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raspberrypi
WANT_64BIT_RT=1 does not activate a real-time kernel
I am on a cm4 booting from emmc flash with raspberry pi OS 64 bit.
I attempt to install real-time kernel with:
sudo SKIP_BOOTLOADER=1 WANT_64BIT_RT=1 rpi-update
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** Performing self-update
*** Relaunching after update
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
FW_REV:286a1d4d7520d7d592d19f64d2daf5160d595a3d
*** We're running for the first time
*** Backing up files (this will take a few minutes)
*** Remove old firmware backup
*** Backing up firmware
*** Remove old modules backup
*** Backing up modules 6.12.37-v8+
WANT_32BIT:0 WANT_64BIT:1 WANT_64BIT_RT:1 WANT_PI4:1 WANT_PI5:1
##############################################################
WARNING: This update bumps to rpi-6.12.y linux tree
See discussions at:
https://forums.raspberrypi.com/viewtopic.php?t=379745
##############################################################
Would you like to proceed? (y/N)
*** Downloading specific firmware revision (this will take a few minutes)
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
100 185M 100 185M 0 0 5089k 0 0:00:37 0:00:37 --:--:-- 4108k
*** Updating firmware
*** Updating kernel modules
*** depmod 6.12.37-v8+
*** depmod 6.12.37-v8-16k+
*** depmod 6.12.37-v8-rt+
*** Updating VideoCore libraries
*** Running ldconfig
*** Storing current firmware revision
*** Deleting downloaded files
*** Syncing changes to disk
*** If no errors appeared, your firmware was successfully updated to 286a1d4d7520d7d592d19f64d2daf5160d595a3d
*** A reboot is needed to activate the new firmware
I then reboot:
sudo reboot
And verify kernel running
uname -a
Linux <redacted> 6.12.37-v8+ #1894 SMP PREEMPT Fri Jul 11 16:47:45 BST 2025 aarch64 GNU/Linux
It says "PREEMPT" when I expect to see "PREEMPT_RT". How do I install the real-time kernel?
Here are the contents of my /boot and /boot/firmware
$ ls -1al /boot/
total 69650
drwxr-xr-x 4 root root 4096 Jul 11 11:27 .
drwxr-xr-x 19 root root 4096 May 31 19:49 ..
-rw-r--r-- 1 root root 92 May 12 17:06 cmdline.txt
-rw-r--r-- 1 root root 246476 Apr 30 05:45 config-6.12.25+rpt-rpi-2712
-rw-r--r-- 1 root root 246452 Apr 30 05:45 config-6.12.25+rpt-rpi-v8
-rw-r--r-- 1 root root 248969 Jun 26 02:26 config-6.12.34+rpt-rpi-2712
-rw-r--r-- 1 root root 248945 Jun 26 02:26 config-6.12.34+rpt-rpi-v8
-rw-r--r-- 1 root root 91 May 12 17:06 config.txt
drwxr-xr-x 4 root root 6144 Dec 31 1969 firmware
drwxr-xr-x 4 root root 4096 Dec 31 1969 firmware.bak
-rw-r--r-- 1 root root 7720825 Jun 28 14:07 initrd.img-6.12.25+rpt-rpi-2712
-rw-r--r-- 1 root root 7721081 Jun 28 14:07 initrd.img-6.12.25+rpt-rpi-v8
-rw-r--r-- 1 root root 7725023 Jul 2 10:00 initrd.img-6.12.34+rpt-rpi-2712
-rw-r--r-- 1 root root 7724915 Jul 2 10:00 initrd.img-6.12.34+rpt-rpi-v8
lrwxrwxrwx 1 root root 18 May 12 17:14 issue.txt -> firmware/issue.txt
lrwxrwxrwx 1 root root 17 May 12 17:06 overlays -> firmware/overlays
-rw-r--r-- 1 root root 83 Apr 30 05:45 System.map-6.12.25+rpt-rpi-2712
-rw-r--r-- 1 root root 83 Apr 30 05:45 System.map-6.12.25+rpt-rpi-v8
-rw-r--r-- 1 root root 83 Jun 26 02:26 System.map-6.12.34+rpt-rpi-2712
-rw-r--r-- 1 root root 83 Jun 26 02:26 System.map-6.12.34+rpt-rpi-v8
-rw-r--r-- 1 root root 9727677 Apr 30 05:45 vmlinuz-6.12.25+rpt-rpi-2712
-rw-r--r-- 1 root root 9726080 Apr 30 05:45 vmlinuz-6.12.25+rpt-rpi-v8
-rw-r--r-- 1 root root 9962173 Jun 26 02:26 vmlinuz-6.12.34+rpt-rpi-2712
-rw-r--r-- 1 root root 9958826 Jun 26 02:26 vmlinuz-6.12.34+rpt-rpi-v8
$ ls -1al /boot/firmware/
total 67328
drwxr-xr-x 4 root root 6144 Dec 31 1969 .
drwxr-xr-x 4 root root 4096 Jul 11 11:27 ..
-rwxr-xr-x 1 root root 30349 Jul 11 11:28 bcm2708-rpi-b.dtb
-rwxr-xr-x 1 root root 30652 Jul 11 11:28 bcm2708-rpi-b-plus.dtb
-rwxr-xr-x 1 root root 30011 Jul 11 11:28 bcm2708-rpi-b-rev1.dtb
-rwxr-xr-x 1 root root 29952 Jul 11 11:28 bcm2708-rpi-cm.dtb
-rwxr-xr-x 1 root root 29962 Jul 11 11:28 bcm2708-rpi-zero.dtb
-rwxr-xr-x 1 root root 31829 Jul 11 11:28 bcm2708-rpi-zero-w.dtb
-rwxr-xr-x 1 root root 32346 Jul 11 11:28 bcm2709-rpi-2-b.dtb
-rwxr-xr-x 1 root root 32121 Jul 11 11:28 bcm2709-rpi-cm2.dtb
-rwxr-xr-x 1 root root 32495 Jul 11 11:28 bcm2710-rpi-2-b.dtb
-rwxr-xr-x 1 root root 34687 Jul 11 11:28 bcm2710-rpi-3-b.dtb
-rwxr-xr-x 1 root root 35322 Jul 11 11:28 bcm2710-rpi-3-b-plus.dtb
-rwxr-xr-x 1 root root 33676 Jul 11 11:28 bcm2710-rpi-cm0.dtb
-rwxr-xr-x 1 root root 32258 Jul 11 11:28 bcm2710-rpi-cm3.dtb
-rwxr-xr-x 1 root root 33664 Jul 11 11:28 bcm2710-rpi-zero-2.dtb
-rwxr-xr-x 1 root root 33664 Jul 11 11:28 bcm2710-rpi-zero-2-w.dtb
-rwxr-xr-x 1 root root 56243 Jul 11 11:28 bcm2711-rpi-400.dtb
-rwxr-xr-x 1 root root 56239 Jul 11 11:28 bcm2711-rpi-4-b.dtb
-rwxr-xr-x 1 root root 56760 Jul 11 11:28 bcm2711-rpi-cm4.dtb
-rwxr-xr-x 1 root root 39913 Jul 11 11:28 bcm2711-rpi-cm4-io.dtb
-rwxr-xr-x 1 root root 53502 Jul 11 11:28 bcm2711-rpi-cm4s.dtb
-rwxr-xr-x 1 root root 78439 Jul 11 11:28 bcm2712d0-rpi-5-b.dtb
-rwxr-xr-x 1 root root 78431 Jul 11 11:28 bcm2712-d-rpi-5-b.dtb
-rwxr-xr-x 1 root root 78391 Jul 11 11:28 bcm2712-rpi-500.dtb
-rwxr-xr-x 1 root root 78435 Jul 11 11:28 bcm2712-rpi-5-b.dtb
-rwxr-xr-x 1 root root 79149 Jul 11 11:28 bcm2712-rpi-cm5-cm4io.dtb
-rwxr-xr-x 1 root root 79215 Jul 11 11:28 bcm2712-rpi-cm5-cm5io.dtb
-rwxr-xr-x 1 root root 79190 Jul 11 11:28 bcm2712-rpi-cm5l-cm4io.dtb
-rwxr-xr-x 1 root root 79256 Jul 11 11:28 bcm2712-rpi-cm5l-cm5io.dtb
-rwxr-xr-x 1 root root 52476 Jul 11 11:28 bootcode.bin
-rwxr-xr-x 1 root root 1 Jul 11 11:28 .bootloader_revision
-rwxr-xr-x 1 root root 102 May 12 17:14 cmdline.txt
-rwxr-xr-x 1 root root 1262 Jul 11 08:52 config.txt
-rwxr-xr-x 1 root root 41 Jul 11 11:28 .firmware_revision
-rwxr-xr-x 1 root root 3232 Jul 11 11:28 fixup4cd.dat
-rwxr-xr-x 1 root root 5458 Jul 11 11:28 fixup4.dat
-rwxr-xr-x 1 root root 8449 Jul 11 11:28 fixup4db.dat
-rwxr-xr-x 1 root root 8449 Jul 11 11:28 fixup4x.dat
-rwxr-xr-x 1 root root 3232 Jul 11 11:28 fixup_cd.dat
-rwxr-xr-x 1 root root 7327 Jul 11 11:28 fixup.dat
-rwxr-xr-x 1 root root 10296 Jul 11 11:28 fixup_db.dat
-rwxr-xr-x 1 root root 10294 Jul 11 11:28 fixup_x.dat
-rwxr-xr-x 1 root root 7725023 Jul 2 10:00 initramfs_2712
-rwxr-xr-x 1 root root 7724915 Jul 2 10:00 initramfs8
-rwxr-xr-x 1 root root 145 May 12 17:14 issue.txt
-rwxr-xr-x 1 root root 9879395 Jul 11 11:28 kernel_2712.img
-rwxr-xr-x 1 root root 9875330 Jul 11 11:28 kernel8.img
-rwxr-xr-x 1 root root 9913762 Jul 11 11:28 kernel8_rt.img
-rwxr-xr-x 1 root root 1594 Jun 28 14:07 LICENCE.broadcom
drwxr-xr-x 2 root root 32768 Jul 2 10:00 overlays
-rwxr-xr-x 1 root root 815516 Jul 11 11:28 start4cd.elf
-rwxr-xr-x 1 root root 3763976 Jul 11 11:28 start4db.elf
-rwxr-xr-x 1 root root 2265920 Jul 11 11:28 start4.elf
-rwxr-xr-x 1 root root 3013576 Jul 11 11:28 start4x.elf
-rwxr-xr-x 1 root root 815516 Jul 11 11:28 start_cd.elf
-rwxr-xr-x 1 root root 4835880 Jul 11 11:28 start_db.elf
-rwxr-xr-x 1 root root 2990080 Jul 11 11:28 start.elf
-rwxr-xr-x 1 root root 3737192 Jul 11 11:28 start_x.elf
drwxr-xr-x 2 root root 2048 May 30 13:17 'System Volume Information'
I figured it out
I had to put the following line in /boot/firmware/config.txt
kernel=kernel8_rt.img
Probably worth putting that in the readme or telling the user that when they install rt kernel.