From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gateway-1237.mvista.com (gateway-1237.mvista.com [63.81.120.158]) by ozlabs.org (Postfix) with ESMTP id 5C8C467A44 for ; Fri, 28 Apr 2006 08:07:29 +1000 (EST) Date: Thu, 27 Apr 2006 15:08:56 -0700 From: "Mark A. Greer" To: Benjamin Herrenschmidt Subject: Re: Not coherent cache DMA for G3/G4 CPUs: clarification needed Message-ID: <20060427220856.GA19401@mag.az.mvista.com> References: <20060420210201.GA25755@gate.ebshome.net> <10026.1145567455@www088.gmx.net> <20060420215514.GE25755@gate.ebshome.net> <1145594285.28014.12.camel@localhost.localdomain> <20060427213134.GA12572@mag.az.mvista.com> <1146174809.30710.14.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1146174809.30710.14.camel@localhost.localdomain> Cc: linuxppc-dev@ozlabs.org, debian-powerpc@lists.debian.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Fri, Apr 28, 2006 at 07:53:29AM +1000, Benjamin Herrenschmidt wrote: > > > There are many mv64x60 based platforms working just fine today with > > CONFIG_NOT_COHERENT_CACHE defined. The reason for turning coherency off > > is that there is a bug in the bridge requiring a hardware workaround. > > Unfortunately, not all of the hardware vendors have implemented that > > workaround and I know of one that considers it infeasible and will > > not implement it. > > Define "working fine" ... With the current implementation, and according > to the spec, it will randomly crap out or checkstop due to the same page > beging accessed via the NCU and being in the L2 unless you disabled > speculative loads and made sure it can't prefetch accross page > boundaries maybe ? Or set the G bit all over the BAT mapping (ouch !). "working fine" == running in a production environment for weeks/months without crapping out. > > If that hardware workaround is not implemented, the options are: > > a) 100% chance of a system hang with coherency on > > or > > b) < 0.0..1% chance of a system hang with coherency off (at least in my > > experience to far). > > > > The choice is simple. > > I disagree. A solution that is known to have a hole in it is no good > even if you haven't managed to trigger it so far. Now it's Gerhard's > choice. TBH, I haven't really looked at what Gerhard is doing yet so I can't comment. No matter what, though, its certainly his choice. Mark