Hi! I noticed I forgot to reply here: On 26.01.22 15:28, Geert Uytterhoeven wrote: > On Tue, Jan 25, 2022 at 5:45 PM Thorsten Leemhuis <linux@xxxxxxxxxxxxx> wrote: >> +How to create a configuration similar to the one of an older kernel? >> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >> + >> +Start a known-good kernel and configure the newer Linux version with ``make >> +olddefconfig``. This makes the kernel's build scripts pick up the configuration >> +file (the `.config` file) from the running kernel as base for the new one you >> +are about to compile; afterwards they set all new configuration options to their >> +default value, which should disable new features that might cause regressions. > > Doing so may actually cause mutations to appear in your .config > when going back and forth (i.e. when bisecting), interfering with > the bisection process. Good point, I knew about this, but hadn't thought of this when writing the text. > To avoid that, I usually start bisecting with > "cp .config <src>/arch/<arch>/configs/bisect_defconfig", and use > "make bisect_defconfig" in every bisection step. That way all steps > are reproducible, and unaffected by config mutations. That's a really cool trick, thx for mentioning it. But I think it's not needed in the text about regressions and instead better be mentioned in Documentation/admin-guide/bug-bisect.rst. I hope to sooner or later improve (rewrite?) that document anyway and will make sure to keep it in mind for that time. I wonder if there is a way to make this work without messing in the source tree? Took a quick look at the sources. It seems to me that it's possible to "cp .config ~/working.config" and then using "make KBUILD_DEFCONFIG=~/working.config defconfig" at every bisection step. It seems to do the trick as well -- but I only tried briefly, so maybe I might have missed something. Ciao, Thorsten
