Order of trying kernels on a headless system, how to see what's happening?

[Chris Green]

I have a BeagleBone Black single board computer running Ubuntu 19.10.
It is a headless system with my only access being via ssh.

There are two kernel versions installed, in /boot I see:-

    -rw-r--r-- 1 root root     528 Jan 14 14:19 SOC.sh
    -rw-r--r-- 1 root root 3449473 Apr  9  2019 System.map-4.14.108-ti-r104
    -rw-r--r-- 1 root root 3939303 Apr  5  2019 System.map-4.19.31-armv7-x31
    -rw-r--r-- 1 root root  161143 Apr  9  2019 config-4.14.108-ti-r104
    -rw-r--r-- 1 root root  170176 Apr  5  2019 config-4.19.31-armv7-x31
    drwxr-xr-x 4 root root    4096 Apr 11  2019 dtbs
    drwxr-xr-x 3 root root    4096 Apr 11  2019 efi
    drwxr-xr-x 2 root root    4096 Jan 15 09:26 grub
    -rw-r--r-- 1 root root 8479516 Jan 14 19:36 initrd.img-4.14.108-ti-r104
    -rw-r--r-- 1 root root 9584252 Jan 14 19:44 initrd.img-4.19.31-armv7-x31
    -rw-r--r-- 1 root root    2213 Jan 14 14:19 uEnv.txt
    drwxr-xr-x 2 root root    4096 Apr 11  2019 uboot
    -rwxr-xr-x 1 root root 9949696 Apr  9  2019 vmlinuz-4.14.108-ti-r104
    -rwxr-xr-x 1 root root 9150976 Apr  5  2019 vmlinuz-4.19.31-armv7-x31

In the /boot/grub/grub.cfg file there are the expected menuentry lines
for both the 4.19 and the 4.14 kernels with the 4.19 ones being first.
However the system boots the 4.14 kernel.

How do I change the default kernel to boot?

I also wonder if there's something preventing the 4.19 kernel from
being booted which makes grub choose the 4.14 kernel.  Is there any
way to log the grub/boot process so that I can see what's happening?

In particular (as regards why 4.19 might not boot) a 'file *' on the
/boot directory returns:-

    SOC.sh:                       POSIX shell script, ASCII text executable
    System.map-4.14.108-ti-r104:  ASCII text
    System.map-4.19.31-armv7-x31: ASCII text
    config-4.14.108-ti-r104:      Linux make config build file, ASCII text
    config-4.19.31-armv7-x31:     Linux make config build file, ASCII text
    dtbs:                         directory
    efi:                          directory
    grub:                         directory
    initrd.img-4.14.108-ti-r104:  gzip compressed data, from Unix, original 
size modulo 2^32 13529600
    initrd.img-4.19.31-armv7-x31: LZ4 compressed data (v0.1-v0.9)
    uEnv.txt:                     ASCII text
    uboot:                        directory
    vmlinuz-4.14.108-ti-r104:     Linux kernel ARM boot executable zImage 
(little-endian)
    vmlinuz-4.19.31-armv7-x31:    Linux kernel ARM boot executable zImage 
(little-endian)

I wonder if the system can't unpack the LZ4 compressed file so chooses
the other.



-- 
Chris Green