This is a different bug to 4629 since in this case NFS is clearly enabled in the kernel. One difference here is that the kernel has been compiled with gcc 4.8. We therefore need to verify this issue is from the 4.8 compiler and there is no other issue.

Bruce: Can you find someone at WR to do a side by side comparison of a gcc 4.7 compiled kernel with the gcc 4.8 compiled kernel and confirm this is the problem. We don't have hardware to reproduce this one.

I'll track someone down to have a look.

This issue still exists in 20130528-2 build.
Log: (notice that GCC version has been changed to 4.7.2)

Trying 128.224.164.67...
Connected to 128.224.164.67.
Escape character is '^]'.
+
AR8316 PHY found on MAC0
AR8316 PHY found on MAC1
Ethernet eth0: MAC address 00:00:00:00:46:61
IP: 128.224.178.127/255.255.254.0, Gateway: 128.224.178.1
Default server: 128.224.178.20

RedBoot(tm) bootstrap and debug environment [ROMRAM]
Ubiquiti Networks certified release, version 0.9.00318M.0905121200 - built 12:01:38, May 12 2009

Copyright (C) 2000, 2001, 2002, 2003, 2004 Red Hat, Inc.

Board: Ubiquiti RouterStation PRO
 Arch: ar7100pro
  RAM: 0x80000000-0x88000000, [0x8003bf00-0x87fe1000] available
Flash: 0xbf000000-0xc0000000, in 256 blocks of 0x00010000 bytes each
== Executing boot script in 10.000 seconds - enter ^C to abort
RedBoot> ip_address -l 128.224.178.127 -h 128.224.178.20
IP: 128.224.178.127/255.255.254.0, Gateway: 128.224.178.1
Default server: 128.224.178.20
RedBoot> load -v vlm-boards/19255/kernel
Using default protocol (TFTP)
Entry point: 0x805863d0, address range: 0x80060000-0x806cae1c
RedBoot> exec -c "console=ttyS0,115200 root=/dev/nfs rw nfsroot=128.224.178.20:/export/pxeboot/vlm-boards/19255/rootfs ip=128.224.178.127:128.224.178.20:128.224.178.1:255.255.254.0::eth0:off board=UBNT-RSPRO"
Now booting linux kernel:
 Base address 0x80050000 Entry 0x805863d0
 Cmdline : console=ttyS0,115200 root=/dev/nfs rw nfsroot=128.224.178.20:/export/pxeboot/vlm-boards/19255/rootfs ip=128.224.178.127:128.224.178.20:128.224.178.1:255.255.254.0::eth0:off board=UBNT-RSPRO
 memsize=0x08000000
 modetty0=0,n,8,1,hw
 board=RouterStation PRO
 ethaddr=00.00.00.00.46.61
Initializing cgroup subsys cpuset
Initializing cgroup subsys cpu
Linux version 3.8.11-yocto-standard (pokybuild@ab06.i.yoctoproject.org) (gcc version 4.7.2 (GCC) ) #1 PREEMPT Fri May 24 16:59:16 GMT 2013
bootconsole [early0] enabled
CPU revision is: 00019374 (MIPS 24Kc)
SoC: Atheros AR7161 rev 2
Clocks: CPU:720.000MHz, DDR:360.000MHz, AHB:180.000MHz, Ref:40.000MHz
Determined physical RAM map:
 memory: 08000000 @ 00000000 (usable)
Initrd not found or empty - disabling initrd
Zone ranges:
  Normal   [mem 0x00000000-0x07ffffff]
Movable zone start for each node
Early memory node ranges
  node   0: [mem 0x00000000-0x07ffffff]
Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
Primary data cache 32kB, 4-way, VIPT, cache aliases, linesize 32 bytes
Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 32512
Kernel command line:   console=ttyS0,115200 root=/dev/nfs rw nfsroot=128.224.178.20:/export/pxeboot/vlm-boards/19255/rootfs ip=128.224.178.127:128.224.178.20:128.224.178.1:255.255.254.0::eth0:off board=UBNT-RSPRO
PID hash table entries: 512 (order: -1, 2048 bytes)
Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
__ex_table already sorted, skipping sort
Writing ErrCtl register=0001e202
Readback ErrCtl register=0001e202
allocated 262144 bytes of page_cgroup
please try 'cgroup_disable=memory' option if you don't want memory cgroups
Memory: 122128k/131072k available (5328k kernel code, 8944k reserved, 1023k data, 236k init, 0k highmem)
SLUB: Genslabs=9, HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
NR_IRQS:48
Calibrating delay loop... 479.23 BogoMIPS (lpj=2396160)
pid_max: default: 32768 minimum: 301
Security Framework initialized
Mount-cache hash table entries: 512
Initializing cgroup subsys debug
Initializing cgroup subsys cpuacct
Initializing cgroup subsys memory
Initializing cgroup subsys devices
Initializing cgroup subsys freezer
Initializing cgroup subsys net_cls
Initializing cgroup subsys blkio
devtmpfs: initialized
NET: Registered protocol family 16
MIPS: machine is Generic AR71XX/AR724X/AR913X based board
bio: create slab <bio-0> at 0
SCSI subsystem initialized
Switching to clocksource MIPS
NET: Registered protocol family 2
TCP established hash table entries: 1024 (order: 1, 8192 bytes)
TCP bind hash table entries: 1024 (order: 0, 4096 bytes)
TCP: Hash tables configured (established 1024 bind 1024)
TCP: reno registered
UDP hash table entries: 256 (order: 0, 4096 bytes)
UDP-Lite hash table entries: 256 (order: 0, 4096 bytes)
NET: Registered protocol family 1
RPC: Registered named UNIX socket transport module.
RPC: Registered udp transport module.
RPC: Registered tcp transport module.
RPC: Registered tcp NFSv4.1 backchannel transport module.
squashfs: version 4.0 (2009/01/31) Phillip Lougher
NFS: Registering the id_resolver key type
Key type id_resolver registered
Key type id_legacy registered
jffs2: version 2.2. (NAND) © 2001-2006 Red Hat, Inc.
Btrfs loaded
msgmni has been set to 238
Block layer SCSI generic (bsg) driver version 0.4 loaded (major 253)
io scheduler noop registered
io scheduler deadline registered
io scheduler cfq registered (default)
Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled
serial8250.0: ttyS0 at MMIO 0x18020000 (irq = 11) is a 16550A
console [ttyS0] enabled, bootconsole disabled
console [ttyS0] enabled, bootconsole disabled
brd: module loaded
i8042: i8042 controller selftest timeout
md: linear personality registered for level -1
md: raid0 personality registered for level 0
md: raid1 personality registered for level 1
md: raid10 personality registered for level 10
md: multipath personality registered for level -4
md: faulty personality registered for level -5
device-mapper: ioctl: 4.23.1-ioctl (2012-12-18) initialised: dm-devel@redhat.com
oprofile: using mips/24K performance monitoring.
TCP: cubic registered
NET: Registered protocol family 10
sit: IPv6 over IPv4 tunneling driver
NET: Registered protocol family 17
Key type dns_resolver registered
console [netcon0] enabled
netconsole: network logging started
md: Waiting for all devices to be available before autodetect
md: If you don't use raid, use raid=noautodetect
md: Autodetecting RAID arrays.
md: Scanned 0 and added 0 devices.
md: autorun ...
md: ... autorun DONE.
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:
VFS: Unable to mount root fs on unknown-block(2,0)
User configuration error - no valid root filesystem found
Kernel panic - not syncing: Invalid configuration from end user prevents continuing
+
AR8316 PHY found on MAC0
AR8316 PHY found on MAC1
Ethernet eth0: MAC address 00:00:00:00:46:61
IP: 128.224.178.127/255.255.254.0, Gateway: 128.224.178.1
Default server: 128.224.178.20

RedBoot(tm) bootstrap and debug environment [ROMRAM]
Ubiquiti Networks certified release, version 0.9.00318M.0905121200 - built 12:01:38, May 12 2009

Copyright (C) 2000, 2001, 2002, 2003, 2004 Red Hat, Inc.

Board: Ubiquiti RouterStation PRO
 Arch: ar7100pro
  RAM: 0x80000000-0x88000000, [0x8003bf00-0x87fe1000] available
Flash: 0xbf000000-0xc0000000, in 256 blocks of 0x00010000 bytes each
== Executing boot script in 10.000 seconds - enter ^C to abort
RedBoot> ip_address -l 128.224.178.127 -h 128.224.178.20
IP: 128.224.178.127/255.255.254.0, Gateway: 128.224.178.1
Default server: 128.224.178.20
RedBoot> load -v vlm-boards/19255/kernel
Using default protocol (TFTP)
Entry point: 0x805863d0, address range: 0x80060000-0x806cae1c
RedBoot> exec -c "console=ttyS0,115200 root=/dev/nfs rw nfsroot=128.224.178.20:/export/pxeboot/vlm-boards/19255/rootfs ip=128.224.178.127:128.224.178.20:128.224.178.1:255.255.254.0::eth0:off board=UBNT-RSPRO"
Now booting linux kernel:
 Base address 0x80050000 Entry 0x805863d0
 Cmdline : console=ttyS0,115200 root=/dev/nfs rw nfsroot=128.224.178.20:/export/pxeboot/vlm-boards/19255/rootfs ip=128.224.178.127:128.224.178.20:128.224.178.1:255.255.254.0::eth0:off board=UBNT-RSPRO
 memsize=0x08000000
 modetty0=0,n,8,1,hw
 board=RouterStation PRO
 ethaddr=00.00.00.00.46.61
Initializing cgroup subsys cpuset
Initializing cgroup subsys cpu
Linux version 3.8.11-yocto-standard (pokybuild@ab06.i.yoctoproject.org) (gcc version 4.7.2 (GCC) ) #1 PREEMPT Fri May 24 16:59:16 GMT 2013
bootconsole [early0] enabled
CPU revision is: 00019374 (MIPS 24Kc)
SoC: Atheros AR7161 rev 2
Clocks: CPU:720.000MHz, DDR:360.000MHz, AHB:180.000MHz, Ref:40.000MHz
Determined physical RAM map:
 memory: 08000000 @ 00000000 (usable)
Initrd not found or empty - disabling initrd
Zone ranges:
  Normal   [mem 0x00000000-0x07ffffff]
Movable zone start for each node
Early memory node ranges
  node   0: [mem 0x00000000-0x07ffffff]
Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
Primary data cache 32kB, 4-way, VIPT, cache aliases, linesize 32 bytes
Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 32512
Kernel command line:   console=ttyS0,115200 root=/dev/nfs rw nfsroot=128.224.178.20:/export/pxeboot/vlm-boards/19255/rootfs ip=128.224.178.127:128.224.178.20:128.224.178.1:255.255.254.0::eth0:off board=UBNT-RSPRO
PID hash table entries: 512 (order: -1, 2048 bytes)
Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
__ex_table already sorted, skipping sort
Writing ErrCtl register=0001e202
Readback ErrCtl register=0001e202
allocated 262144 bytes of page_cgroup
please try 'cgroup_disable=memory' option if you don't want memory cgroups
Memory: 122128k/131072k available (5328k kernel code, 8944k reserved, 1023k data, 236k init, 0k highmem)
SLUB: Genslabs=9, HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
NR_IRQS:48
Calibrating delay loop... 479.23 BogoMIPS (lpj=2396160)
pid_max: default: 32768 minimum: 301
Security Framework initialized
Mount-cache hash table entries: 512
Initializing cgroup subsys debug
Initializing cgroup subsys cpuacct
Initializing cgroup subsys memory
Initializing cgroup subsys devices
Initializing cgroup subsys freezer
Initializing cgroup subsys net_cls
Initializing cgroup subsys blkio
devtmpfs: initialized
NET: Registered protocol family 16
MIPS: machine is Generic AR71XX/AR724X/AR913X based board
bio: create slab <bio-0> at 0
SCSI subsystem initialized
Switching to clocksource MIPS
NET: Registered protocol family 2
TCP established hash table entries: 1024 (order: 1, 8192 bytes)
TCP bind hash table entries: 1024 (order: 0, 4096 bytes)
TCP: Hash tables configured (established 1024 bind 1024)
TCP: reno registered
UDP hash table entries: 256 (order: 0, 4096 bytes)
UDP-Lite hash table entries: 256 (order: 0, 4096 bytes)
NET: Registered protocol family 1
RPC: Registered named UNIX socket transport module.
RPC: Registered udp transport module.
RPC: Registered tcp transport module.
RPC: Registered tcp NFSv4.1 backchannel transport module.
squashfs: version 4.0 (2009/01/31) Phillip Lougher
NFS: Registering the id_resolver key type
Key type id_resolver registered
Key type id_legacy registered
jffs2: version 2.2. (NAND) © 2001-2006 Red Hat, Inc.
Btrfs loaded
msgmni has been set to 238
Block layer SCSI generic (bsg) driver version 0.4 loaded (major 253)
io scheduler noop registered
io scheduler deadline registered
io scheduler cfq registered (default)
Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled
serial8250.0: ttyS0 at MMIO 0x18020000 (irq = 11) is a 16550A
console [ttyS0] enabled, bootconsole disabled
console [ttyS0] enabled, bootconsole disabled
brd: module loaded
i8042: i8042 controller selftest timeout
md: linear personality registered for level -1
md: raid0 personality registered for level 0
md: raid1 personality registered for level 1
md: raid10 personality registered for level 10
md: multipath personality registered for level -4
md: faulty personality registered for level -5
device-mapper: ioctl: 4.23.1-ioctl (2012-12-18) initialised: dm-devel@redhat.com
oprofile: using mips/24K performance monitoring.
TCP: cubic registered
NET: Registered protocol family 10
sit: IPv6 over IPv4 tunneling driver
NET: Registered protocol family 17
Key type dns_resolver registered
console [netcon0] enabled
netconsole: network logging started
md: Waiting for all devices to be available before autodetect
md: If you don't use raid, use raid=noautodetect
md: Autodetecting RAID arrays.
md: Scanned 0 and added 0 devices.
md: autorun ...
md: ... autorun DONE.
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:
VFS: Unable to mount root fs on unknown-block(2,0)
User configuration error - no valid root filesystem found
Kernel panic - not syncing: Invalid configuration from end user prevents continuing

issue still exists in 20130604-5 build.

same log as above except release information.

I diff the current kernel (3.8.x) and the yocto 1.4 kernel (3.4.x), and found the following configurations are missing in the current kernel config: 

CONFIG_ATH79_MACH_UBNT=y
CONFIG_SOC_AR71XX=y                                                      CONFIG_ATH79_DEV_ETH=y                                                        CONFIG_ATH79_DEV_M25P80=y                                                       CONFIG_ATH79_DEV_GPIO_BUTTONS=y                                            CONFIG_ATH79_DEV_LEDS_GPIO=y                                                     CONFIG_ATH79_DEV_SPI=y                                                           CONFIG_ATH79_DEV_USB=y 

CONFIG_USB_EHCI_ATH79=y                                                          CONFIG_USB_OHCI_ATH79=y

And the above configurations are missing in menuconfig when I try to configure the kernel manually. It looks that there are missing some patches with the 3.8.x kernel.

Looks like the 3.8.x kernel bump missed a NFS test, I swear that
I saw that in the report.

Assuming that this boots from disk, I made this "normal" priority
and will have someone look into it.

issue still exists in 20130618-2 build.

same log as above except release information.

Using 20130618-2 build,  booting from local disk also get failed.
But using yocto 1.4 (3.4.36) kernel, it works well.

PS: I tried to add EHCI & OHCI in kernel config options as 3.4.36 did, it still doesn't work.

Log:

AR8316 PHY found on MAC0
AR8316 PHY found on MAC1
Ethernet eth0: MAC address 00:00:00:00:46:61
IP: 128.224.178.127/255.255.254.0, Gateway: 128.224.178.1
Default server: 128.224.178.20

RedBoot(tm) bootstrap and debug environment [ROMRAM]
Ubiquiti Networks certified release, version 0.9.00318M.0905121200 - built 12:01:38, May 12 2009

Copyright (C) 2000, 2001, 2002, 2003, 2004 Red Hat, Inc.

Board: Ubiquiti RouterStation PRO
 Arch: ar7100pro
  RAM: 0x80000000-0x88000000, [0x8003bf00-0x87fe1000] available
Flash: 0xbf000000-0xc0000000, in 256 blocks of 0x00010000 bytes each
== Executing boot script in 10.000 seconds - enter ^C to abort
^C
RedBoot> ip_address -l 128.224.178.127 -h 128.224.178.20 
IP: 128.224.178.127/255.255.254.0, Gateway: 128.224.178.1
Default server: 128.224.178.20
RedBoot> load -v vlm-boards/19255/kernel 
Using default protocol (TFTP)
TFTP timed out 1/15
TFTP timed out 2/15
Entry point: 0x805b01c0, address range: 0x80060000-0x806fcaac
RedBoot> exec -c "console=ttyS0,115200 root=/dev/sda1 rw rootdelay=2 board=UBNT-RSPRO"
Now booting linux kernel:
 Base address 0x80050000 Entry 0x805b01c0
 Cmdline : console=ttyS0,115200 root=/dev/sda1 rw rootdelay=2 board=UBNT-RSPRO
 memsize=0x08000000
 modetty0=0,n,8,1,hw
 board=RouterStation PRO
 ethaddr=00.00.00.00.46.61
Initializing cgroup subsys cpuset
Initializing cgroup subsys cpu
Linux version 3.8.13-yocto-standard (builder@pek-yocto-build2) (gcc version 4.7.2 (GCC) ) #5 PREEMPT Tue Jun 25 16:08:59 CST 2013
bootconsole [early0] enabled
CPU revision is: 00019374 (MIPS 24Kc)
SoC: Atheros AR7161 rev 2
Clocks: CPU:720.000MHz, DDR:360.000MHz, AHB:180.000MHz, Ref:40.000MHz
Determined physical RAM map:
 memory: 08000000 @ 00000000 (usable)
Initrd not found or empty - disabling initrd
Zone ranges:
  Normal   [mem 0x00000000-0x07ffffff]
Movable zone start for each node
Early memory node ranges
  node   0: [mem 0x00000000-0x07ffffff]
Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
Primary data cache 32kB, 4-way, VIPT, cache aliases, linesize 32 bytes
Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 32512
Kernel command line:   console=ttyS0,115200 root=/dev/sda1 rw rootdelay=2 board=UBNT-RSPRO
PID hash table entries: 512 (order: -1, 2048 bytes)
Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
__ex_table already sorted, skipping sort
Writing ErrCtl register=0001e202
Readback ErrCtl register=0001e202
allocated 262144 bytes of page_cgroup
please try 'cgroup_disable=memory' option if you don't want memory cgroups
Memory: 121932k/131072k available (5496k kernel code, 9140k reserved, 1054k data, 232k init, 0k highmem)
SLUB: Genslabs=9, HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
NR_IRQS:48
Calibrating delay loop... 479.23 BogoMIPS (lpj=2396160)
pid_max: default: 32768 minimum: 301
Security Framework initialized
Mount-cache hash table entries: 512
Initializing cgroup subsys debug
Initializing cgroup subsys cpuacct
Initializing cgroup subsys memory
Initializing cgroup subsys devices
Initializing cgroup subsys freezer
Initializing cgroup subsys net_cls
Initializing cgroup subsys blkio
devtmpfs: initialized
NET: Registered protocol family 16
MIPS: machine is Generic AR71XX/AR724X/AR913X based board
bio: create slab <bio-0> at 0
SCSI subsystem initialized
usbcore: registered new interface driver usbfs
usbcore: registered new interface driver hub
usbcore: registered new device driver usb
Switching to clocksource MIPS
NET: Registered protocol family 2
TCP established hash table entries: 1024 (order: 1, 8192 bytes)
TCP bind hash table entries: 1024 (order: 0, 4096 bytes)
TCP: Hash tables configured (established 1024 bind 1024)
TCP: reno registered
UDP hash table entries: 256 (order: 0, 4096 bytes)
UDP-Lite hash table entries: 256 (order: 0, 4096 bytes)
NET: Registered protocol family 1
RPC: Registered named UNIX socket transport module.
RPC: Registered udp transport module.
RPC: Registered tcp transport module.
RPC: Registered tcp NFSv4.1 backchannel transport module.
squashfs: version 4.0 (2009/01/31) Phillip Lougher
NFS: Registering the id_resolver key type
Key type id_resolver registered
Key type id_legacy registered
jffs2: version 2.2. (NAND) © 2001-2006 Red Hat, Inc.
Btrfs loaded
msgmni has been set to 238
Block layer SCSI generic (bsg) driver version 0.4 loaded (major 253)
io scheduler noop registered
io scheduler deadline registered
io scheduler cfq registered (default)
Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled
serial8250.0: ttyS0 at MMIO 0x18020000 (irq = 11) is a 16550A
console [ttyS0] enabled, bootconsole disabled
console [ttyS0] enabled, bootconsole disabled
brd: module loaded
ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
ehci-platform: EHCI generic platform driver
ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver
Initializing USB Mass Storage driver...
usbcore: registered new interface driver usb-storage
USB Mass Storage support registered.
i8042: i8042 controller selftest timeout
md: linear personality registered for level -1
md: raid0 personality registered for level 0
md: raid1 personality registered for level 1
md: raid10 personality registered for level 10
md: multipath personality registered for level -4
md: faulty personality registered for level -5
device-mapper: ioctl: 4.23.1-ioctl (2012-12-18) initialised: dm-devel@redhat.com
oprofile: using mips/24K performance monitoring.
TCP: cubic registered
NET: Registered protocol family 10
sit: IPv6 over IPv4 tunneling driver
NET: Registered protocol family 17
Key type dns_resolver registered
console [netcon0] enabled
netconsole: network logging started
Waiting 2sec before mounting root device...
md: Waiting for all devices to be available before autodetect
md: If you don't use raid, use raid=noautodetect
md: Autodetecting RAID arrays.
md: Scanned 0 and added 0 devices.
md: autorun ...
md: ... autorun DONE.
VFS: Cannot open root device "sda1" or unknown-block(0,0): error -6
Please append a correct "root=" boot option; here are the available partitions:
VFS: Unable to mount root fs on unknown-block(0,0)
User configuration error - no valid root filesystem found
Kernel panic - not syncing: Invalid configuration from end user prevents continuing

issue still exists in 20130625-2 build.

same log as above except release information.

Issue still exists in 20130703-3  1.5_M2 rc1 build

issue fixed in 20130710-1 build

POKY rev: a63229917a5708de2d161aba0d67168ce0da6365