FreeBSD: hdac0 Unexpected unsolicited response from address 1: 0000000

The problem

I upgraded my test system from FreeBSD 14.3 to FreeBSD 15.0 without much issues, so I thought do also upgrade my production system. But after rebooting the system, it didn’t get back up and when I looked at the console I saw the following messages on the screen:

hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000
hdac0: Unexpected unsolicited response from address 1: 00000000

And those messages just kept coming.

When searching for a cause or solution I found an email thread about an almost identical issue, but sadly without a solution.

Old kernel fails as well

Since the system was not working I gave it a reset and tried again, but the problem stayed the same. So next I tried to boot an old (14.3) kernel (option 6 in the bootloader), which also gave the same messages.

I also tried to disable ACPI and Single User mode, but those also didn’t work. disabling ACPI did not even get to the init process.

Sound driver issue

Searching the internet I found some indication that it has to do with a sound driver. And there are even comments mentioning that it is still present in 14.2-p1, but no solution.

So back to searching. This finally lead me to a solution that could help me:

Add something along the lines

hint.hdac.0.disabled=1

to the /boot/loader.conf

hdac being hw you want to prevent from attaching to, 0 being its unit.

but since the system is not booting, I can not change the loader.conf

This is also documented in the FreeBSD handbook but at the time I found that I didn’t know what it could do, or help me.

Final piece of the puzzle

On the FreeBSD.org forums I found a post which explained how to blacklist a FreeBSD kernel module from the bootloader. The steps are as follows:

1. When in the bootloader screen, press 3 “Escape to loader prompt” This will show you a OK prompt.
2. Type show to show a list of variables. Here you can see what options are all set.
3. Type set hint.hdac.0.disabled=1 to change the setting that would normally go into the /boot/loader.conf.
4. Type boot (without any arguments) to boot the system.

More details are available at the post

With these steps, I was able to boot the system and add the setting to the /boot/loader.conf after which a reboot worked as expected.