Subject: Re: [PATCH] fix gcc4 warning in asm-ppc/time.h

From: Gabriel Paubert <[email-masked]>
Date: Thu, 4 Aug 2005 07:54:59 +0200

On Wed, Aug 03, 2005 at 10:15:17PM -0700, Andrew Morton wrote:
> Christoph Hellwig <[email-masked]> wrote:
> >
> > A function must not return a const value, instead use
> > __attribute_const__.
> > 
> > 
> > Index: linux-2.6/include/asm/time.h
> > ===================================================================
> > --- linux-2.6.orig/include/asm/time.h	2005-04-30 10:17:19.000000000 +0200
> > +++ linux-2.6/include/asm/time.h	2005-08-01 11:57:12.000000000 +0200
> > @@ -10,6 +10,7 @@
> >  #define __ASM_TIME_H__
> >  
> >  #include <linux/config.h>
> > +#include <linux/compiler.h>
> >  #include <linux/types.h>
> >  #include <linux/rtc.h>
> >  #include <linux/threads.h>
> > @@ -58,7 +59,7 @@
> >  /* Accessor functions for the timebase (RTC on 601) registers. */
> >  /* If one day CONFIG_POWER is added just define __USE_RTC as 1 */
> >  #ifdef CONFIG_6xx
> > -extern __inline__ int const __USE_RTC(void) {
> > +extern __inline__ int __attribute_const__ __USE_RTC(void) {
> >  	return (mfspr(SPRN_PVR)>>16) == 1;
> >  }
> >  #else
> 
> Do we really want to do this?  The above implies that mfspr(SPRN_PVR) will
> return the same value across the entire uptime of the kernel.  Is that
> true?

Yes, it is a read-only register. PVR means processor version register
and is linked to the revision of the silicon. Besides that it is
only used in this context to check for processors of the 601 family
(the very first PPC processors). Nobody expects them to be hotpluggable,
nor even used in SMP systems for the matter.

> 
> Why is this function paying with const anyway?

I believe that it was originally to tell the compiler to be
able to cache it across several tests (a good compiler
on PPC would even cache the result of the test in a condition
register to allow branch prediction to work optimally).

Otherwise the compiler would never cache it since mfspr
is a volatile asm.

However this was written in pre gcc2.95 days so my 
memory may be failing.

	Gabriel