconds to fail (when not present).
2. b.
  Then USB mode boot will be tried when enabled by OTP (this requires at least 3 seconds to allow for drive spinup and enumeration).
6.
The second-stage boot-loader firmware named bootcode.bin is loaded into the local 128k cache.
7.
The GPU control passes to the loaded bootcode.bin firmware and it enables SDRAM.
8.
The file start.elf is loaded by the GPU into RAM from the same partition and the GPU gives it control.
9.
The file config.txt is examined for configuration parameters that need to be processed.
10.
Information found in cmdline.txt is also processed by start.elf.
11.
The GPU allows the ARM CPU to execute the program start.elf.
12.
The kernel image is loaded into RAM by the GPU running start.elf.
13.
Finally, the GPU starts the kernel executing on the ARM CPU.

Boot Files

The FAT partition containing the boot files is normally mounted as /boot when Raspbian Linux has come up. Table 17-1 lists the files that apply to the boot process. The text files can be edited to affect new configurations. The binary files can also be replaced by new revisions of the same.

Table 17-1

/boot Files

File Name	Purpose	Format
bootcode.bin	Second-stage boot loader	Binary
config.txt	Configuration parameters	Text
cmdline.txt	Command-line parameters for kernel	Text
fixup*.dat	Partitions the SDRAM between ARM CPU and GPU	Binary
start.elf	ARM CPU code to be launched	Binary
kernel.img	Kernel to be loaded	Binary
	Name can be overridden with kernel= parameter in config.txt

config.txt

Unlike many PCs that contain a BIOS system, the Raspberry Pi uses the config.txt file. This file is optional, so when it is missing, defaults apply. When Raspbian Linux is booted, this file is found in /boot/config.txt.

What is supported by vcgencmd in this file can be displayed with:

# vcgencmd get_config int

aphy_params_current=819

arm_freq=1400

audio_pwm_mode=514

config_hdmi_boost=5

core_freq=250

desired_osc_freq=0x33e140

desired_osc_freq_boost=0x3c45b0

disable_commandline_tags=2

disable_l2cache=1

display_hdmi_rotate=-1

display_lcd_rotate=-1

dphy_params_current=547

enable_uart=1

force_eeprom_read=1

force_pwm_open=1

framebuffer_ignore_alpha=1

framebuffer_swap=1

gpu_freq=300

hdmi_force_cec_address=65535

init_uart_clock=0x2dc6c00

lcd_framerate=60

over_voltage_avs=31250

over_voltage_avs_boost=0x200b2

pause_burst_frames=1

program_serial_random=1

sdram_freq=450

For string options use:

# vcgencmd get_config str

device_tree=-

Additionally, there are additional options including the device tree settings:

dtparam=spi=on

and

dtoverlay=mcp23017,gpiopin=4,addr=0x20

Because the support for the original and new options keep changing, with each new model that is released, you can review this resource:

https://elinux.org/RPiconfig

Composite Aspect Ratio

The sdtv_aspect parameter configures the composite video aspect ratio.

sdtv_aspect	Description
1	4:3 (default)
2	14:9
3	16:9

Color Burst

By default, color burst is enabled. This permits the generation of color out of the composite video jack. Setting the video for monochrome may be desirable for a sharper display in some situations.

sdtv_disable_colourburst	Description
0	Color burst enabled (default)
1	Color burst disabled (monochrome)

High-Definition Video

This section covers config.txt settings that affect HDMI operation.

HDMI Safe Mode

The hdmi_safe parameter enables support of automatic HDMI configuration for optimal compatibility.

hdmi_safe	Description
0	Disabled (default)
1	Enabled

When hdmi_safe=1 (enabled), the following settings are implied:

hdmi_force_hotplug=1
config_hdmi_boost=4
hdmi_group=1
hdmi_mode=1
disable_overscan=0

HDMI Force Hot-Plug

This configuration setting allows you to force a hot-plug signal for the HDMI display, whether the display is connected or not. The NOOBS distribution enables this setting by default.

hdmi_force_hotplug	Description
0	Disabled (non-NOOBS default)
1	Use HDMI mode even when no HDMI monitor is detected (NOOBS default)

HDMI Ignore Hot-Plug

Enabling the hdmi_ignore_hotplug setting causes it to appear to the system that no HDMI display is attached, even if there is. This can help force composite video output, while the HDMI display is plugged in.

hdmi_ignore_hotplug	Description
0	Disabled (default)
1	Use composite video even if an HDMI display is detected

HDMI Drive

This mode allows you to choose between DVI (no sound) and HDMI mode (with sound, when supported).

hdmi_drive	Description
1	Normal DVI mode (no sound)
2	Normal HDMI mode (sound will be sent if supported and enabled)

HDMI Ignore EDID

Enabling this option causes the EDID information from the display to be ignored. Normally, this information is helpful and is used.

hdmi_ignore_edid	Description
Unspecified	Read EDID information
0xa5000080	Ignore EDID information

HDMI EDID File

When hdmi_edid_file is enabled, the EDID information is taken from the file named edid.txt. Otherwise, it is taken from the display, when available.

hdmi_edid_file	Description
0	Read EDID data from device (default)
1	Read EDID data from edid.txt file

HDMI Force EDID Audio

Enabling this option forces the support of all audio formats even if the display does not support them. This permits pass-through of DTS/AC3 when reported as unsupported.

hdmi_force_edid_audio	Description
0	Use EDID-provided values (default)
1	Pretend all audio formats are supported

Avoid EDID Fuzzy Match

Avoid fuzzy matching of modes described in the EDID.

avoid_edid_fuzzy_match	Description
0	Use fuzzy matching (default)
1	Avoid fuzzy matching

HDMI Group

The hdmi_group option defines the HDMI type.

hdmi_group	Description
0	Use the preferred group reported by the EDID (default)
1	CEA
2	DMT

HDMI Mode

This option defines the screen resolution to use in CEA or DMT format (see the parameter hdmi_group in the preceding subsection “HDMI Group”). In Table 17-2, the modifiers shown have the following meanings:

H means 16:9 variant of a normally 4:3 mode.
2x means pixel doubled (higher clock rate).
4x means pixel quadrupled (higher clock rate).
R means reduced blanking (fewer bytes are used for blanking within the data stream, resulting in lower clock rates).

Table 17-2

HDMI Mode Settings

Group	CEA			DMT
Mode	Resolution	Refresh	Modifiers	Resolution	Refresh	Notes
1	VGA			640×350	85 Hz
2	480 p	60 Hz		640×400	85 Hz
3	480 p	60 Hz	H	720×400	85 Hz
4	720 p	60 Hz		640×480	60 Hz
5	1080 i	60 Hz		640×480	72 Hz
6	480 i	60 Hz		640×480	75 Hz
7	480 i	60 Hz	H	640×480	85 Hz
8	240 p	60 Hz		800×600	56 Hz
9	240 p	60 Hz	H	800×600	60 Hz
10	480 i	60 Hz	4x	800×600	72 Hz
11	480 i	60 Hz	4x H	800×600	75 Hz
12	240 p	60 Hz	4x	800×600	85 Hz
13	240 p	60 Hz	4x H	800×600	120 Hz
14	480 p	60 Hz	2x	848×480	60 Hz
15	480 p	60 Hz	2x H	1024×768	43 Hz	Don’t use
16	1080 p	60 Hz		1024×768	60 Hz
17	576 p	50 Hz		1024×768	70 Hz
18	576 p	50 Hz	H	1024×768	75 Hz
19	720 p	50 Hz		1024×768	85 Hz
20	1080 i	50 Hz		1024×768	120 Hz
21	576 i	50 Hz		1152×864	75 Hz
22	576 i	50 Hz	H	1280×768		R
23	288 p	50 Hz		1280×768	60 Hz
24	288 p	50 Hz	H	1280×768	75 Hz
25	576 i	50 Hz	4x	1280×768	85 Hz
26	576 i	50 Hz	4x H	1280×768	120 Hz	R
27	288 p	50 Hz	4x	1280×800		R
28	288 p	50 Hz	4x H	1280×800	60 Hz
29	576 p	50 Hz	2x	1280×800	75 Hz
30	576 p	50 Hz	2x H	1280×800	85 Hz
31	1080 p	50 Hz		1280×800	120 Hz	R
32	1080 p	24 Hz		1280×960	60 Hz
33	1080 p	25 Hz		1280×960	85 Hz
34	1080 p	30 Hz		1280×960	120 Hz	R
35	480 p	60 Hz	4x	1280×1024	60 Hz
36	480 p	60 Hz	4x H	1280×1024	75 Hz
37	576 p	50 Hz	4x	1280×1024	85 Hz
38	576 p	50 Hz	4x H	1280×1024	120 Hz	R
39	1080 i	50 Hz	R	1360×768	60 Hz
40	1080 i	100 Hz		1360×768	120 Hz	R
41	720 p	100 Hz		1400×1050		R
42	576 p	100 Hz		1400×1050	60 Hz
43	576 p	100 Hz	H	1400×1050	75 Hz
44	576 i	100 Hz		1400×1050	85 Hz
45	576 i	100 Hz	H	1400×1050	120 Hz	R
46	1080 i	120 Hz		1440×900		R
47	720 p	120 Hz		1440×900	60 Hz
48	480 p	120 Hz		1440×900	75 Hz
49	480 p	120 Hz	H	1440×900	85 Hz
50	480 i	120 Hz		1440×900	120 Hz	R
51	480 i	120 Hz	H	1600×1200	60 Hz
52	576 p	200 Hz		1600×1200	65 Hz
53	576 p	200 Hz	H	1600×1200	70 Hz
54	576 i	200 Hz		1600×1200	75 Hz
55	576 i	200 Hz	H	1600×1200	85 Hz
56	480 p	240 Hz		1600×1200	120 Hz	R
57	480 p	240 Hz	H	1680×1050		R
58	480 i	240 Hz		1680×1050	60 Hz
59	480 i	240 Hz	H	1680×1050	75 Hz
60				1680×1050	85 Hz
61				1680×1050	120 Hz	R
62				1792×1344	60 Hz
63				1792×1344	75 Hz
64				1792×1344	120 Hz	R
65				1856×1392	60 Hz
66				1856×1392	75 Hz
67				1856×1392	120 Hz	R
68				1920×1200		R
69				1920×1200	60 Hz
70				1920×1200	75 Hz
71				1920×1200	85 Hz
72				1920×1200	120 Hz	R
73				1920×1440	60 Hz
74				1920×1440	75 Hz
75				1920×1440	120 Hz	R
76				2560×1600		R
77				2560×1600	60 Hz
78				2560×1600	75 Hz
79				2560×1600	85 Hz
80				2560×1600	120 Hz	R
81				1366×768	60 Hz
82	1080 p	60 Hz
83				1600×900		R
84				2048×1152		R
85	720 p	60 Hz
86				1366×768		R

HDMI Boost

The config_hdmi_boost parameter allows you to tweak the HDMI signal strength.

config_hdmi_boost	Description
0	Non-NOOBS default
1
2
3
4	Use if you have interference issues (NOOBS default setting)
5
6
7	Maximum strength

HDMI Ignore CEC Init

When this option is enabled, the CEC initialization is not sent to the device. This avoids bringing the TV out of standby and channel switch when rebooting.

hdmi_ignore_cec_init	Description
0	Normal (default)
1	Don’t send initial active source message

HDMI Ignore CEC

When this option is enabled, the assumption made is that CEC is not supported at all by the HDMI device, even if the device does have support. As a result, no CEC functions will be supported.

hdmi_ignore_cec	Description
0	Normal (default)
1	Disable CEC support

Overscan Video

A few options control the overscan support of the composite video output. When overscan is enabled, a certain number of pixels are skipped at the sides of the screen as configured.

Disable Overscan

The disable_overscan option can disable the overscan feature. It is enabled by default:

disable_overscan	Description
0	Overscan enabled (default)
1	Overscan disabled

Overscan Left, Right, Top, and Bottom

These parameters control the number of pixels to skip at the left, right, top, and bottom of the screen.

Parameter	Pixels to Skip
overscan_left=0	At left
overscan_right=0	At right
overscan_top=0	At top
overscan_bottom=0	At bottom

Frame Buffer Settings

The Linux frame buffer support is configured by a few configuration options described in this section.

Frame Buffer Width

The default is to define the width of the frame buffer as the display’s width minus the overscan pixels.

framebuffer_width	Description
default	Display width overscan
framebuffer_width=n	Set width to n pixels

Frame Buffer Height

The default is to define the height of the frame buffer as the display’s height minus the overscan pixels.

framebuffer_height	Description
default	Display height overscan
framebuffer_height=n	Set height to n pixels

Frame Buffer Depth

This parameter defines the number of bits per pixel.

framebuffer_depth	Description
8	Valid, but default RGB palette makes an unreadable screen
16	Default
24	Looks better but has corruption issues as of 6/15/2012
32	No corruption, but requires framebuffer_ignore_alpha=1, and shows wrong colors as of 6/15/2012

Frame Buffer Ignore Alpha

The alpha channel can be disabled with this option. As of this writing, this option must be used when using a frame buffer depth of 32 bits.

framebuffer_ignore_alpha	Description
0	Alpha channel enabled (default)
1	Alpha channel disabled

General Video Options

The display can be flipped or rotated in different ways, according to the display_rotate option . You should be able to do both a flip and a rotate by adding the flip values to the rotate value.

The 90° and 270° rotations require additional memory on the GPU, so these options won’t work with a 16 MB GPU split.

display_rotate	Description
0	0° (default)
1	90°
2	180°
3	270°
0x1000	Horizontal flip
0x2000	Vertical flip

While the flip options are documented, I was unable to get them to work. The rotations, however, were confirmed as working.

Licensed Codecs

The following options permit you to configure the purchased license key codes for the codecs they affect.

Option	Notes
decode_MPG2=0x12345678	License key for hardware MPEG-2 decoding
decode_WVC1=0x12345678	License key for hardware VC-1 decoding

Testing

The following test option enables image/sound tests during boot. This is intended for manufacturer testing.

test_mode	Description
0	Disable test mode (default)
1	Enable test mode

Memory

This section summarizes configuration settings pertaining to memory.

Disable GPU L2 Cache

The disable_l2cache option allows the ARM CPU access to the GPU L2 cache to be disabled. This needs the corresponding L2 disabled in the kernel.

disable_l2cache	Description
0	Enable GPU L2 cache access
1	Disable GPU L2 cache access

Boot Options

Several options in this section affect the boot process. Many options pertain to the kernel being started, while others affect file systems and devices.

Command Line

The cmdline option allows you to configure the kernel command-line parameters within the config.txt file, instead of the cmdline.txt file.

cmdline	Description
unspecified	Command line is taken from cmdline.txt
cmdline=“command”	Command line is taken from parameter

Kernel

By default, start.elf loads the kernel from the file named kernel.img or kernel7.img depending on the Raspberry Pi model. This can be overridden to specify a specific image.

Kernel Address

This parameter determines the memory address where the kernel image is loaded into.

kernel_address	Description
0x00000000	Default

RAM File System File

The ramfsfile parameter names the file for the RAM FS file, to be used with the kernel.

ramfsfile	Description
unspecified	No RAM FS file used
ramfsfile=“ramfs.file”	File ramfs.file is used

RAM File System Address

The ramfsaddr parameter specifies where the RAM file system image is to be loaded into memory.

ramfsaddr	Description
0x00000000	Default address

Init RAM File System

This option is a convenience option, which combines the options ramfsfile and ramfsaddr.

initramfs	Arg 1	Arg 2	Description
initramfs	initram.gz	0x00800000	Example

Device Tree Address

The device_tree_address option defines where the device tree address is loaded.

device_tree_address	Description
0x00000000	Default

Init UART Baud

The init_uart_baud option allows the user to reconfigure the serial console to use a baud rate that is different from the default.

init_uart_baud	Description
115200	Default baud rate

Init UART Clock

The init_uart_clock parameter permits the user to reconfigure the UART to use a different clock rate.

init_uart_clock	Description
3000000	Default

Init EMMC Clock

The init_emmc_clock parameter allows the user to tweak the EMMC clock, which can improve the SD card performance.

init_emmc_clock	Description
100000000	Default

Boot Delay

The boot_delay and boot_delay_ms options allow the user to reconfigure the delay used by start.elf prior to loading the kernel. The actual delay time used is computed from the following:

$D=1000\times b+m$

where

D is the computed delay in milliseconds.
b is the boot_delay value.
m is the boot_delay_ms value.

boot_delay (b)	Description
1	Default

The boot_delay_ms augments the boot_delay parameter.

boot_delay_ms (m)	Description
0	Default

Avoid Safe Mode

A jumper or switch can be placed between pins P1-05 (GPIO 1) and P1-06 (ground) to cause start.elf to initiate a safe mode boot. If GPIO 1 is being used for some other I/O function, the safe mode check should be disabled.

avoid_safe_mode	Description
0	Default (check P1-05 for safe mode)
1	Disable safe mode check

cmdline.txt

The cmdline.txt file is used to supply command-line arguments to the kernel. The Raspbian values supplied in the standard image are broken into multiple lines here for easier reading (this sample was for the Raspberry Pi 3 B+):

# cat cmdline.txt

dwc_otg.lpm_enable=0 \

console=tty1 \

console=serial0,115200 \

root=PARTUUID=2383b4bd-02 \

rootfstype=ext4 \

elevator=deadline \

fsck.repair=yes \

rootwait \

quiet \

splash \

plymouth.ignore-serial-consoles

This file is provided as a convenience, since the parameters can be configured in the config.txt file, using the cmdline="text" option. When the config.txt option is provided, it supersedes the cmdline.txt file.

Once the Raspbian Linux kernel comes up, you can review the command-line options used as follows (this is the same Raspberry Pi 3 B+):

$ cat /proc/cmdline

8250.nr_uarts=1 \

bcm2708_fb.fbwidth=1680 \

bcm2708_fb.fbheight=1050 \

bcm2708_fb.fbswap=1 \

vc_mem.mem_base=0x3ec00000 \

vc_mem.mem_size=0x40000000 \

dwc_otg.lpm_enable=0 \

console=tty1 \

console=ttyS0,115200 \

root=PARTUUID=2383b4bd-02 \

rootfstype=ext4 \

elevator=deadline \

fsck.repair=yes \

rootwait \

quiet \

splash \

plymouth.ignore-serial-consoles

This output is interesting because it shows details that are not present in the cmdline.txt file. Options of the format name.option=values are specific to kernel-loadable modules.

Serial console=

The Linux console parameter specifies to Linux what device to use for a console. In this example, it was:

console=ttyS0,115200

This references the serial device that is made available after boot-up as /dev/ttyS0. The parameter following the device name is the baud rate.

The general form of the serial console option is as follows:

console=ttyDevice,bbbbpnf

The second parameter is the options field:

Zone	Description	Value	Raspbian Notes
bbbb	Baud rate	115200	Can be more than four digits
p	Parity	n	No parity
		o	Odd parity
		e	Even parity
n	Number of bits	7	7 data bits
		8	8 data bits
f	Flow control	r	RTS
		omitted	No RTS

Virtual console=

Linux supports a virtual console, which is also configurable from the console= parameter. Raspbian Linux specifies the following:

console=tty1

This device is available from /dev/tty1, after the kernel boots up. The tty parameters used for this virtual console can be listed (edited here for readability):

$ sudo −i

# stty −a </dev/tty1

speed 38400 baud ; rows 26; columns 82; line = 0;

intr = ^C; quit = ^\; erase = ^?; kill = ^U; \

eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>;

start = ^Q; stop = ^S ; susp = ^Z; rprnt = ^R; werase = ^W; \

lnext = ^V; flush = ^O; min = 1; time = 0;

−parenb −parodd cs8 hupcl −cstopb cread −clocal −crtscts

−ignbrk brkint −ignpar −parmrk −inpck −istrip −inlcr \

−igncr icrnl ixon −ixoff −iuclc −ixany imaxbel iutf8

opost −o lcuc −ocrnl onlcr −onocr −onlret −ofill −ofdel \

nl0 cr0 tab0 bs0 vt0 ff0

isig icanon iexten echo echoe echok −echonl −noflsh \

−xcase −tostop −echoprt −echoctl echoke

root=

The Linux kernel needs to know what device holds the root file system. The standard Raspbian image supplies the following:

root=PARTUUID=2383b4bd-02

This information can be obtained by the blkid command:

# blkid

/dev/mmcblk0p1: LABEL="boot" UUID="A75B-DC79" TYPE="vfat" PARTUUID="2383b4bd-01"

/dev/mmcblk0p2: LABEL="rootfs" UUID="485ec5bf-9c78-45a6-9314-32be1d0dea38" \

TYPE="ext4" PARTUUID="2383b4bd-02"

/dev/mmcblk0: PTUUID="2383b4bd" PTTYPE="dos"

Reviewing that output confirms that the partition named "rootfs" is being used as the root file system. The partition in this example is specified by the partition UUID (a shortened version of a UUID, which is a universally unique identifier).

The general form of the root= parameter supports the following forms:

root=MMmm: Boot from major device MM, minor mm (hexadecimal).
root=/dev/nfs: Boot a NFS disk specified by nfsroot (see also nfs-root= and ip=).
root=/dev/name: Boot from a device named /dev/name.
root=PARTUUID=: Boot from a locally unique partition identified by as shortened UUID.

rootfstype=

In addition to specifying the device holding the root file system, the Linux kernel sometimes needs to know the file system type. This is configured through the rootfstype parameter. The standard Raspbian image supplies the following:

rootfstype=ext4

This example indicates that the root file system is the ext4 type.

The Linux kernel can examine the device given in the root parameter to determine the file system type. But there are scenarios where the kernel cannot resolve the type or gets confused. Otherwise, you may need to force a certain file system type.

elevator=

This option selects the I/O scheduler scheme to be used within the kernel. The standard Raspbian image specifies the following:

elevator=deadline

To find out the I/O scheduler option being used and the other available choices (in your kernel), we can consult the /sys pseudo file system:

$ cat /sys/block/mmcblk0/queue/scheduler

noop [deadline] cfq

The name mmcblk0 is the name of the device that your root file system is on (review the example blkid output earlier). The output shows in square brackets that the deadline I/O scheduler is being used. The other choices are noop and cfq. These I/O schedulers are as follows:

Name	Description	Notes
noop	No special ordering of requests
cfq	Completely fair scheduler	Older
deadline	Cyclic scheduler, but requests have deadlines	Newest

The deadline I/O scheduler is the newest implementation, designed for greater efficiency and fairness. The deadline scheduler uses a cyclic elevator, except that it additionally logs a deadline for the request. A cyclic elevator is one where the requests are ordered according to sector numbers and head movement (forward and backward). The deadline scheduler will use the cyclic elevator behavior, but if it looks like the request is about to expire, it is given immediate priority.

rootwait=

This option is used when the device used for the root file system is a device that is started asynchronously with other kernel boot functions. This is usually needed for USB and MMC devices, which may take extra time to initialize. The rootwait option forces the kernel to wait until the root device becomes ready.

Given that the root file system is on the SD card (a MMC device), the Raspbian image uses the following:

rootwait

nfsroot=

The nfsroot option permits you to define a kernel that boots from an NFS mount (assuming that NFS support is compiled into the kernel). The square brackets show placement of optional values:

nfsroot=[server−ip:]root−dir[,nfs−options]

Field	Description
server-ip	NFS server IP number (default uses ip=)
root-dir	Root dir on NFS server. If there is a %s present, the IP address will be inserted there.
nfs-options	NFS options like ro, separated by commas

When unspecified, the default of /tftpboot/client_ip_address will be used. This requires that root=/dev/nfs be specified and optionally ip= may be added.

To test whether you have NFS support in your kernel, you can query the /proc file system when the system has booted:

# cat /proc/filesystems

nodev sysfs

nodev rootfs

nodev ramfs

nodev bdev

nodev proc

nodev cpuset

nodev cgroup

nodev cgroup2

nodev tmpfs

nodev devtmpfs

nodev configfs

nodev debugfs

nodev tracefs

nodev sockfs

nodev pipefs

nodev rpc_pipefs

nodev devpts

ext3

ext2

ext4

vfat

msdos

nodev nfs

nodev nfs4

nodev autofs

f2fs

nodev mqueue

fuseblk

nodev fuse

nodev fusectl

From this example, we see that both the older NFS (nfs) and the newer NFS4 file systems are supported.

ip=

This option permits the user to configure the IP address of a network device, or to specify how the IP number is assigned. See also the root= and nfsroot= options.

ip=client−ip:server−ip:gw−ip:netmask:hostname:device:autoconf

Table 17-3 describes the fields within this option. The autoconf value can appear by itself, without the intervening colons if required. When ip=off or ip=none is given, no autoconfiguration takes place. The autoconfiguration protocols are listed in Table 17-4.

Table 17-3

ip= Kernel Parameter

Field	Description	Default
ip-client	IP address of the client	Autoconfigured
ip-server	IP address of NFS server, required only for NFS root	Autoconfigured
gw-ip	IP address of server if on a separate subnet	Autoconfigured
netmask	Netmask for local IP address	Autoconfigured
hostname	Hostname to provide to DHCP	Client IP address
device	Name of interface to use	When more than one is available, autoconf
autoconf	Autoconfiguration method	Any

Table 17-4

Autoconfiguration Protocols

Protocol	Description
off or none	Don’t autoconfigure
on or any	Use any protocol available (default)
dhcp	Use DHCP
bootp	Use BOOTP
rarp	Use RARP
both	Use BOOTP or RARP but not DHCP

With the kernel exchanged and the configuration restored to safe options, it should now be possible to boot the emergency kernel. Log in and rescue.

To restore your system back to its normal state, you’ll need to follow these steps:

1.
Rename kernel.img to kernel_emergency.img (for future rescues).
2.
Rename kernel.bak to kernel.img (reinstate your normal kernel).
3.
Restore/alter your config.txt configuration, if necessary.
4.
Restore/alter your cmdline.txt configuration, if necessary.

At this point, you can reboot with your original kernel and configuration.

Table of Contents for Advanced Raspberry Pi: Raspbian Linux and GPIO Integration