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 ?

Thanks -- Daniel