2/27/19 update: These procedures have been successfully tested on an Arria 10 board using the Achilles Dev Kit from REFLEX CES. [DN]

Why boot from TFTP/NFS?

There are a number of advantages to booting your target board from a network, especially during development.

• The device tree, kernel, and root file system can be tested and modified during development without repeatedly burning images to SD cards.
• Changes made to the root file system (cross compiled binaries, drivers, etc) on the development host (also used as the file server) are immediately available to the target hardware.
• Different kernel versions or root file systems (Angstrom, Ubuntu, Linaro Nano) can easily be loaded and tested without burning SD cards.
• Target boards with Ethernet interface and eMMC (but no SD card slot) can be booted from network and eMMC can then be formatted and programmed using standard Linux tools.

This example demonstrates booting an Arrow SoCKit development board using TFTP (Trivial File Transfer Protocol) and NFS (Network File System), however the process is applicable to any Altera FPGA SoC device. This example uses U-boot v2013.01.01 and Angstrom v2014.12.

A Linux host is setup as a TFTP and NFS server. U-boot is loaded on the target hardware (SoCKit in this example). The FPGA configuration file, device tree, and kernel image are downloaded from the server using TFTP in u-boot. The Linux kernel is booted and configured to use NFS as the root file system resident on the NFS server. The entire process can be run using u-boot command line or scripts.

To follow along with this example, you will need a SoCKit development board and should be familiar with booting it with an SD card. This will be used to load u-boot. The files required are the same used to create a bootable SD card. Precompiled binaries can be downloaded from the latest Arrow SoCKit GSRD v15.1 Release Contents page. All files needed are contained in linux-sockit-gsrd-15.1-bin.tar.gz. You can also use your own compiled binaries following the instructions for Compiling the Linux kernel. This example uses a network with gateway IP address at 192.168.1.1 and DHCP clients connecting from 192.168.1.100 to 192.168.1.249. Modify the IP addresses in the instructions below where appropriate to match your network settings.

Linux Host Setup

A Linux host running Ubuntu 14.04 will act as the TFTP and NFS server in this example. These TFTP and NFS utilities need to be installed and some of the system configuration files will need to be modified. This is a one time setup process. Modifying the configuration files will require root privileges. Where the instructions indicate to edit a file as root user, you can use your preferred text editor such as vi, vim, nano, gedit, etc. To edit foo.txt in gedit as root user, at a terminal prompt you would enter:

$ sudo gedit foo.txt

and enter your password when prompted.

During development, it is ideal (but not required) to set a static IP address on your Linux host. We will need to set u-boot variables that contain the host IP address and it becomes inconvenient to change these variables if your host IP address changes as a result of DHCP after a host reboot.

If you choose not to set a static IP, find and record the current IP address of your host:

$ ifconfig

To set a static IP address on an Ubuntu host, edit the /etc/network/interfaces file as root user, adding the lines below to the interfaces file. This example assumes a wired connection on the eth0 interface with a static IP address of 192.168.1.155. Modify the netmask and gateway values as appropriate for your network.

auto eth0
iface eth0 inet static
address 192.168.1.155
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 8.8.8.8 8.8.4.4

If you are unsure of your netmask and gateway values, you can find them by entering:

$ ifconfig eth0 | grep Mask
$ ip route show

Save and close the interfaces file, then restart your networking services:

$ sudo service network-manager restart

Server Directory Structure

For both TFTP and NFS, you need to setup server directories that will be accessed by u-boot and Linux on the target hardware over the network. There are multiple options (and opinions) on how to do this and the method you use might depend on how you plan to utilize booting over the network.

Depending on the distribution, your Linux host root file system should have a /srv directory. As defined by the Filesystem Heirarchy Standard (FHS):
/srv contains site-specific data which is served by this system.

Therefore it is common to use this directory to store files served by utilities such as TFTP and NFS. Alternatively, you could create an /export directory, or any other directory you choose and copy the required files there. You could also keep the files where they are generated by the build tools and just point to them in the server configuration files. Or you could use the /srv structure proposed here, but using symbolic links to point to the files. Since this example will show how one can easily load different kernel versions and different root filesystems for different target hardware, the specific /srv directory structure shown below will be used, although it is not necessary to create all the directories at this time.

/srv
├── nfs
│   ├── angstrom-v2014.12
│   │   └── root file system extracted from build tools .tar.gx archive
│   ├── ubuntu-core
│   │   └── root file system extracted from build tools .tar.gx archive
└── tftp
    ├── bootscripts
    │   ├── sockit-angstrom.scr
    │   ├── sockit-angstrom.script
    │   ├── sockit-ubuntu.scr
    │   └── sockit-ubuntu.script
    ├── kernel
    │   ├── kernel-3.10-ltsi
    │   │   └── zImage
    │   ├── kernel-4.4
    │   │   └── zImage
    ├── my-custom-board
    │   ├── my-custom.dtb
    │   └── my-custom.rbf    
    └── sockit
        ├── soc_system.dtb
        └── soc_system.rbf

The files contained in the directories should look familiar from creating a bootable SD card. The table below summarizes the files required for this example and can be obtained from compiling the GSRD or using the prebuilt binaries. Following the steps below, you will create the directories and then copy the required files into these directories. Because the /srv directory is part of the Linux host root file system (/), all operations to create and copy files to these directories will require root user privileges. The directories will be created and files copied in the following sections.

Table 1

Item	Source	File Location	Copy to server directory
Compressed FPGA configuration file	Quartus GHRD Build	/sockit_ghrd/output_files/soc_system.rbf	soc_system.rbf
Device Tree Blob file	^	/sockit_ghrd/soc_system.dtb	soc_system.dtb
Compressed Linux kernel image file	Yocto Build	/angstrom-socfpga/deploy/glibc/images/cyclone5/zImage	zImage
Linux root filesystem	^	/angstrom-socfpga/deploy/glibc/images/cyclone5/sockit-gsrd-console-image.tar.xz	extract to /rootfs (see below)

Setup the TFTP Server

Install TFTP server if not already done:

$ sudo apt-get install tftpd-hpa

After installation, the TFTP service should start automatically. Verify by running:

$ sudo service tftpd-hpa status

If it is not running, start it by entering:

$ sudo service tftpd-hpa start

Next create the following directories that will be used to hold the FPGA configuration file, device tree, and kernel image. The sockit directory created below will store the device tree and FPGA configuration files specific to the Arrow SoCKit development kit running the GHRD. You might also have a different development kit or custom board with its own device tree and FPGA configuration file, as shown in the example directory structure above.

$ sudo mkdir -p /srv/tftp
$ sudo mkdir -p /srv/tftp/sockit
$ sudo mkdir -p /srv/tftp/kernel

If you have compiled multiple kernel versions with your build tool, you can create additional version directories for the zImage files under the kernel directory. Otherwise, if you are following this example with a single zImage, you can just copy that under kernel. If you do so, be sure to modify the subsequent instructions using your correct path.

Next edit the TFTP_DIRECTORY variable in /etc/default/tftpd-hpa as root user. This variable will likely default to /var/lib/tftpboot. We will change it to the directory created above.

TFTP_DIRECTORY="/srv/tftp/"

Save and close the file. Check the tftpd-hpa status again to make sure it's still running. The status should display

tftpd-hpa start/running, process xxxx

If it displays

tftpd-hpa stop/waiting

there is probably an error with the directory you created and/or the TFTP_DIRECTORY variable is not pointing to a valid directory.

Setup the NFS Server

Install NFS server if not already done:

$ sudo apt-get install nfs-kernel-server

Next create the following directories that will be used to hold the root file system(s).

$ sudo mkdir -p /srv/nfs
$ sudo mkdir -p /srv/nfs/angstrom-v2014.12
$ sudo mkdir -p /srv/nfs/angstrom-v2014.12/rootfs

If you plan to add other root file systems, such as ubuntu-core, you can create those directories now. You can download a copy of ubuntu-core rootfs for the SoCKit from here.

$ sudo mkdir -p /srv/nfs/ubuntu-core
$ sudo mkdir -p /srv/nfs/ubuntu-core/rootfs

Edit the /etc/exports file as root user, adding the text shown below. There are 2 options shown; choose only one appropriate for how you want to connect to the NFS server, as described in the comments (preceeded by #). You will also need to modify the network IP address to match yours and if you are using a different directory structure, modify the entries accordingly.

#Share access to particular network, in this case 192.168.1.1
#This gives any IP address on this network access to the server
#This should be fine on a private network
/srv/nfs/angstrom-v2014.12/rootfs/ 192.168.1.1/24(rw,no_subtree_check,sync,no_root_squash)
/srv/nfs/ubuntu-core/rootfs/ 192.168.1.1/24(rw,no_subtree_check,sync,no_root_squash)

OR

#Share access to particular host, in this case 192.168.1.222 (which is the IP address we will later assign to the !SoCKit in u-boot)
#Only the host specified can connect to the server
#This might be more suitable for a corporate network
/srv/nfs/angstrom-v2014.12/rootfs/ 192.168.1.222(rw,no_subtree_check,sync,no_root_squash)
/srv/nfs/ubuntu-core/rootfs/ 192.168.1.222(rw,no_subtree_check,sync,no_root_squash)

Save the exports file and then restart the NFS server:

$ sudo service nfs-kernel-server restart

To show all currently exported filesystems with their options:

$ sudo exportfs -v

You should see the paths and options as specified in the /etc/exports file. The output should look something like this:

/srv/nfs/angstrom-v2014.12/rootfs
                192.168.1.1/24(rw,wdelay,no_root_squash,no_subtree_check,sec=sys,rw,no_root_squash,no_all_squash)
/srv/nfs/ubuntu-core/rootfs
                192.168.1.1/24(rw,wdelay,no_root_squash,no_subtree_check,sec=sys,rw,no_root_squash,no_all_squash)

Copy Files to the Server Directories

Now that the file servers are setup, we need to copy the files shown in Table 1 above. In a terminal, browse to the directory where each file is located and copy each file to the /srv directories created in the previous steps. Since you are copying into the root-owned /srv directory, the commands must be run as sudo.

$ sudo cp soc_system.dtb /srv/tftp/sockit/
$ sudo cp soc_system.rbf /srv/tftp/sockit/
$ sudo cp zImage /srv/tftp/kernel/kernel-3.10-ltsi/
$ sudo cp sockit-gsrd-15.1-rootfs.tar.xz /srv/nfs/angstrom-v2014.12/rootfs/

After copying the rootfs archive, extract the files:

$ cd /srv/nfs/angstrom-v2014.12/rootfs/
$ sudo tar xf sockit-gsrd-15.1-rootfs.tar.xz

Disable the Angstrom Connection Manager to prevent loss of connection to the NFS server during Angstrom boot (see Troubleshooting section for more information):

sudo mv /srv/nfs/angstrom-v2014.12/rootfs/lib/systemd/system/connman.service /srv/nfs/angstrom-v2014.12/rootfs/lib/systemd/system/connman.service.nouse

After copying the files, restart the TFTP server:

$ sudo service tftpd-hpa restart

We have just copied the minimum set of files required to boot the v15.1 GSRD on the SoCKit. If you have additional root file systems or other kernel version images to use, they can be copied at this time using similar commands shown above.

Load U-Boot

Now boot the SoCKit to u-boot. This can be done using either DS-5 (following instructions here) or a GSRD SD Card used for booting Linux as described in the introduction. If you need help with this step, please refer to the Getting Started page (see Tab 6 or 7). If using an SD card, with the board powered off, insert the SD card. Start up PuTTy or another terminal program. Press and hold the WARM_RST button on the SoCKit, power on the SoCKit, connect PuTTY to the SoCKit, and then release the WARM_RST button. Interrupt the boot process by pressing any key when prompted. Holding down the WARM_RST button while connecting ensures you don't miss the prompt to interrupt the boot process.

For a Cyclone 5 board, the u-boot prompt should look like this:

SOCFPGA_CYCLONE5 #

If you load u-boot from an SD card prepared for the SoCKit, you should notice the following message as u-boot loads:

Skipped ethaddr assignment due to invalid EMAC address in EEPROM
Net:   mii0
Warning: failed to set MAC address

The SoCKit does not have an EEPROM that provides a MAC address for the Ethernet PHY, so we will assign one below. If you are building a custom board and need to assign a MAC address, see here.

From the precompiled binaries SD card, there should also be several u-boot environment variables predefined. You can examine these using:

SOCFPGA_CYCLONE5 # printenv

In order to test the network interface within u-boot on the SoCKit, the following u-boot environment variables need to be setup. You can assign any valid IP address on your network to the SoCKit, but it should match what you defined above in the NFS export folder if you setup access for a single host. The MAC address is also somewhat arbitrary. On a small network, it is likely you can make up a unique MAC address, but for a custom production board you should obtain a valid unique MAC address.

SOCFPGA_CYCLONE5 # setenv ipaddr 192.168.1.222
SOCFPGA_CYCLONE5 # setenv ethaddr 02:aa:bb:cc:dd:ee

To verify that your network connection is now active, ping your router or Linux host:

SOCFPGA_CYCLONE5 # ping 192.168.1.1

You should see

Using mii0 device
host 192.168.1.1 is alive

For TFTP commands to work in u-boot, the serverip variable needs to be set. Substitute your Linux host (TFTP and NFS server) IP address obtained from the ifconfig command above for the IP address below:

SOCFPGA_CYCLONE5 # setenv serverip 192.168.1.155

Save these new environment variables so they are available next time you start u-boot from the SD card.

SOCFPGA_CYCLONE5 # saveenv

Booting Manually from the U-Boot Command Prompt

The following u-boot commands can be run individually from the u-boot prompt or can be added to a script. Scripts can be customized by modifying the variables to load specific FPGA configuration files, device trees, kernel images, and root file systems. An example of creating and using a script will be shown later. For now we will run the commands individually to ensure that everything is setup and working correctly.

The example below loads the 3.10 kernel and the Angstrom 2014.12 root file system from the GSRD. The comment lines preceded by # can be skipped and are shown to provide clarity for the instructions. They can also be included or omitted from your script files as you choose. All of these commands are run from the u-boot prompt, but the SOCFPGA_CYCLONE5 # is omitted for brevity.

#set variables for the directories where the boot files are saved
setenv tftp-kernel-dir kernel/kernel-3.10-ltsi
setenv tftp-sockit-dir sockit
setenv nfs-rootfs-dir /srv/nfs/angstrom-v2014.12/rootfs/

#serverip and nfsip are the same here, but the TFTP and NFS servers could be 2 separate servers with different IP addresses
setenv serverip 192.168.1.155
setenv nfsip 192.168.1.155
#these arguments will be passed to the linux kernel during boot
#this sets up the serial terminal and baud rate, tells the kernel to obtain an IP address via dhcp,
#and sets the nfs information for the kernel rootfs
setenv bootargs console=ttyS0,115200 ip=dhcp root=/dev/nfs rw nfsroot=${nfsip}:${nfs-rootfs-dir}

#set variables for the files to be transferred
setenv fpga-image soc_system.rbf
setenv dtb-image soc_system.dtb
setenv kernel-image zImage

#set the address where the files will be loaded into RAM when transferred via tftp
setenv fpgadata 0x2000000
setenv fpgadatasize 0x700000
setenv dtbaddr 0x00000100
setenv kerneladdr 0x8000

#create commands to transfer files, load fpga, and boot kernel
#tftp server dir was set as /srv/tftp/ in the tftp config file, so tftp transfers are relative to this directory
setenv get-fpgadata 'tftp ${fpgadata} ${tftp-sockit-dir}/${fpga-image}'
setenv load-fpga 'fpga load 0 ${fpgadata} ${fpgadatasize}'
setenv get-dtb 'tftp ${dtbaddr} ${tftp-sockit-dir}/${dtb-image}'
setenv get-kernel 'tftp ${kerneladdr} ${tftp-kernel-dir}/${kernel-image}'
setenv bootnfs 'bootz ${kerneladdr} - ${dtbaddr}'

#run all the file transfer commands and boot
#bridge_enable_handoff is predefined with u-boot loaded from gsrd sd card
#and is required if the fpga design uses the bridges between the fpga and hps
#if u-boot was loaded from DS-5, then bridge_enable_handoff should be defined prior to running this command
run get-fpgadata load-fpga get-dtb get-kernel bridge_enable_handoff bootnfs

If you loaded u-boot via DS-5 or bridge_enable_handoff was otherwise undefined, then the following commands could be used to create the function:

#Commands to run bridge_enable_handoff:
setenv axibridge=ffd0501c
setenv axibridge_handoff=0x00000000
setenv fpgaintf=ffd08028
setenv fpgaintf_handoff=0x00000000
setenv fpga2sdram_apply=3ff7952c
setenv fpga2sdram=ffc25080
setenv fpga2sdram_handoff=0x000001ff
setenv l3remap=ff800000
setenv l3remap_handoff=0x00000019
setenv bridge_enable_handoff=mw $fpgaintf ${fpgaintf_handoff}; go $fpga2sdram_apply; mw $fpga2sdram ${fpga2sdram_handoff}; mw $axibridge ${axibridge_handoff}; mw $l3remap ${l3remap_handoff}
run bridge_enable_handoff

Successfully running the above commands should look like this:

SOCFPGA_CYCLONE5 # run get-fpgadata load-fpga get-dtb get-kernel bridge_enable_handoff bootnfs
Configuring PHY skew timing for Micrel ksz9021
Waiting for PHY auto negotiation to complete. done
ENET Speed is 1000 Mbps - FULL duplex connection
Using mii0 device
TFTP from server 192.168.1.155; our IP address is 192.168.1.222
Filename 'sockit/soc_system.rbf'.
Load address: 0x2000000
Loading: #################################################################
         #################################################################
         ############################
         460 KiB/s
done
Bytes transferred = 2311760 (234650 hex)
Using mii0 device
TFTP from server 192.168.1.155; our IP address is 192.168.1.222
Filename 'sockit/soc_system.dtb'.
Load address: 0x100
Loading: ##
         411.1 KiB/s
done
Bytes transferred = 23170 (5a82 hex)
Using mii0 device
TFTP from server 192.168.1.155; our IP address is 192.168.1.222
Filename 'kernel/kernel-3.10-ltsi/zImage'.
Load address: 0x8000
Loading: #################################################################
         #################################################################
         #################################################################
         ##########################################
         460 KiB/s
done
Bytes transferred = 3466256 (34e410 hex)

From here the kernel should begin loading and the NFS file system will be mounted and you will eventually get to the familiar Linux login prompt.

If you made it to the Linux prompt, CONGRATULATIONS! Now you can move on to the next sections to create a u-boot script.

If you did not get the Linux prompt but encountered some errors, please scroll down to the Troubleshooting section below to try to resolve the problem.

Booting From a U-Boot Script

The u-boot commands above can be saved into a script text file that is then converted to a script image file using the u-boot mkimage utility. This is the same process used to create the u-boot.scr file as part of the GHRD make utilities when building the GHRD. To convert the script text files on your Linux host, first install the mkimage utility:

$ sudo apt-get install u-boot-tools

Create the directories for the scripts using the example directory structure from above, or other directory of your choice:

$ sudo mkdir -p /srv/tftp/bootscripts

Change to the new directory and then copy & paste the commands above into a text file and save it to a descriptive file name such as sockit-angstrom.script. You will need to open your text editor as root user if you choose to save the files to the /srv/bootscripts directory.

$ cd /srv/tftp/bootscripts
$ sudo your-text-editor sockit-angstrom.script

Make any necessary modifcations to the commands and then save the file. Next, convert the .script text file to a u-boot image script.

$ sudo mkimage -A arm -O u-boot -T script -C none -a 0 -e 0 -n "TFTP Boot Script" -d sockit-angstrom.script /srv/tftp/bootscripts/sockit-angstrom.scr

Now if you had loaded u-boot with DS-5 or via an SD card with no previously saved u-boot environment variables, from the u-boot prompt you could run:

setenv ipaddr 192.168.1.222
setenv ethaddr 02:aa:bb:cc:dd:ee
setenv serverip 192.168.1.155
#tftp the script file to address 0x100000 and run it
setenv boot-angstrom 'tftp 0x100000 bootscripts/sockit-angstrom.scr ; source 0x100000'
run boot-angstrom

Troubleshooting

Below are some typical problems you might encounter if the Linux host or target hardware is not setup properly.

Problem: TFTP files will not transfer

Example:

SOCFPGA_CYCLONE5 # run get-fpgadata
Using mii0 device
TFTP from server 192.168.1.131; our IP address is 192.168.1.222
Filename 'sockit/soc_system.rbf'.
Load address: 0x2000000
Loading: T T T T T T T T T

Possible solution: If running Ubuntu 14.04 on the Linux host, turn off Uncomplicated Firewall:

$ sudo ufw disable

To enable it again at a later time:

$ sudo ufw enable

Possible solution: If you have modified or added files in the /srv/tftp/ directories, try restarting the tftp server:

$ sudo service tftpd-hpa restart

Problem: TFTP error: 'File not found' (1)

Example:

SOCFPGA_CYCLONE5 # run get-fpgadata
Configuring PHY skew timing for Micrel ksz9021
Waiting for PHY auto negotiation to complete... done
ENET Speed is 1000 Mbps - FULL duplex connection
Using mii0 device
TFTP from server 192.168.1.155; our IP address is 192.168.1.222
Filename 'sockit/soc_system.rbf'.
Load address: 0x2000000
Loading: T
TFTP error: 'File not found' (1)
Not retrying...

Possible solutions:
1. Verify that the file you are trying to transfer is actually in the TFTP directory specified.
2. Verify that your TFTP_DIRECTORY variable is correctly set in /etc/default/tftpd-hpa. In this example we set it to "/srv/tftp/". The last / after tftp is important since this becomes the root directory for the TFTP service and all other subdirectories used are specified relative to this TFTP root directory.
3. If you have modified or added files in the /srv/tftp/ directories, try restarting the tftp server:

$ sudo service tftpd-hpa restart

4. Try changing the file permissions in the TFTP directory. This was not necessary on the Ubuntu 14.04 host, but might be required on other distributions or older versions. From terminal prompt at /srv:

$ sudo chmod -R 777 tftp

Yes this makes everything in tftp read/write/executable recursively, but it's OK for our purposes. You might also need to run:

$ sudo chown -R nobody tftp

Problem: kernel message during boot:

VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "nfs" or unknown-block(2,0): error -6

Possible solutions:

If you added a new filesystem to the /srv/nfs directory, you need to update the /etc/exports file with the path to the new filesystem, then restart the NFS server:

$ sudo service nfs-kernel-server restart

Problem: various kernel errors during boot

Example 1:

platform leds.1: Driver leds-gpio requests probe deferral
libphy: stmmac-1:01 - Link is Up - 1000/Full
., OK
IP-Config: Got DHCP answer from 192.168.1.1, my address is 192.168.1.114
IP-Config: Complete:
     device=eth0, hwaddr=02:aa:bb:cc:dd:ee, ipaddr=192.168.1.114, mask=255.255.255.0, gw=192.168.1.1
     host=192.168.1.114, domain=lan, nis-domain=(none)
     bootserver=192.168.1.1, rootserver=192.168.1.131, rootpath=
     nameserver0=192.168.1.1
VFS: Mounted root (nfs filesystem) on device 0:11.
devtmpfs: error mounting -2
Freeing unused kernel memory: 332K (8067c000 - 806cf000)
Kernel panic - not syncing: No init found.  Try passing init= option to kernel. See Linux Documentation/init.txt for guidance.

Possible solution: Verify that the NFS exports file at /etc/exports is properly set to one of the valid options described in the instructions above, such as:

/srv/nfs/angstrom-v2014.12/rootfs 192.168.1.1/24(rw,no_subtree_check,sync,no_root_squash)

Example 2: You get further into kernel boot and loading rootfs, but encounter other errors. Look for clues in the boot messages:

Unhandled fault: imprecise external abort (0x406) at 0x7f000e00
Internal error: : 406 [#1] SMP ARM
Modules linked in: altera_sysid gpio_altera
CPU: 1 PID: 6 Comm: kworker/u4:0 Not tainted 3.10.31-ltsi-05172-g28bac3e #1

OR

Internal error: Oops: 17 [#2] SMP ARM
Modules linked in: altera_sysid gpio_altera
CPU: 1 PID: 6 Comm: kworker/u4:0 Tainted: G      D      3.10.31-ltsi-05172-g28bac3e #1
task: bf8b52c0 ti: bf8d0000 task.ti: bf8d0000

OR

IP-Config: Got DHCP answer from 192.168.1.1, my address is 192.168.1.115
IP-Config: Complete:
     device=eth0, hwaddr=02:aa:bb:cc:dd:ee, ipaddr=192.168.1.115, mask=255.255.255.0, gw=192.168.1.1
     host=192.168.1.115, domain=lan, nis-domain=(none)
     bootserver=192.168.1.1, rootserver=192.168.1.155, rootpath=
     nameserver0=192.168.1.1
VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "nfs" or unknown-block(2,0): error -6
Please append a correct "root=" boot option; here are the available partitions:

Possible solutions: In these cases the kernel was trying to load a module that required the FPGA/HPS bridges to be enabled. Run the bridge_enable_handoff command to resolve this.

Problem: NFS halts with message:

nfs: server 192.168.1.155 not responding, still trying
systemd[1]: systemd-udevd.service: Watchdog timeout (limit 1min)!

Possible solution: If you are booting Angstrom, the NFS network connection is being reconfigured. Once Angstrom starts, Connection Manager (connman) assigns a new IP address, even if you pass a fixed ip address vs. dhcp as a bootarg (ip=dhcp) in u-boot. To resolve this, either remove or rename the connman.service file in the NFS rootfs directory: /lib/systemd/system/connman.service. The full path to this file, following the directory structure used in this example, would be /srv/nfs/angstrom-v2014.12/rootfs/lib/systemd/system/connman.service and connman.service could be renamed to connman.service.nouse (rename requires root privileges).