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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Gabriel Paubert <paubert@iram.es>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
        Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH] (comments requested) adding finer-grained timing to PPC
 add_timer_randomness()
Date: Thu, 23 Aug 2001 15:11:04 +0200
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>> In the 2.4 tree we have code that works out a cpu features word from
>> the PVR value.  The cpu features word has bits for things like does
>> the cpu have the TB register, does the MMU use a hash table, does the
>> cpu have separate I and D caches, etc.
>
>Reading the PVR is probably faster in this case, since you avoid a
>potential cache miss. As I said in an earlier message the __USE_RTC macro
>should be made dependent on whether the kernel supports 601 or not.

The CPU features will nop-out sections of code that don't apply for
a given CPU if you use the assembly macros. If you nop'out a couple
of instructions, you win over reading the PVR, masking out bits,
comparing, and doing a conditional branch.

Ben.