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Re: mips32_flush_cache routine corrupts CP0_STATUS with gcc-2.96

To: Jon Burgess <Jon_Burgess@eur.3com.com>
Subject: Re: mips32_flush_cache routine corrupts CP0_STATUS with gcc-2.96
From: Jun Sun <jsun@mvista.com>
Date: Thu, 11 Jul 2002 09:53:58 -0700
Cc: Carsten Langgaard <carstenl@mips.com>, "Gleb O. Raiko" <raiko@niisi.msk.ru>, Ralf Baechle <ralf@oss.sgi.com>, linux-mips@oss.sgi.com
References: <80256BF3.00435388.00@notesmta.eur.3com.com>
Sender: owner-linux-mips@oss.sgi.com
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:0.9.2.1) Gecko/20010901
Jon Burgess wrote:


I don't wonder if other IDT CPUs also require this, including those that
conform MIPS32.
Basically, requirement of uncached run makes hadrware logic much simpler
and allows  to save silicon a bit.

That could be true, but then again I suggest making specific cache routines for

those

CPUs.
It would be a real performance hit for the rest of us, if we have to operate

from

uncached space.


I pulled together the relevant code to generate a module to test this problem
and it looks like the CPU always misses 1 instruction following the end of the
cache loop. If I add some nop's to change the alignment of the code it doesn't
seem to make any difference. The same thing seems to happen even if I change the
cache flush to a 'Hit_invalidate' of some completely different memory region.
One thing I thought might happen is the CPU ending the loop early as soon as it
invalidates the cacheline containing the current instructions, but this doesn't
seem to be the case, the 'end' address is always correct. Perhaps this really is
a hardware problem.

The test module below does a blast_icache then a few well known instructions and
signifies if anything has been missed. I typically get the following on our
board.
     Cacheop skipped 1 instructions, end = 0x80004000



Here is the test results from Malta 4kc

Cacheop skipped 0 instructions, end = 0x80004000

root@10.0.18.6:~# cat /proc/cpuinfo
processor               : 0
cpu model               : MIPS 4Kc V0.1
BogoMIPS                : 79.66
wait instruction        : no
microsecond timers      : yes
extra interrupt vector  : yes
hardware watchpoint     : yes
VCED exceptions         : not available
VCEI exceptions         : not available


Jun



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