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=-5.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 580DDC4332B for ; Tue, 26 Jan 2021 13:53:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 354D42255F for ; Tue, 26 Jan 2021 13:53:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2405270AbhAZNxp (ORCPT ); Tue, 26 Jan 2021 08:53:45 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36636 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2405322AbhAZNuW (ORCPT ); Tue, 26 Jan 2021 08:50:22 -0500 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [IPv6:2001:4d48:ad52:32c8:5054:ff:fe00:142]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2121DC0611C2; Tue, 26 Jan 2021 05:49:42 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=J5ZqF3ROoAZvc79uZMB4KxJkDo/pwToOQ5rY1t//3ZM=; b=KZChSPk+5hypw/XJh9A75gVhv JDD6HhjfEwWfJ+zoV2ECb2EymAIkJ9h4MarIq8lf/u5o8Bj6n7pJ6LKaX/jQMYmC7WzRyf65ZwxTv wzs3KBDTDsE54WsaJ14tbbJ65G2Leh0i4DGXXY0MFGlpWyAhgH32owmUzfbxEfiYg0awvCPhbv1cN uVCEGtVHmJbb9Qi5kDbtzTJCNLbQLg8TLwejyJbTscSQ61H5nXLy95K6qMPzoE+tnNioCmB3oo9/K whzN7Fkio789ZBSbVnVmg/cZd06vt+AwujSxEGBxpzy3mN0evJNDlxIw++8JaYfjW9YhcIWwI+kws XNsOU3yvQ==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:52974) by pandora.armlinux.org.uk with esmtpsa (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1l4Oix-0004Ud-Ly; Tue, 26 Jan 2021 13:49:39 +0000 Received: from linux by shell.armlinux.org.uk with local (Exim 4.92) (envelope-from ) id 1l4Oiw-0003qo-3H; Tue, 26 Jan 2021 13:49:38 +0000 Date: Tue, 26 Jan 2021 13:49:38 +0000 From: Russell King - ARM Linux admin To: Andrew Lunn Cc: Mike Looijmans , netdev@vger.kernel.org, "David S. Miller" , Heiner Kallweit , Jakub Kicinski , linux-kernel@vger.kernel.org Subject: Re: [PATCH] net: mdiobus: Prevent spike on MDIO bus reset signal Message-ID: <20210126134937.GI1551@shell.armlinux.org.uk> References: <20210126073337.20393-1-mike.looijmans@topic.nl> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: Russell King - ARM Linux admin Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 26, 2021 at 02:14:40PM +0100, Andrew Lunn wrote: > On Tue, Jan 26, 2021 at 08:33:37AM +0100, Mike Looijmans wrote: > > The mdio_bus reset code first de-asserted the reset by allocating with > > GPIOD_OUT_LOW, then asserted and de-asserted again. In other words, if > > the reset signal defaulted to asserted, there'd be a short "spike" > > before the reset. > > > > Instead, directly assert the reset signal using GPIOD_OUT_HIGH, this > > removes the spike and also removes a line of code since the signal > > is already high. > > Hi Mike > > This however appears to remove the reset pulse, if the reset line was > already low to start with. Notice you left > > fsleep(bus->reset_delay_us); > > without any action before it? What are we now waiting for? Most data > sheets talk of a reset pulse. Take the reset line high, wait for some > time, take the reset low, wait for some time, and then start talking > to the PHY. I think with this patch, we have lost the guarantee of a > low to high transition. > > Is this spike, followed by a pulse actually causing you problems? If > so, i would actually suggest adding another delay, to stretch the > spike. We have no control over the initial state of the reset line, it > is how the bootloader left it, we have to handle both states. Andrew, I don't get what you're saying. Here is what happens depending on the pre-existing state of the reset signal: Reset (previously asserted): ~~~|_|~~~~|_______ Reset (previously deasserted): _____|~~~~|_______ ^ ^ ^ A B C At point A, the low going transition is because the reset line is requested using GPIOD_OUT_LOW. If the line is successfully requested, the first thing we do is set it high _without_ any delay. This is point B. So, a glitch occurs between A and B. We then fsleep() and finally set the GPIO low at point C. Requesting the line using GPIOD_OUT_HIGH eliminates the A and B transitions. Instead we get: Reset (previously asserted) : ~~~~~~~~~~|______ Reset (previously deasserted): ____|~~~~~|______ ^ ^ A C Where A and C are the points described above in the code. Point B has been eliminated. Therefore, to me the patch looks entirely reasonable and correct. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!