From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2CB75C43331 for ; Fri, 6 Sep 2019 10:42:52 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id F3660207FC for ; Fri, 6 Sep 2019 10:42:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="Svd64WX6" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F3660207FC Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=bootlin.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=SBvLNl2JCNstO0pIh3xvzYSkYVWEq+SZ+WOwUAXtwCw=; b=Svd64WX6X0bBTP Zf5/dcHPtMpWH2+v62BOAlxyUb0WpNeQaJszPI5Wv6jx9gor/n/BK0TaC4V9ssvGIE5pqXq2qjnJ8 uvg31fxH5J1FShv5eopXXL/HBhBT0b8wfYRbRB6koVnA/BtnChDXWqz8bhrpEJS7FCeLqZzG5TbQS izYNu2NHCI5I4nrqTNE7toqbxN8LFnKULtJtofFA7imETdkwmdNaiibfV+fm9YC8kYgnUsRU/Y7F9 NyEsmvPm3dkF2nHnfhRthiXZh2YPRxwBNVtoFZnyzdNktIBYcpVI6J9+42OHZILwqQl6GJx4RtNs0 K2Cyd24Ovf8lrwbFueVg==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1i6BhW-0000Nr-6A; Fri, 06 Sep 2019 10:42:46 +0000 Received: from relay1-d.mail.gandi.net ([217.70.183.193]) by bombadil.infradead.org with esmtps (Exim 4.92 #3 (Red Hat Linux)) id 1i6BhR-0000NA-NF for linux-arm-kernel@lists.infradead.org; Fri, 06 Sep 2019 10:42:44 +0000 X-Originating-IP: 86.207.98.53 Received: from localhost (aclermont-ferrand-651-1-259-53.w86-207.abo.wanadoo.fr [86.207.98.53]) (Authenticated sender: alexandre.belloni@bootlin.com) by relay1-d.mail.gandi.net (Postfix) with ESMTPSA id BE71F240004; Fri, 6 Sep 2019 10:42:30 +0000 (UTC) Date: Fri, 6 Sep 2019 12:42:24 +0200 From: Alexandre Belloni To: David Laight Subject: Re: [PATCH v2] pinctrl: at91-pio4: implement .get_multiple and .set_multiple Message-ID: <20190906104224.GG21254@piout.net> References: <20190905144849.24882-1-alexandre.belloni@bootlin.com> <2261eadf98584d13a490f2abd8777d4a@AcuMS.aculab.com> <20190906091212.GF21254@piout.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190906_034241_912723_E3844AFA X-CRM114-Status: GOOD ( 20.16 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Linus Walleij , "linux-kernel@vger.kernel.org" , "linux-gpio@vger.kernel.org" , Ludovic Desroches , "Claudiu.Beznea@microchip.com" , "linux-arm-kernel@lists.infradead.org" Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 06/09/2019 09:46:02+0000, David Laight wrote: > From: Alexandre Belloni > > Sent: 06 September 2019 10:12 > > On 06/09/2019 09:05:36+0000, David Laight wrote: > > > From: Alexandre Belloni > > > > Implement .get_multiple and .set_multiple to allow reading or setting > > > > multiple pins simultaneously. Pins in the same bank will all be switched at > > > > the same time, improving synchronization and performances. > > > > > > Actually it won't 'improve synchronisation', instead it will lead to > > > random synchronisation errors and potential metastability if one > > > pin is used as a clock and another as data, or if the code is reading > > > a free-flowing counter. > > > > > > > It does improve gpio switching synchronisation when they are in the same > > bank as it will remove the 250ns delay. Of course, if you need this > > delay between clk and data, then the consumer driver should ensure the > > delay is present. > > With multiple requests the output pin changes will always be in the > same order and will be separated by (say) 250ns. > This is a guaranteed synchronisation. > > If you change multiple pins with the same 'iowrite()' then the pins > will change at approximately the same time. > But the actual order will depend on internal device delays (which > may depend on the actual silicon and temperature). > You then have to take account of varying track lengths and the > target devices input stage properties before knowing which change > arrives first. > The delays might be sub-nanosecond, but they matter if you are > talking about synchronisation. > And my point is that this means that your gpio consumer driver is buggy if it doesn't do multiple requests if it requires a delay between two pin changes. > IIRC both SMBus and I2C now quote 0ns setup time. > Changing both clock and data with the same IOW isn't enough to > guarantee this. > (In practise the I2C setup time required by a device is probably > slightly negative (In order to support 0ns inputs) so a very small > -ve setup will (mostly) work.) I'm not sure what is your point exactly as this patch doesn't break any existing use cases. -- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel