From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with ESMTP id A22AF6B009C for ; Tue, 19 Oct 2010 16:48:26 -0400 (EDT) Message-ID: <56111.84.105.60.153.1287521237.squirrel@gate.crashing.org> In-Reply-To: <20101019181021.22456.qmail@kosh.dhis.org> References: <20101019181021.22456.qmail@kosh.dhis.org> Date: Tue, 19 Oct 2010 22:47:17 +0200 (CEST) Subject: Re: PROBLEM: memory corrupting bug, bisected to 6dda9d55 From: "Segher Boessenkool" MIME-Version: 1.0 Content-Type: text/plain;charset=iso-8859-1 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org To: pacman@kosh.dhis.org Cc: Benjamin Herrenschmidt , Mel Gorman , linux-mm@kvack.org, Andrew Morton , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org List-ID: > 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org