Hi Alex, Andrei,<br /><br />thanks for your reply. Based on your feedback I've tried the following:<br />- I have downloaded the patch patch-5.15.86-rt56.patch from https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/ and stored in ./meta-raspberrypi/recipes-kernel/linux/files<br />- I have created a file linux-raspberrypi_%.bbappend in ./meta-raspberrypi/recipes-kernel/linux<br />- I have created a .cfg file with CONFIG_PREEMPT_RT_FULL = y in ./meta-raspberrypi/recipes-kernel/linux/files<br />- I have added both patch and cfg file in bbappend using SRC_URI:append:rpi.<br /><br />I can observe the following:<br />- patch and cfg files are available in ./build/tmp/work/raspberrypi4_64-agl-linux/linux-raspberrypi/1_5.15.34+gitAUTOINC+e1b976ee4f_0086da6acd-r0<br />- new folder linux-raspberrypi4_64-preempt-rt-build is available inside the folder above. But the problem seems to be that CONFIG_PREEMPT_RT = y is not applied to the .config file. So it seems the preempt-rt kernel is built but without the full preempt-rt support.<br />- When I do bitbake linux-raspberrypi -c menuconfig I cannot select the full real time preempt kernel, only preemptible option available is --&gt; Preemptible Kernel (Low-Latency Desktop). Fully Preemptible Kernel (Real-Time) is not available.<br />- When I flash the image in the Rpi4 and run uname -r I see that the rt kernel has been built --&gt; 5.15.34.rt56.v8 <br />- but only with PREEMPT option but without RT when I do uname -v --&gt; #1 SMP PREEMPT Tue Aug 9 21:20:00 UTC 2022 (without RT).<br /><br />I have tried building linux-yocto-rt for qemu and there I see that CONFIG_PREEMP_RT = y is available in the .config file. Also if I open menuconfig I have the option to select in General setup --&gt; Preemption Model --&gt; Fully Preemptible Kernel (Real-Time) <br /><br />If you can provide any hints on what am I missing it would be highly appreciated.<br /><br />Best Regards,<br />Carles