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Date: Fri, 26 Mar 2010 10:01:21 +0100 (CET)
Subject: Re: Continual reading from the PowerPc time base register is not   
	stable
From: "Segher Boessenkool" <segher@kernel.crashing.org>
To: "Csdncannon" <csdncannon@gmail.com>,
	"Segher Boessenkool" <segher@kernel.crashing.org>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	linuxppc-dev@ozlabs.org, "Arnd Bergmann" <arnd@arndb.de>
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> Yes indeed.  Could you post the relevant piece if disassembly from
> your original binary (the one that has the problem)?  Or send me the
> binary (not to the mailing list), I'll do it then.

Ah scratch that.  I compiled your original code (after fixing the
compile errors -- there is no such type as "bool" in C).

The problem is that  (upper << 32) | lower  thing.  "upper" is a 32-bit
type, so shifting it by 32 or more bits is undefined.  GCC compiles this
to (shortened):

0: mftbu 9 ; mftbl 11 ; mftbu 0 ; cmpw 0,9 ; bne 0b  # so far so good
   slwi 0,0,0 ; or 4,0,11 ; li 3,0 ; blr

so it shifts by 0, i.e. it does  upper | lower .

Case closed, no hardware problem :-)


Segher