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Re: Strange, strange occurence

To: ralf@linux-mips.org
Subject: Re: Strange, strange occurence
From: "S C" <theansweriz42@hotmail.com>
Date: Mon, 12 Jul 2004 21:23:20 +0000
Cc: linux-mips@linux-mips.org
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
And thinking about it a little more, I might as well clarfy my understanding while we're on the topic.

Here's a code snippet from r4k_tlb_init() in arch/mips/mm/c-r4k.c

memcpy((void *)KSEG0, &except_vec0_r4000, 0x80);
flush_icache_range(KSEG0, KSEG0 + 0x80);

So my understanding is that the TLB exception handler is being copied to the right memory location, and just in case some other TLB exception handler (YAMON's presumably) is residing in I-cache, to flush ( hit invalidate) it. Is this correct?

Shouldn't there be code to writeback_invalidate the exception handler from the data cache ? Presumably the memcpy causes the TLB handler to lodge itself in the D cache till it is moved to RAM (either explicitly or when some other lines replace the cache lines where the handler is), right?

Thanks in advance for helping me understand the issue here.

Regards,
-Steve.





From: Ralf Baechle <ralf@linux-mips.org>
To: S C <theansweriz42@hotmail.com>
CC: linux-mips@linux-mips.org
Subject: Re: Strange, strange occurence
Date: Sat, 10 Jul 2004 12:04:12 +0200

On Fri, Jul 09, 2004 at 06:50:00PM +0000, S C wrote:

> Using MontaVista Linux 3.1 on a Toshiba RBTx4938 board. Using YAMON, when I > download the kernel via the debug ethernet port it runs fine. If I download
> the kernel via the Tx4938 inbuilt ethernet controller, it crashes!

If you're using a Montavista kernel you should report to Montavista.  We
don't have the source so any comment here is speculation.

> The crash is occuring inside the function r4k_flush_icache_range().
>
> I tried 'flush -i' and 'flush -d' on YAMON after the download but before
> the 'go', but that didn't help. I also tried completely disabling caches
> and loading/running uncached, but it gave the same error.
>
> Now, the final twist! Using an ICE, I set a breakpoint at the
> r4k_flush_icache_range function. Then I loaded the kernel as usual, ran it
> with the ICE, stepped through a few instructions inside the
> r4k_flush_icache_range function and then did a 'cont'. The kernel now
> booted fine!

As already pointed out by the other poster Niels Sterrenburg using a
debugger unavoidably changes the state of the system to be debugged.

For at least some of the TX49xx processors there is a problem under certain
circumstances if a flush of an I-cache line flushes that cache instruction
itself.  Make sure you're not getting hit by that one.

  Ralf

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