On Mon, 19 Sep 2016 16:20:12 +0530, Madhu K said:

That means kernel maintainer will define default configuration for his architecture not for all architecture, I mean kernel maintainer might have defined configuration for x86, not for ARM, POWERPC and all, is my understanding correct?, if not please correct me.

There's more than one defconfig file. Even more than one per architecture. For many, there's a separate defconfig for each board: [/usr/src/linux-next] for i in arch/*/configs; do echo -n "$i: "; find $i -name '*defconfig' | wc -l; done arch/arc/configs: 15 arch/arm/configs: 112 arch/arm64/configs: 1 arch/avr32/configs: 15 arch/blackfin/configs: 29 arch/c6x/configs: 5 arch/cris/configs: 4 arch/h8300/configs: 3 arch/hexagon/configs: 1 arch/ia64/configs: 6 arch/m32r/configs: 12 arch/m68k/configs: 19 arch/metag/configs: 4 arch/microblaze/configs: 2 arch/mips/configs: 61 arch/mn10300/configs: 2 arch/nios2/configs: 2 arch/openrisc/configs: 1 arch/parisc/configs: 8 arch/powerpc/configs: 99 arch/s390/configs: 4 arch/score/configs: 1 arch/sh/configs: 56 arch/sparc/configs: 2 arch/tile/configs: 2 arch/um/configs: 2 arch/unicore32/configs: 1 arch/x86/configs: 2 arch/xtensa/configs: 6

On Mon, 19 Sep 2016 10:58:35 -0400, Aruna Hewapathirane said:

make menuconfig: An ncurses-based pseudo-graphical menu (only text input). Navigate through the menu to modify the desired options. Within menuconfig, use the / key to search modules by name.

Actually, / searches (in a case-insensitive manner, so feel free to type in lower case) for CONFIG_WHATEVER variables, not modules. The distinction is important for 2 reasons: 1) Sometimes, the config variable that controls building a module isn't the uppercase version of the module name. It's rare, but does happen. For example, a Fedora 4.3.0 kernel has 2,739 =m entries in its .config. Of those: [/lib/modules/4.3.0-0.rc0.git14.1.fc24.x86_64] for i in `grep =m /boot/config-4.3.0-0.rc0.git14.1.fc24.x86_64 | sed -e 's/^CONFIG_//' -e 's/=m$//' | tr 'A-Z' 'a-z'`; do find . -name ${i}.ko.xz; done | wc -l 531 Only 531 are the lowercase of the config name. [/lib/modules/4.3.0-0.rc0.git14.1.fc24.x86_64] for i in `grep =m /boot/config-4.3.0-0.rc0.git14.1.fc24.x86_64 | sed -e 's/^CONFIG_//' -e 's/=m$//' | tr '_A-Z' '-a-z'`; do find . -name ${i}.ko.xz; done | wc 593 And some more have underscores changed to dashes (CONFIG_FOO_BAR -> foo-bar.ko) And the *majority* of them play games of one form or another: obj-$(CONFIG_BATTERY_OLPC) += olpc_battery.o obj-$(CONFIG_BLK_DEV_LOOP) += loop.o obj-$(CONFIG_LOOPBACK_TARGET) += tcm_loop.o obj-$(CONFIG_NVME_TARGET_LOOP) += nvme-loop.o and so on... 2) It's also good for finding yes/no variables that don't control building a module. For example: CONFIG_ALLOW_DEV_COREDUMP=y

make defconfig: Generates a new config with default from the ARCH supplied defconfig file. Use this option to get back the default configuration file that came with the sources.

Actually, that *won't* get you back "the config that came with the sources". If you look in a release source tarball from kernel.org, there *isn't* a .config in there, you need to create one somehow (copying a distro .config and then running 'make localmodconfig' is a popular choice). And there's no guarantee that 'make defconfig' will re-create whatever your distro shipped - in fact, it probably *won't* do so, because distros rarely, if ever, ship a kernel that's built with a Linus-approved defconfig.

On Mon, 19 Sep 2016 16:02:11 -0400, Aruna Hewapathirane said:

Many times I have seriously screwed up my kernel to a point where it suddenly refuses to boot. Every single time this has happened to me what saved me was make defconfig. You may not have your internet connection working, your sound may be kaput, the webcam not functional BUT defconfig has invariably *every single time* given me back a kernel I can boot into.

Of course, the *correct* answer here is to keep around the known-working config files for several previous kernels. And the easiest way to do that is to: 1) keep several kernels in /boot (I have enough room for the current kernel, 3 or 4 previous, and enough slack to do 12 or 13 builds for a 'git bisect' without having to stop and clean up). 2) Set the following two things in your .config, so you can always boot into an older kernel and retrieve its config from /proc/config.gz: CONFIG_IKCONFIG=y CONFIG_IKCONFIG_PROC=y 3) Make sure your kernel install process copies the .config used to /boot/config-`uname -r` so you have it handy. And of course: 4) Take regular backups of your system. That will protect you from lots of screw-ups - and hardware failures as well. A 1 or 2 tera USB hard drive isn't that expensive.