Subject: cpu_relax in busy loops question

From: [email-masked] (Daniel Lezcano)
Date: Wed, 01 Feb 2012 12:34:29 +0100

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

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