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Date: Thu, 13 May 2004 12:11:41 -0700
From: Andrew Morton <akpm@osdl.org>
To: Paul Wagland <paul@wagland.net>
Cc: wli@holomorphy.com, greg@kroah.com, linux-kernel@vger.kernel.org,
       jgarzik@pobox.com, mingo@elte.hu, netdev@oss.sgi.com,
       davidel@xmailserver.org, Valdis.Kletnieks@vt.edu
Subject: Re: MSEC_TO_JIFFIES is messed up...
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Paul Wagland <paul@wagland.net> wrote:
>
> > -#if HZ == 1000
>  > -# define JIFFIES_TO_MSECS(x)	(x)
>  > -# define MSECS_TO_JIFFIES(x)	(x)
>  > -#elif HZ == 100
>  > -# define JIFFIES_TO_MSECS(x)	((x) * 10)
>  > -# define MSECS_TO_JIFFIES(x)	(((x) + 9) / 10)
>  > +#if HZ <= 1000 && !(1000 % HZ)
>  > +# define JIFFIES_TO_MSECS(j)	((1000/HZ)*(j))
>  > +# define MSECS_TO_JIFFIES(m)	(((m) + (1000/HZ) - 1)/(1000/HZ))
>  > +#elif HZ > 1000 && !(HZ % 1000)
>  > +# define JIFFIES_TO_MSECS(j)	(((j) + (HZ/1000) - 1)/(HZ/1000))
>  > +# define MSECS_TO_JIFFIES(m)	((m)*(HZ/1000))
>  >  #else
>  >  # define JIFFIES_TO_MSECS(x)	(((x) * 1000) / HZ)
>  >  # define MSECS_TO_JIFFIES(x)	(((x) * HZ + 999) / 1000)
> 
>  Also, can we keep the same parameter name across all of the macros?

Fair enough.

--- 25/include/linux/time.h~MSEC_TO_JIFFIES-fixups-tidy	2004-05-13 12:09:27.463273344 -0700
+++ 25-akpm/include/linux/time.h	2004-05-13 12:09:41.300169816 -0700
@@ -191,8 +191,8 @@ struct timezone {
 # define JIFFIES_TO_MSECS(j)	(((j) + (HZ/1000) - 1)/(HZ/1000))
 # define MSECS_TO_JIFFIES(m)	((m)*(HZ/1000))
 #else
-# define JIFFIES_TO_MSECS(x)	(((x) * 1000) / HZ)
-# define MSECS_TO_JIFFIES(x)	(((x) * HZ + 999) / 1000)
+# define JIFFIES_TO_MSECS(j)	(((j) * 1000) / HZ)
+# define MSECS_TO_JIFFIES(m)	(((m) * HZ + 999) / 1000)
 #endif
 
 /*

>  This changes behaviour when HZ==(z)000
> 
>  JIFFIES_TO_MSECS  goes from
>  ((x) * 1000) / (z)000  to (((x) + (z) - 1)/(z))
> 
>  i.e. for x=1, z=2 this goes from ((1)*1000)/2000)=0 to (((1)+(2)-1)/2)=1

hm, so you're saying that we now round 0.5 up to 1 rather than down to zero?

>  However, MSECS_TO_JIFFIES remains the same going from
>  (((x) * (z)000 + 999) / 1000) to ((x)*(z))
> 
>  I.e. they basically reduce down to the same thing (modulo overflows)
> 
>  All of the other permuations look correct to me...