RE: kernel doesn't start booting after chaning to minimum config (on qemu arm64 machine)

[Chan Kim]

Hi Greg K-H,

> There are two different ways of getting a "minimal" kernel
> configuration:
>   - start with a working one and remove items until something
> breaks
>   - start with a minimal config and keep adding things until it
> works.
> 
> Usually the first option is the easier one, as it should go faster and you
> can figure out exactly what is needed.  The second way is just blindly
> guessing, like it seems you are doing here.
> 
> Best of luck!
> 
> greg k-h

After reading your email I thought I'd try just a couple of more additions 
(because gradually removing will take so long..) and tried adding the most 
suspicious ones. Then I found CONFIG_SERIAL_AMBA_PL011, 
CONFIG_SERIAL_AMBA_PL011_CONSOLE and CONFIG_SERIAL_EARLYCON_ARM_SEMIHOST were 
not set! (qemu virt machine uses PL011 not PL010 which was set). After this 
addition, it boots ok to the shell! I was very luck.. 
Thank you,
Best regards,

Chan Kim





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Re: kernel doesn't start booting after chaning to minimum config (on qemu arm64 machine)

[Greg KH]

On Tue, Oct 12, 2021 at 11:06:53PM +0900, Chan Kim wrote:
> Hello all,
> 
> To make a minimal kernel size (with almost no driver), I copied a .config
> that was used for building kernel for sparc machine using linux 3.3 several
> years ago to current linux 5.4.21 version build tree.
> 
> (That .config was for kernel with interrupt controller, timer and uart so a
> very basic scheduling and shell is possible.)
> 
> Then I ran 
> 
> Yes "" | make ARCH=arm64 CROSS_COMPILE=aarch64-none-elf- oldconfig
> 
> This copies the configs and sets all the new config settings with default
> values. 
> 
> (see
> https://serverfault.com/questions/116299/automatically-answer-defaults-when-
> doing-make-oldconfig-on-a-kernel-tree)
> 
>  
> 
> Now I can see the start_kernel function runs to the end and it's running
> inside the cpu_idle function. but I don't see the first scrolling message
> below that I normally should see.
> 
> [0.00] Booting Linux on physical CPU 0x00 [0x410fd083]
> 
> I tried adding several CONFIG variables using menuconfig but it's still the
> same.
> 
> (CONFIG_SERIAL_AMBA_PL010, CONFIG_SERIAL_AMBA_PL010_CONSOLE,
> CONFIG_SERIAL_CORE, CONFIG_SERIAL_CORE_CONSOLE were already automatically
> set. 
> 
> And later I tried adding CONFIG_CPU_IDLE, CONFIG_CPU_IDLE_GOV_LADDR,
> CONFIG_ACPI and other ACPI related things, CONFIG_PCI_ECAM, CONFIG_PNP,
> CONFIG_PNP_DEBUG_MESSAGES, .. )
> 
> And the qemu command is like this.
> 
> ${QEMU_DIR}/qemu-system-aarch64 -M ${QMACHINE} -cpu cortex-a72 -kernel
> ${LINUX_DIR}/arch/arm64/boot/Image -initrd ${BUSYBOX_DIR}/initramfs.cpio.gz
> --append "root=/dev/ram init=/init nokaslr earlycon ip=dhcp" -m 2048M
> -nographic -netdev user,id=n1 -device e1000,netdev=n1
> 
> Has anyone any idea what I can add in the config or try? (with original
> .config, the kernel boots ok to the bash in initramfs)
> 
> Any suggestion will be appreciated.

There are two different ways of getting a "minimal" kernel
configuration:
- start with a working one and remove items until something
  breaks
- start with a minimal config and keep adding things until it
  works.

Usually the first option is the easier one, as it should go faster and
you can figure out exactly what is needed.  The second way is just
blindly guessing, like it seems you are doing here.

Best of luck!

greg k-h

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