cpu_relax in busy loops question

Hi Linus,

While looking at the drivers/mfd/db8500-prcmu.c file I noticed:

?595 ? ? ? ? /* Grab the HW semaphore. */
?596 ? ? ? ? while ((readl(PRCM_SEM) & PRCM_SEM_PRCM_SEM) != 0) ?597 ? ? ? ? ? ? ? ? cpu_relax();

I was wondering why is cpu_relax needed here as readl does a memory barrier ? I thought the cpu_relax function was related to the x86 to consume less power with an optimization of the "rep nop" loops or/and do a memory barrier.

Should a busy-loop be always with cpu_relax ?

Quoted 16 lines · Daniel Lezcano

Hi Linus,

While looking at the drivers/mfd/db8500-prcmu.c file I noticed:

  595         /* Grab the HW semaphore. */
  596         while ((readl(PRCM_SEM)&  PRCM_SEM_PRCM_SEM) != 0)
  597                 cpu_relax();

I was wondering why is cpu_relax needed here as readl does a memory barrier ? I thought the cpu_relax function was related to the x86 to consume less power with an optimization of the "rep nop" loops or/and do a memory barrier.

Quoted 1 line

Should a busy-loop be always with cpu_relax ?

Last time we discussed it ar LAKML I think we came to the conclusion that while cpu_relax() has very little semantic meaning on ARM, it is still nice to have from a readability and maintenance point of view, since it is pretty clear what we're trying to do.