On Wed, 24 Feb 2021, Song Bao Hua (Barry Song) wrote:
Realtime requirement is definitely a true requirement on ARM Linux. I once talked/worked with some guys who were using ARM for realtime system. The feasible approaches include: 1. Dual OS(RTOS + Linux): e.g. QNX+Linux XENOMAI+Linux L4+Linux 2. preempt-rt Which is continuously maintained like: https://lore.kernel.org/lkml/20210218201041.65fknr7bdplwqbez@xxxxxxxxxxxxx/ 3. bootargs isolcpus= to isolate a cpu for a specific realtime task or interrupt https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux_for_real_time/7/html/tuning_guide/isolating_cpus_using_tuned-profiles-realtime 4. ARM FIQ which has separate fiq API, an example in fsl sound: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/sound/soc/fsl/imx-pcm-fiq.c 5. Let one core invisible to Linux Running non-os system and rtos on the core
Regarding Linux systems, it appears that approach 3 could theoretically achieve minimal interrupt latency for a given device without requiring any interrupt nesting. But the price is one CPU core which is not going to work on a uniprocessor system.
Honestly, I've never seen anyone who depends on irq priority to support realtime in ARM Linux though ARM's RTOS-es use it quite commonly.
Perhaps you don't work with uniprocessor ARM Linux systems?
Once preempt_rt is enabled, those who want a fast irq environment need a no_thread flag, or need to set its irq thread to higher sched_fifo/rr priority.
Thanks for the tip.
[...] Anyway, the debate is long enough, let's move to some more important things. I appreciate that you shared a lot of knowledge of m68k.
No problem.
Thanks Barry
