ECE 440 · Week 3
Modern Computer: Full Stack
No Monday lecture.
Week 3 of 10 · Solitude
ECE 440 · Week 3 · In class
No Monday lecture.
- One lecture hour remains: a serial-console pin, claimed by a driver, drops the console.
- The revert puts the console back.
- Do this only on spare media.
- On this section the spare is a card.
ECE 440 · Week 3 · In class
No Monday lecture. (continued)
- Write the spare-card label before you start, and write one command you will not run on the station's main card.
- That pair is the homework.
ECE 440 · Week 3 · In class
Capture the console dying, revert the claim, and confirm the console answers after a second reboot of that spare card.
- Leave the main card untouched.
ECE 440 · Week 3 · Lecture
The console has its own connector
- Console TX
- Console RX
- Console ground
Other pins are dimmed. Colours as on the Solitude pinout, where each pin's name is. The square pad is pin 1.
ECE 440 · Week 3 · Lecture
Why a pin claim can kill the console
- The serial console is a UART whose pads are muxed to the UART function.
- If a driver claims one of those pads for another function, the UART loses its line.
- The kernel keeps running; you just stop seeing it. Silence is not a crash.
- The pin controller refuses a second owner, so the order in which drivers probe matters.
ECE 440 · Week 3 · Lecture
What the log says when two owners collide
pin PAD already requested by OWNER; cannot claim for DEV
could not request pin N (PAD) from group GRP on device PCTL- Upper-case words are placeholders the kernel fills in.
- A refused claim is a log line; a granted one is silence on the console.
- The driver whose claim was refused does not probe.
ECE 440 · Week 3 · Lecture
Capture from the other end of the cable
# on the lab PC, before powering the board
picocom -b 115200 -g console.log /dev/ttyUSB0- The board's own log cannot record its console dying; the lab PC can.
-gwrites everything picocom shows to a file. Exit with Ctrl-A Ctrl-X.- Start the capture first, then power on, so the last line before silence is kept.
ECE 440 · Week 3 · Lecture
Spare media first, revert planned before
- Label the spare card and write the label down before it goes in the board.
- Run
lsblkand read the size and label before any write: the wrong device is one letter away. - Know where the claim lives (an overlay, a boot setting) so the revert is one edit away.
- A revert counts when the console answers after a second reboot, not just once.
ECE 440 · Week 3 · Lab
Lab
- No Monday lecture. Claim a pin the serial console uses, capture the console dying, and revert the claim so the console returns.
- Do this on spare media. Second pass: confirm the console still answers after a second reboot of that spare card.
- Assemble the lab hand-in: The console log of the pin claim that breaks the serial console, and the revert that brings the console back.
ECE 440 · Week 3 · Lab
Hand in
The console log of the pin claim that breaks the serial console, and the revert that brings the console back.
ECE 440 · Week 3 · Homework
Homework
- Write the spare-media label you will use, and one command you will not run on the station's main card.
ECE 440 · Week 3 · Homework
Turn in
The label and the command you refuse.
ECE 440 · Week 3 · Read
Read
On the lab site: Module 9, Device tree and pin multiplexing, does not grade a claim that breaks the console and the revert. This week does.
ECE 440 · Week 3 · Next
Week 4
What a bind line looks like in the kernel log, and which job stays in userspace.