There was an 'unanswered question' about what has to be set to make u-boot perform a netboot. I've dug into the 'doc' i had at work, and -for the toradex/colibri module- we've used the following u-boot commands to get a netboot:
setenv bootargs "root=/dev/nfs ip=:::::eth0: console=ttyS0,9600n8"
dhcp
bootm
The 1-st command is to prepare the kernel for net-booting, and tell the kernel to use a root-filesystem 'on nfs'. The contents of 'bootargs' is a set of parameters that will be passed to the kernel.
The 2-nd command is to let u-boot request an IP address and a boot-file (the kernel) from the dhcp/tftp server
The 3-rd command is to actually boot the kernel
Note that this is *only* the 'small brother' part of net-booting. The server ('big brother') needs some information to make the entire story work:
- a configured dhcp server
- a kernel file
- a nfs-exported root-filesystem
But: the good news is that -when these commands are 'passed' to u-boot- it will *attempt* to netboot. As you will have seen, there is not a word about over-writing any flash-filesystem, nor is there a 'saveenv' command used on the 'small brother's u-boot, just some instructions to use the network. When you'd monitor the network -with tcpdump for instance- you should 'see' the 'small brother' attempting to get an IP address and boot-info. Without the services installed and configured, that *will* fail, but -if 'small brother' does try to netboot- that shows that the u-boot in the 'small brother' is compiled with support for this operation, and for networking. U-boot is a way more potent bootloader than any BIOS i've ever found on any 'normal' computer. To set up the actual services to perform a successfull netboot is not quite trivial, but only a matter of 'work'
I'm going to see if i can 'open' the small machine and connect an oscilloscope to the TX 'pad', to see if u-boot is attempting to 'talk' to the serial port, and -if so- to see what voltage it generates there. I'm assuming that the RX pin will accept the same voltages that it generates on the TX pin. All assuming that these nice 'pins' are indeed the connection to the serial console and that u-boot is actually 'talking' there of course
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