Software development notes
by Andrew
You’re doing bringup of a board or SoC or have got yourself in a tight spot; you have no networking and are unable to write to local storage in the runtime environment. How do you boot a custom firmware or kernel? If you’re local the obvious approach is to use an external tool to write the boot storage, but lets add to the challenge and say you’re doing this remotely. You have your board hooked up to a terminal server, and are at the u-boot prompt.
The historic approach has been to use the *MODEM protocols to do this
over serial, but we have the added impediment of the terminal server. Tools
like CKermit can drive the lrzsz tools as demonstrated by the u-boot
documentation and CKermit also supports e.g. telnet for the terminal
server, but modern Linux distributions (Ubuntu 19.04) don’t ship CKermit and
naively attempting to build it from source with modern compilers fails.
Telnet is a bit sketchy for several reasons, but ignoring security we also have the issue of data mangling. Thankfully our terminal server offers a “raw” port where no mangling takes place. With this context, we can dodge building and learning CKermit with some socat magic:
In u-boot, prepare the environment for data transfer via e.g. XMODEM:
# loadx 0x90000000
Then locally, once we have socat and lrzsz installed:
$ socat TCP:ts.example.com:2100 EXEC:"sx -b image"
What we should see is:
$ socat TCP:ts.example.com:2100 EXEC:"sx -b image"
Sending image, 16 blocks: Give your local XMODEM receive command now.
Bytes Sent: 2176 BPS:269
Transfer complete
$
And on the remote end:
# loadx 0x90000000
## Ready for binary (xmodem) download to 0x90000000 at 115200 bps...
CCxyzModem - CRC mode, 17(SOH)/0(STX)/0(CAN) packets, 1 retries
## Total Size = 0x00000809 = 2057 Bytes
#
The key insight is that the [XYZ]MODEM tools use synchronization on stdin and
stdout to ensure data is being sent and received correctly. This means we
can’t simply construct a shell pipeline like sx -b image | socat
TCP:9.41.165.152:2114 - and expect it to work. If we take this naive approach,
we see output like:
Retry 0: Timeout on pathname
Transfer incomplete
Or:
Sending s12983.lsz, 0 blocks: Give your local XMODEM receive command now.
Xmodem sectors/kbytes sent: 0/ 0kRetry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Timeout on sector ACK
Retry 0: Retry Count Exceeded
Transfer incomplete