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Re: No sound on laptop after linux-libre 5.15

From: Thiago Jung Bauermann
Subject: Re: No sound on laptop after linux-libre 5.15
Date: Wed, 08 Dec 2021 23:34:24 -0300

Hello Paxton,

Em quarta-feira, 8 de dezembro de 2021, às 01:55:13 -03, Paxton Evans 
> Rolling back to linux-libre 5.14.21 (guix
> ea7233befb9570cce47e5ca71725b285a580cd22) totally fixes this issue.
> Troubleshooting sound issues on GNU/Linux is such a nightmare that I'm
> hoping this list can help me, as I don't really know much about how
> sound on our favorite OS really works, and don't have enough information
> to file a bug. Can anyone give me pointers on where to start looking?
> dmesg on 5.15.6 didn't seem to report anything strange.

I’m not particularly familiar with the moving parts that make sound work 
either, so I’ll focus on the kernel side of things. My first idea would be 
trying to boot with the “debug” kernel command line option, which increases 
the verbosity of kernel messages. Perhaps something helpful will show up in 
dmesg that way?

If not, and if the only change in the working setup vs the non-working 
setup is the kernel version, then it’s a regression in the kernel. If you 
are willing to run the upstream Linux kernel on your machine (i.e., the 
non-libre kernel), the first step would be to run Linux v5.15.6 and see if 
sound works.

If it does, then it’s a bug in linux-libre itself and you should report the 
problem to them. If it doesn’t, it’s a bug in Linux itself and the most 
straightforward (but tedious and time-intensive) way to find the kernel 
commit that caused the bug is to use “git bisect”:

That is, assuming that you are comfortable building kernels.
You need to use v5.14 as your known-good version not v5.14.21 because your 
range should be a “straight line” in the version history, and v5.14.x 
versions branch off from mainline and thus aren’t ancestors of v5.15.x.

Also, since you’d be running random kernel commits, it’s important to be 
aware that there’s a small but non-zero risk of running a bad kernel 
version which could corrupt something on your system. E.g., not long ago 
during the v5.12 development cycle there was a filesystem corruption bug:

You wouldn’t be at risk of hitting that particular bug since your range 
starts at v5.14, but it’s not inconceivable that there could be something 
bad in your range (though I’m not aware of anything). The safest way to 
address that is to run the bisect kernels in a temporary Linux installation 
on your machine, such as in a USB flash drive rather than on your main Linux 


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