dmesg can be empty about boot while the journal still has everything

scope: generic · severity: trap · confidence: proven · subsystem: diagnosis

Symptom: dmesg | grep -i adreno returns nothing on a device whose GPU is demonstrably working. Nothing errors. The empty result reads as “the driver never probed”.

The kernel log ring is a fixed-size buffer. An ath10k_snoc WARN storm — 12 × “Unbalanced enable for IRQ” with a full register dump and backtrace on every WiFi firmware restart — pushed the oldest surviving dmesg line to t=195 s of a ~2000 s uptime. Everything about boot was gone. journalctl -b 0 -k still had all of it, because journald had already persisted it.

dmesg | head -1    # if this is not near [    0.0], dmesg cannot answer any
                   # question about boot -- use journalctl -b 0 -k

Check that one line before concluding anything from a dmesg grep. Any device that logs a repeating WARN puts every agent after it in this position, which makes this one of the most reusable checks here.