From mboxrd@z Thu Jan 1 00:00:00 1970 From: "David S. Miller" Date: Thu, 25 Apr 2002 01:00:23 +0000 Subject: [Linux-ia64] Re: PCI DAC routines for SN Message-Id: List-Id: References: In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org From: Jesse Barnes Date: Wed, 24 Apr 2002 18:01:59 -0700 I fully understand and agree with that, and I don't propose adding the call just for the fun of it. It's just that most of the time, it's easier software-wise to use the 64 bit consistent mappings if your device can support it (e.g. generic ia64, ia64/sn). It isn't easier, in fact it's a lot harder. The DMA api is confusing enough, and you want to add yet another interface for device driver author Joe Blow to have to learn? I don't think the performance impact of those extra cycles would be measurable, but I'd be happy to see any numbers you might have to the contrary. You already showed me the numbers. Hardware folks don't make hardware so you can waste cycles, every one counts. I guess this is a fundamental disagreement in approach between you and me. So let's just agree to disagree. You have to show me a significant advantage to add a completely new API to the mix. It has to be important enough that it is worth making every driver author learn the new interface. This basically boils down to the fact that it must do one of two things: 1) It must make performance phenominally better. In the DAC consistent case, it actaully will decrease bus utilization slightly, we agree on this. 2) It must make some DMA performance optimization possible which currently is not possible at all. I do not believe this is the case. You've tried to argue that SAC consistent memory is precious for mappings, and I've tried to show you that the consistent part of the mappings used by devices won't even show up on the radar. We are at an impasse' and I really doubt this is going to change. All you can do by arguing further is just make me type the same things a few more times, and I'd like to avoid that, I type enough already. :-)