linux-kernel - Re: PROBLEM: memory corrupting bug, bisected to 6dda9d55

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Message-ID: <56111.84.105.60.153.1287521237.squirrel@gate.crashing.org>
Date:	Tue, 19 Oct 2010 22:47:17 +0200 (CEST)
From:	"Segher Boessenkool" <segher@...nel.crashing.org>
To:	pacman@...h.dhis.org
Cc:	"Benjamin Herrenschmidt" <benh@...nel.crashing.org>,
	"Mel Gorman" <mel@....ul.ie>, linux-mm@...ck.org,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: memory corrupting bug, bisected to 6dda9d55

> I made a new discovery.

And this nails it :-)

> So then I ran
>   dd if=/dev/mem bs=4 count=1 skip=$((0xfc5c080/4)) | od -t x4
> a few times very fast, plucking the first affected word directly out of
> memory by its physical address. The result:
>
> The low 16 bits are always zero as before. The high 16 bits are a counter,
> being incremented at about 1000Hz (as close as I could measure with a
> crude
> shell script. 1024Hz would also be within the margin of error). And it's
> little-endian.

> So what type of driver, firmware, or hardware bug puts a 16-bit 1000Hz
> timer
> in memory, and does it in little-endian instead of the CPU's native byte
> order? And why does it stop doing it some time during the early init
> scripts,
> shortly after the root filesystem fsck?

It looks like it is the frame counter in an USB OHCI HCCA.
16-bit, 1kHz update, offset x'80 in a page.

So either the kernel forgot to call quiesce on it, or the firmware
doesn't implement that, or the firmware messed up some other way.


Segher

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