From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754106Ab0IOOIz (ORCPT ); Wed, 15 Sep 2010 10:08:55 -0400 Received: from void.printf.net ([89.145.121.20]:54706 "EHLO void.printf.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753181Ab0IOOIy (ORCPT ); Wed, 15 Sep 2010 10:08:54 -0400 Date: Wed, 15 Sep 2010 15:08:40 +0100 From: Chris Ball To: Haavard Skinnemoen Cc: Ben Nizette , linux-mmc@vger.kernel.org, Sascha Hauer , Linux Kernel Mailing List , Pierre Ossman Subject: Re: [PATCH] mmc: Reduce fOD to 200 kHz if possible Message-ID: <20100915140840.GB1677@void.printf.net> References: <1284541140-7328-1-git-send-email-haavard.skinnemoen@atmel.com> <9348108E-1108-46FE-8437-6F1271025C61@niasdigital.com> <20100915125138.1f25e4b1@hskinnemoen-d830> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100915125138.1f25e4b1@hskinnemoen-d830> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Haavard, On Wed, Sep 15, 2010 at 12:51:38PM +0200, Haavard Skinnemoen wrote: > Thanks for the references. IMO Hein's patch is overkill. There is > absolutely no reason why 200 kHz should be a problem on any setup, and > I haven't found any indication in any discussions that it is. Pierre was worried that we'd ping-pong between having an f_min too high for some cards and too low for others, and be breaking a new set of cards each time we changed the value. He may have been being overly pessimistic, but I don't think being cautious comes with any significant downsides here. There's also a (small) performance concern when f_min gets low, though I agree that it wouldn't be a problem at 200 kHz. > The reason why fOD was set to 400 kHz in the first place is that some > controllers have a very low f_min so running the initialization at that > frequency causes problems. Which makes sense because the SD standard > clearly says that the clock can't be slower than 100 kHz. > > But I have never seen any reasons why we absolutely _have_ to run the > clock at the maximum frequency allowed by the spec. In fact, Sascha > Hauer, who was the one who changed the minimum clock frequency to 400 > kHz, said he would be fine with any frequency between 50 kHz and 400 > kHz [1]. I agree that we aren't trying to run at the maximum possible frequency, just trying to avoid having to choose a single perfect value without enough information on what it is. Thanks, -- Chris Ball One Laptop Per Child