Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932503AbaLBPMz (ORCPT ); Tue, 2 Dec 2014 10:12:55 -0500 Received: from smtp-out-006.synserver.de ([212.40.185.6]:1098 "EHLO smtp-out-006.synserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932177AbaLBPMx (ORCPT ); Tue, 2 Dec 2014 10:12:53 -0500 X-SynServer-TrustedSrc: 1 X-SynServer-AuthUser: lars@metafoo.de X-SynServer-PPID: 27141 Message-ID: <547DD6F0.6060006@metafoo.de> Date: Tue, 02 Dec 2014 16:12:48 +0100 From: Lars-Peter Clausen User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.2.0 MIME-Version: 1.0 To: Wolfram Sang , Harini Katakam CC: Mark Rutland , "grant.likely@linaro.org" , "robh+dt@kernel.org" , Pawel Moll , "ijc+devicetree@hellion.org.uk" , "galak@codeaurora.org" , "michal.simek@xilinx.com" , "soren.brinkmann@xilinx.com" , "linux-arm-kernel@lists.infradead.org" , "linux-i2c@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "devicetree@vger.kernel.org" , "vishnum@xilinx.com" Subject: Re: [PATCH 3/4] devicetree: bindings: Add defeature-repeated-start property for Cadence I2C References: <1417514749-24319-1-git-send-email-harinik@xilinx.com> <1417514749-24319-4-git-send-email-harinik@xilinx.com> <20141202111907.GC23671@leverpostej> <20141202125203.GA4072@katana> <20141202131611.GC4072@katana> <20141202141548.GA5883@katana> In-Reply-To: <20141202141548.GA5883@katana> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/02/2014 03:15 PM, Wolfram Sang wrote: >>> What do you do when disable repeated start? Sending STOP and START? If >>> so, this is really something different than repeated start. By using >>> I2C_FUNC_I2C a user expects repeated start, so if the HW does not >>> support it, we should say so and don't try to emulate it with something >>> different. >> >> Yes, we send stop. > > As said before, this is wrong. Another master could interfere between > the messages when using stop+start. This is no replacement for repeated > start. More importantly a lot of I2C slaves also reset their internal state machine on a stop. So e.g. if reading a register is implemented by doing start,write,repeated start,read,stop and you replace that with start,write,stop,start,read,stop you'll always read register zero instead of the register you wanted to read. - Lars -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/