What does make oldconfig do exactly in the Linux kernel makefile

Question

Can anyone explain what the target  oldconfig  does exactly in the Linux kernel makefile   I see it referenced in some build documentation but never explained what it does exactly

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Before you run make oldconfig  you need to copy a kernel configuration file from an older kernel into the root directory of the new kernel   You can find a copy of the old kernel configuration file on a running system at  boot config-3 11 0  Alternatively  kernel source code has configs in linux-3 11 0 arch x86 configs  i386 defconfig   x86 64 defconfig   If your kernel source is located at  usr src linux   cd  usr src linux cp  boot config-3 9 6-gentoo  config make oldconfig

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It s torture  Instead of including a generic conf file  they make you hit return 9000 times to generate one

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Summary  As mentioned by Ignacio  it updates your  config for you after you update the kernel source  e g  with git pull   It tries to keep your existing options   Having a script for that is helpful because    new options may have been added  or old ones removed the kernel s Kconfig configuration format has options that    imply one another via select depend on another via depends   Those option relationships make manual config resolution even harder        Let s modify  config manually to understand how it resolves configurations  First generate a default configuration with   make defconfig   Now edit the generated  config file manually to emulate a kernel update and run   make oldconfig   to see what happens  Some conclusions    Lines of type     CONFIG XXX is not set   are not mere comments  but actually indicate that the parameter is not set   For example  if we remove the line     CONFIG DEBUG INFO is not set   and run make oldconfig  it will ask us   Compile the kernel with debug info  DEBUG INFO   N y     NEW    When it is over  the  config file will be updated   If you change any character of the line  e g  to   CONFIG DEBUG INFO  it does not count  Lines of type     CONFIG XXX is not set   are always used for the negation of a property  although   CONFIG XXX n   is also understood as the negation   For example  if you remove   CONFIG DEBUG INFO is not set and answer   Compile the kernel with debug info  DEBUG INFO   N y     NEW    with N  then the output file contains     CONFIG DEBUG INFO is not set   and not   CONFIG DEBUG INFO n   Also  if we manually modify the line to   CONFIG DEBUG INFO n   and run make oldconfig  then the line gets modified to     CONFIG DEBUG INFO is not set   without oldconfig asking us  Configs whose dependencies are not met  do not appear on the  config  All others do   For example  set   CONFIG DEBUG INFO y   and run make oldconfig  It will now ask us for  DEBUG INFO REDUCED  DEBUG INFO SPLIT  etc  configs   Those properties did not appear on the defconfig before   If we look under lib Kconfig debug where they are defined  we see that they depend on DEBUG INFO   config DEBUG INFO REDUCED     bool  Reduce debugging information      depends on DEBUG INFO   So when DEBUG INFO was off  they did not show up at all  Configs which are selected by turned on configs are automatically set without asking the user   For example  if CONFIG X86 y and we remove the line   CONFIG ARCH MIGHT HAVE PC PARPORT y   and run make oldconfig  the line gets recreated without asking us  unlike DEBUG INFO   This happens because arch x86 Kconfig  contains   config X86     def bool y               select ARCH MIGHT HAVE PC PARPORT   and select forces that option to be true  See also  https   unix stackexchange com questions 117521 select-vs-depends-in-kernel-kconfig Configs whose constraints are not met are asked for   For example  defconfig had set   CONFIG 64BIT y CONFIG RCU FANOUT 64   If we edit   CONFIG 64BIT n   and run make oldconfig  it will ask us   Tree-based hierarchical RCU fanout value  RCU FANOUT   32   NEW    This is because RCU FANOUT is defined at init Kconfig as   config RCU FANOUT     int  Tree-based hierarchical RCU fanout value      range 2 64 if 64BIT     range 2 32 if  64BIT   Therefore  without 64BIT  the maximum value is 32  but we had 64 set on the  config  which would make it inconsistent    Bonuses  make olddefconfig sets every option to their default value without asking interactively  It gets run automatically on make to ensure that the  config is consistent in case you ve modified it manually like we did  See also  https   serverfault com questions 116299 automatically-answer-defaults-when-doing-make-oldconfig-on-a-kernel-tree  make alldefconfig is like make olddefconfig  but it also accepts a config fragment to merge  This target is used by the merge config sh script  https   stackoverflow com a 39440863 895245  And if you want to automate the  config modification  that is not too simple  How do you non-interactively turn on features in a Linux kernel  config file

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From this page        Make oldconfig takes the  config and runs it through the rules of the   Kconfig files and produces a  config which is consistant with the   Kconfig rules  If there are CONFIG values which are missing  the make   oldconfig will ask for them         If the  config is already consistant with the rules found in Kconfig    then make oldconfig is essentially a no-op       If you were to run make oldconfig  and then run make oldconfig a   second time  the second time won t cause any additional changes to be   made

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It reads the existing  config file that was used for an old kernel and prompts the user for options in the current kernel source that are not found in the file  This is useful when taking an existing configuration and moving it to a new kernel

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Updates an old config with new changed removed options

[linux] What does "make oldconfig" do exactly in the Linux kernel makefile?

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