How to identify root partition via UUID without initramfs/initrd

I found the answer burried in another thread:

A UUID identifies a filesystems, whereas a PARTUUID identifies a partition (i.e. remains intact after reformatting). Without initramfs/initrd the kernel only supports PARTUUID.

To find the PARTUUID of the block devices in your machine use

sudo blkid

This will print, for example

/dev/sda1: UUID="XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX" TYPE="ext2" PARTUUID="f3f4g3f4-02"

You can now modify you linux command line as follows:

linux   /bzImage root=PARTUUID=f3f4g3f4-02 ro

This will boot from the partition with PARTUUID f3f4g3f4-02, which in this case is /dev/sda1.

lsblk with various options can show you what disk/partition/uuid are in use

eg

% sudo lsblk -o UUID,PARTUUID,NAME,MOUNTPOINT 
UUID                                   PARTUUID           NAME                MOUNTPOINT
                                                          sda                 
d634adc8-69de-edd8-d491-a79e69aeff78   0008500a-01        |-sda1
195237da-8825-45fb-abf7-a62895bd0967                      | `-md0             /boot
d2cf1bcc-d51d-bf37-9723-3b505172fe5f   0008500a-02        `-sda2              
24bvXN-PVU1-kubI-Zgj5-W82i-3Z07-v80lME                      `-md1             
67fe5039-de46-4629-bd03-ee65a5dd0132                          |-godzilla-root /
ba70f1d1-89f0-4dd9-83a4-8bc9a74a6548                          `-godzilla-swap [SWAP]

So I can see that UUID d634adc8-69de-edd8-d491-a79e69aeff78 corresponds to /dev/sda3 and partition UUID 0008500a-01

Depending on your setup you can then do

root=/dev/sda1

or

root=PARTUUID=0008600a-01

(In my case root is part of an LVM and so can't be mounted this way, but the concept applies)