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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 C65DCC433E0 for ; Wed, 13 May 2020 22:47:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D3D4F20693 for ; Wed, 13 May 2020 22:47:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731499AbgEMWrk (ORCPT ); Wed, 13 May 2020 18:47:40 -0400 Received: from esa5.hgst.iphmx.com ([216.71.153.144]:27751 "EHLO esa5.hgst.iphmx.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726383AbgEMWrj (ORCPT ); Wed, 13 May 2020 18:47:39 -0400 IronPort-SDR: pBxB5QgUKyIGt1FMAYgYxIytp5jhmHmUHOvixm1z7sZn9lBguvN2LA6UUFkmw92UAFQubUqrOB IC6um+uZK7cwUIINxmbq3lqk9KAs0pAVefbBZrZD65W+UuFRweiMpL9kgWmEpzN0XhZyZaYAzL lceGdzNsxzvr/SpzSHuJKm+LoMQBHXdErF05ujTfY/vJ5XVx82HfbTW1Ele7IZlCtsV9NzRpY2 5iMrPIxyM+X6I1P+v7LFIcr3dXHneQyrwauzicVNU7B2PouOUphwnu8gTHKAtMaIhL9I8NsCL2 ZpA= X-IronPort-AV: E=Sophos;i="5.73,389,1583164800"; d="scan'208";a="137958329" Received: from uls-op-cesaip02.wdc.com (HELO uls-op-cesaep02.wdc.com) ([199.255.45.15]) by ob1.hgst.iphmx.com with ESMTP; 14 May 2020 06:47:35 +0800 IronPort-SDR: bZmDWVM3MQkpeg485VE5S5dRwyZgdpWY8aDW36iyesE6Q512JtR32eHk49TWmSs7z67a5FQkk5 UEvCzAodXf0n+YtakYKmrc8k3xXBgmFl4= Received: from uls-op-cesaip01.wdc.com ([10.248.3.36]) by uls-op-cesaep02.wdc.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 May 2020 15:37:16 -0700 IronPort-SDR: 2EySntN24zxi0edUzpgZhV+CpTP0Z3zImfdH57LckXl1Ua9E2Ptxzweq/LDVPFX2H3aQJp5EIM dlSmuNPUu7dQ== WDCIronportException: Internal Received: from unknown (HELO redsun52) ([10.149.66.28]) by uls-op-cesaip01.wdc.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 May 2020 15:47:34 -0700 X-Received: from SN6PR04MB3824.namprd04.prod.outlook.com (2603:10b6:a03:255::8) by BYAPR04MB5176.namprd04.prod.outlook.com with HTTPS via BY3PR10CA0003.NAMPRD10.PROD.OUTLOOK.COM; Wed, 13 May 2020 22:35:15 +0000 X-Received: from BN3PR05CA0003.namprd05.prod.outlook.com (2603:10b6:400::13) by SN6PR04MB3824.namprd04.prod.outlook.com (2603:10b6:805:4f::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2979.33; Wed, 13 May 2020 22:35:13 +0000 X-Received: from BN3NAM04FT044.eop-NAM04.prod.protection.outlook.com (2603:10b6:400:0:cafe::b2) by BN3PR05CA0003.outlook.office365.com (2603:10b6:400::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3000.12 via Frontend Transport; Wed, 13 May 2020 22:35:13 +0000 Authentication-Results: spf=none (sender IP is 216.71.154.42) smtp.mailfrom=linux-mips.org; wdc.com; dkim=none (message not signed) header.d=none;wdc.com; dmarc=none action=none header.from=linux-mips.org; X-Received: from esa4.hgst.iphmx.com (216.71.154.42) by BN3NAM04FT044.mail.protection.outlook.com (10.152.93.100) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3000.19 via Frontend Transport; Wed, 13 May 2020 22:35:12 +0000 Authentication-Results-Original: esa4.hgst.iphmx.com; dkim=none (message not signed) header.i=none; spf=None smtp.mailfrom=macro@linux-mips.org; spf=None smtp.helo=postmaster@cvs.linux-mips.org X-Ironport-Dmarc-Check-Result: validskip IronPort-SDR: 6ITrYayVJ17T5nzs64uONfWC8P6FWqxy4BUmjDsoNXwRfLas9emPEydOW8aPjY1dEbx9+vCZIL uZLsQNqlfPwt+LS2UhTSGx0wWVNhAFbGv9RDckSUi41kCg+vnv4TIUI+7fmf81+ggvLWKxxui1 tuWaZmu6soXjicD/nWzMEeKonuXlCfx7OAMq3s3ZsA6RM/nc64l8GUaQRS4hxnYCSPhQOAOIRI JYlFGdml10lEbWqV+X7U22aFdUweC4nm96frDaaCWgy6MnEYQfr61wRfOTUBtGuXfIDMWGBnNI zbvrYUF6/Hf/HeagLHCaNwOu IronPort-PHdr: =?us-ascii?q?9a23=3Axuei2hAX2/Ao/GJLPNa3UyQJP3N1i/DPJgcQr6?= =?us-ascii?q?AfoPdwSPT/ocbcNUDSrc9gkEXOFd2Cra4d1qyH7uu4CCQp2tWoiDg6aptCVh?= =?us-ascii?q?sI2409vjcLJ4q7M3D9N+PgdCcgHc5PBxdP9nC/NlVJSo6lPwWB6nK94iQPFR?= =?us-ascii?q?rhKAF7Ovr6GpLIj8Swyuu+54Dfbx9HiTagYb5+NhG7oAXeusUKhYZpN7o8xA?= =?us-ascii?q?bOrnZUYepd2HlmJUiUnxby58ew+IBs/iFNsP8/9MBOTLv3cb0gQbNXEDopPW?= =?us-ascii?q?Y15Nb2tRbYVguA+mEcUmQNnRVWBQXO8Qz3UY3wsiv+sep9xTWaMMjrRr06RT?= =?us-ascii?q?iu86FmQwLzhSwZKzA27n3Yis1ojKJavh2hoQB/w5XJa42RLfZyY7/Rcc8fSW?= =?us-ascii?q?dHQ81fVTFOApmkYoUBD+QPI/tWoIv+qVsAohSxGRKhBP/zxjNUmnP6wbE23/?= =?us-ascii?q?onHArb3AIgBdUOsHHModn7M6kdS/y6zKjWwjXBcvhZ2S3x45XUfBAmoPCNXa?= =?us-ascii?q?5/ccnQyUUhCgjIiVCQppb5MDOP0eQAqGyb7+tmVeKyhG8rsgRxojm3xsc3kI?= =?us-ascii?q?XJgZgVyl/d+Ch/3Y06KsG2RlRhbt64DJtfqTuaN41uT84iQGxluyU3xLIHtJ?= =?us-ascii?q?O/YiUH1YkrygDcZvKHbYWF4g7uWfqPLTtlmX5odq6yiwiv/EauxeDxV8e53E?= =?us-ascii?q?tKoCdGj9XBsG0G2R/L6sWfV/dw8USs1SyR2w3T9+1IO0Q5mKrBJ5I8zLM9lZ?= =?us-ascii?q?weulnZECDsgkX5lqqWe10k+ue27+TnZa3rqYOCN49vkA3+NqUumsqiAeU3Kw?= =?us-ascii?q?QOWmmb+fyn1LH5+k35WrpKguU3kqnfrp/aOdwWqrO2DgJWyIov9RayAyq83N?= =?us-ascii?q?kcnXQLNk9JdRyEgoTxPlHBOvH4DfOxg1S2lzdrwujLPrj7DZrRNHjDl6zsfb?= =?us-ascii?q?J8605a1QoywslT55dVCrEZO/L/QEjxtMbXDhMhKQy73/7nCMlh1oMZQW+PB7?= =?us-ascii?q?WWP7/IsV+P/e8uLfSDa5MPtDnhLPgl4q2msXhsvF4eYK2gxtMvYWy/Ffdvax?= =?us-ascii?q?GceXPwhdwCGE8QswY+RfCsg1qHB219fXG3CugZ7zo6BYDuMsGLfIe/kbeC3W?= =?us-ascii?q?3zSphfbXpBIlSNF2r4MoCFQ+xKbziddJwy2gcYXKSsHtdynSqlsxX3nv8+dr?= =?us-ascii?q?KOoH8xqI7j2d5p5ubaiRA183lOAt+A132WFj8szG4TTmNw3K9kvQp80leGwe?= =?us-ascii?q?5zhPkLXdBQ5vYcVAA8ONacyuFhENn9V0rHecvBU1epRNiqQHkxQ9s9zsVIYh?= =?us-ascii?q?N7HNOv3VjYxyT/JbgTmvSQAYAstKfV3nz/PcF4nnPP37Mmp10nRNZfcGinnr?= =?us-ascii?q?U58BLcVMbSi0vMsaGseOwH2TLVsmeOyW3bpEZDTAt5SrnIR1gUeFCQqdX/+1?= =?us-ascii?q?OHSr62EvIgKAQYk5y4J6BHa8PkgRB9fNm5aY6MRWW3li/wCA2BmvWMY5H2PW?= =?us-ascii?q?QF3CDNTkMDllJb+3HOLgU4Ciq762vDEDxjE07uaELw4Iwc4Du6SEEz1QSDc0?= =?us-ascii?q?xm0fK85BcUgfWWT/5b0KgDvW8trDB9HVD12NyeDtOLvQ1JeKRafMN45VZfzi?= =?us-ascii?q?TerQM8W/7oZ6Fui1gCchhm6kbn1hF5EIJFwoAhqHInyhY3KLrNiQ8RMWnFhN?= =?us-ascii?q?ahZ/uOcTmhmXLnI7Tb0VzfztuMr6oU4ah+q1r4oEeoDkon4zNs1NwGthnUrp?= =?us-ascii?q?jMEgcWVor8F0gt8B0v7bTTaTM6z4fV02B8dKy1qiSE3MgmTrhAqF7ob5JEPa?= =?us-ascii?q?WIGRWnWcgTDNKpAOgrnUW5KxcFJPsU87Q7dZDuZ76N36ilO/xllTStgDFc+I?= =?us-ascii?q?5K1USI5nMmGNTF1JsE3fyUmzC/eWyk3QWHtcb60cBJfjhOWGqw0jSiBZZRY7?= =?us-ascii?q?00doEOWy+oJIWsy9NyioSlUmNE9FOlG1IN2dO4HHjaJ1370wpK0E0LoHuh0S?= =?us-ascii?q?Kmxj1wmjsto+KRxivLi+jlcRMGPCZMSgwAxR/0JpOoitkBQEWyRwM3j12h5E?= =?us-ascii?q?Dn1+1epbxkaW7JTgYAfiT7KX1jTrrltreGZJ0qittguiFWXeKgJFGCH+eh81?= =?us-ascii?q?1AjnqlTjYYnWtrJHmwt570ngJ3kjeQN38v6nrWYt02xA/f4cyaTvlUjV9kDG?= =?us-ascii?q?F1jyfaAl+kMpyn59KRwt3Gu+ukUEqlV5tOYWzuzJub8iyh6ic5ZH/31+D2gd?= =?us-ascii?q?DhHQUghGXj0MJ2UCzTsBvmSoL1zOK+MOR6bgxtBUfnrc1gFcst9+l4zIFV0n?= =?us-ascii?q?8ciJKP+HMBmmqmKs1V7qX4aGtUFGw7huXN6Q2g43VNa3eAxob3THKYm5UzY9?= =?us-ascii?q?i8f3NT3yMl9IZNEqjGtOUZzxswmUKxqEfqWdY4njoZzqF2unUEmKcNuQww3m?= =?us-ascii?q?CYA6gJW05CMn60mg=3D=3D?= X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: =?us-ascii?q?A0FgAwC0dbxefYpf+5RmHQEBAQEJARI?= =?us-ascii?q?BBQUBQIFHgiWBawMEC41Sh2Alm2APATsEAQGERAKCKwY6EwIDAQEBAwIFAQE?= =?us-ascii?q?BBAEBAQIBAQEDAgEBAQECEAEBFjo0YFOBR4FBLAkGAwE7gjspAYNiAQEBAgE?= =?us-ascii?q?SKD8FCwsOOFcGNYVcBSCkJgGBKD4CIwE/AQyBBYl+gTSKcoE4jFYGgUE/gRE?= =?us-ascii?q?zA4Fcfj6KQgSOOoJfAYcomjwHA4JNjiCKFB2dSS2vSCGBWH0IgyVMAQIBAgE?= =?us-ascii?q?MAQICAQEBAgEBCAEBAQGfBkNnAgYIAQEDCY9EAQE?= X-IPAS-Result: =?us-ascii?q?A0FgAwC0dbxefYpf+5RmHQEBAQEJARIBBQUBQIFHgiWBa?= =?us-ascii?q?wMEC41Sh2Alm2APATsEAQGERAKCKwY6EwIDAQEBAwIFAQEBBAEBAQIBAQEDA?= =?us-ascii?q?gEBAQECEAEBFjo0YFOBR4FBLAkGAwE7gjspAYNiAQEBAgESKD8FCwsOOFcGN?= =?us-ascii?q?YVcBSCkJgGBKD4CIwE/AQyBBYl+gTSKcoE4jFYGgUE/gREzA4Fcfj6KQgSOO?= =?us-ascii?q?oJfAYcomjwHA4JNjiCKFB2dSS2vSCGBWH0IgyVMAQIBAgEMAQICAQEBAgEBC?= =?us-ascii?q?AEBAQGfBkNnAgYIAQEDCY9EAQE?= X-IronPort-AV: E=Sophos;i="5.73,389,1583164800"; d="scan'208";a="137621459" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-Received: from eddie.linux-mips.org (HELO cvs.linux-mips.org) ([148.251.95.138]) by esa4.hgst.iphmx.com with ESMTP; 14 May 2020 06:35:02 +0800 X-Received: (from localhost user: 'macro', uid#1010) by eddie.linux-mips.org with ESMTP id S23994956AbgEMWezdCe7c (ORCPT ); Thu, 14 May 2020 00:34:55 +0200 Date: Mon, 11 May 2020 15:58:24 +0100 (BST) From: "Maciej W. Rozycki" To: Mikulas Patocka cc: Arnd Bergmann , Richard Henderson , Ivan Kokshaysky , Matt Turner , Greg Kroah-Hartman , alpha , linux-serial@vger.kernel.org, linux-rtc@vger.kernel.org Subject: Re: [PATCH 1/2 v3] alpha: add a delay to inb_p, inb_w and inb_l In-Reply-To: Message-ID: References: Content-Type: text/plain; charset=US-ASCII X-ReSent-Date: Wed, 13 May 2020 23:34:55 +0100 (BST) X-Resent-Sender: "Maciej W. Rozycki" X-ReSent-From: "Maciej W. Rozycki" X-ReSent-To: "Maciej W. Rozycki" X-ReSent-Subject: Re: [PATCH 1/2 v3] alpha: add a delay to inb_p, inb_w and inb_l X-ReSent-Message-ID: X-Return-Path: macro@linux-mips.org X-MS-Exchange-Organization-ExpirationStartTime: 13 May 2020 22:35:12.6544 (UTC) X-MS-Exchange-Organization-ExpirationStartTimeReason: OriginalSubmit X-MS-Exchange-Organization-ExpirationInterval: 1:00:00:00.0000000 X-MS-Exchange-Organization-ExpirationIntervalReason: OriginalSubmit X-MS-Exchange-Organization-Network-Message-Id: 92a7625c-0900-49a9-2bf7-08d7f78de68b X-EOPAttributedMessage: 0 X-MS-Exchange-Organization-MessageDirectionality: Originating X-MS-Office365-Filtering-HT: Tenant X-Forefront-Antispam-Report: CIP:216.71.154.42;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:esa4.hgst.iphmx.com;PTR:esa4.hgst.iphmx.com;CAT:NONE;SFTY:;SFS:(4636009)(33430700002)(86362001)(82310400002)(7636003)(33440700001)(6666004)(7596003)(6862004)(107886003)(4326008)(5660300002)(8676002)(356005)(34756004)(1096003)(26005)(8936002)(54906003)(42186006)(336012)(1736002);DIR:INB;SFP:; X-MS-PublicTrafficType: Email X-MS-Exchange-Organization-AuthSource: BN3NAM04FT044.eop-NAM04.prod.protection.outlook.com X-MS-Exchange-Organization-AuthAs: Anonymous X-OriginatorOrg: sharedspace.onmicrosoft.com X-MS-Office365-Filtering-Correlation-Id: 92a7625c-0900-49a9-2bf7-08d7f78de68b X-MS-TrafficTypeDiagnostic: SN6PR04MB3824: X-MS-Oob-TLC-OOBClassifiers: OLM:9508; X-MS-Exchange-Organization-SCL: 1 X-Microsoft-Antispam: BCL:0; X-MS-Exchange-CrossTenant-OriginalArrivalTime: 13 May 2020 22:35:12.4095 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 92a7625c-0900-49a9-2bf7-08d7f78de68b X-MS-Exchange-CrossTenant-Id: b61c8803-16f3-4c35-9b17-6f65f441df86 X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=b61c8803-16f3-4c35-9b17-6f65f441df86;Ip=[216.71.154.42];Helo=[esa4.hgst.iphmx.com] X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN6PR04MB3824 X-MS-Exchange-Transport-EndToEndLatency: 00:00:02.7540472 X-MS-Exchange-Processed-By-BccFoldering: 15.20.2979.027 X-Microsoft-Antispam-Mailbox-Delivery: ucf:1;jmr:0;auth:0;dest:C;OFR:CustomRules;ENG:(20160514016)(750127)(520002050)(944506383)(944626516); X-Microsoft-Antispam-Message-Info: =?us-ascii?Q?tS5cGOkS7BKblWxrMJDESsfhRaK/n+NgCOGz84uy0anebKmprI4/bTbGYEO+?= =?us-ascii?Q?yethRZAWbWDD6mD303DiWnk1Gv2UUB6Z8VAW+VXS+DJwXac6TxJ/f3dDZuzP?= =?us-ascii?Q?yzqVmfgEJPUT5uxYlTWqpgVj+Rrt070k47EB4Ak+jst5F6QxLTwrTNvHhY+h?= =?us-ascii?Q?CTLfURpD6M6p+2msdce1ZX5SgfmKSEAPff18CPdns1SJ8wTnptw6tP/MENhC?= =?us-ascii?Q?leynS+8jQk3QnmWvdZE3JolVfJnX46EnI5MaRjZRdPRqPN68TfVl7TAc9kTZ?= =?us-ascii?Q?xiyvhl6RwefMYJOb33JXBxJnMRDI7jzesBHbAOLCACINJICrO/CMJ/qRAEX7?= =?us-ascii?Q?XMYl/7kzZikbwG6SEfUb36wDWJ92dCgac34RvcI+3HDllQY8zv3Qi92OWYb2?= =?us-ascii?Q?P82e1RPOBXlToyJaDTRnceOsaoPI6VPpAYzRFESqaTVCoKZuSJWx7YXzqA4Y?= =?us-ascii?Q?R54RFlQ4nGs/lnJsowEqvTvO0sD3XG4WCspGCQgVWmhtXfnS5zluChKZejsN?= =?us-ascii?Q?n8alsyQ515PyW2qk5gfizCnwXKDVWkcMAi/DQJn7ztW43gQeqQ1meVqQAU5+?= =?us-ascii?Q?CE3f9vvH7keg7mzW2aSzHZFzmqX57KBkcGk4pYN8GtOd1FISXBOYAIavaiGt?= =?us-ascii?Q?Q/CJwtYYUCGD2wqiPrT8LdnaPy5PrGe6KnP7UYpE2G9hWhxeLRg3mgYSXogf?= =?us-ascii?Q?avsovxhOmguSus40V08gDkjlVxfwvJx9seCGPLO+LdpYvf3pHgQWt700q61B?= =?us-ascii?Q?Ug9yDcU6J3xcg5BLmSQtdJyrYScrNlp/tNiSfyVlo/a6NS+H5JJYU8V6iGSV?= =?us-ascii?Q?Jr225pIiQuP47uGJKkBkg7KmXBKdVeOaE99zZJ2/oBjliqs5IpaaePkR1HtF?= =?us-ascii?Q?QIiVdBPBTiJE8ubwo9UZo4p/TDO156MaM7w4wxWZa1icAn2Ti6e/BKbf7ln0?= =?us-ascii?Q?6qkaf/N3KKpVPOh/Gbf1cN/kR4Rvy8j37wCIyBZXQdnuShuBDgXDBBWVBp33?= =?us-ascii?Q?CZk4i5UKpCZh/9zr+N7rIjxMo4RMqz/rujXyuE8FtSQGW0Zue/lBDoCGaEtV?= =?us-ascii?Q?15FY4F22v5AzZHSavUzbXZHPEbUl3ReWTZ5prfqsZwwmvwjITp9xS++KPRaN?= =?us-ascii?Q?pn07KzfxzrIA4Vwh4bn7dwnH5S3dmqL5J/hHakUXu84WD1ialoXZieepnVDR?= =?us-ascii?Q?7IznM1HINHygJnpcMKnhqEJbF2V3Gl4yz2iW7IrFm3O0gX6eEF5zwa5n9niv?= =?us-ascii?Q?qFmBPBUInZ2Ua3vd9MppfX28OzFR0k5q3T7egrl7DqQ212NSb5vwhBOCYCEA?= =?us-ascii?Q?oDFvVekcGK0pJtDIkGdNSn6iWz3qY+j3R4889JtSKeSndjv46nbVo97HjQsY?= =?us-ascii?Q?UqUXjZxirAWbrQStzfGBe9Kuv7m0?= MIME-Version: 1.0 Sender: linux-serial-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org On Sun, 10 May 2020, Mikulas Patocka wrote: > > > That's what we do on some other architectures to emulate the non-posted > > > behavior of out[bwl], as required by PCI. I can't think of any reasons to > > > have a barrier before in[bwl], or after write[bwl], but we generally want > > > one after out[bwl] > > > > Alpha is weakly ordered, also WRT MMIO. The details are a bit obscure > > (and were discussed before in a previous iteration of these patches), but > > my understanding is multiple writes can be merged and writes can be > > reordered WRT reads, even on UP. It's generally better for performance to > > We discussed it some times ago, and the conclusion was that reads and > writes to the same device are not reordered on Alpha. Reads and writes to > different devices or to memory may be reordered. Except that "device" in this context is a particular MMIO location; most peripherals span multiple locations, including the RTC and the UART in the Avanti line in particular. > In these problematic cases, we only access serial port or real time clock > using a few ports (and these devices don't have DMA, so there's not any > interaction with memory) - so I conclude that it is timing problem and not > I/O reordering problem. Individual PCI port locations correspond to different MMIO locations, so yes, accesses to these can be reordered (merging won't happen due to the use of the sparse address space). As I noted using a small program to verify actual behaviour ought to reveal what the problem really is. And /dev/mem can be mmapped for PCI port I/O access on Alpha (some X-servers do this), so it can be done even in the userland with a running system. And if timing is indeed the culprit, then I think it will be best fixed in the 82378IB southbridge, i.e.[1]: "The I/O recovery mechanism in the SIO is used to add additional recovery delay between PCI originated 8-bit and 16-bit I/O cycles to the ISA Bus. The SIO automatically forces a minimum delay of four SYSCLKs between back-to-back 8 and 16 bit I/O cycles to the ISA Bus. The delay is measured from the rising edge of the I/O command (IOR# or IOW#) to the falling edge of the next BALE. If a delay of greater than four SYSCLKs is required, the ISA I/O Recovery Time Register can be programmed to increase the delay in increments of SYSCLKs. Note that no additional delay is inserted for back-to-back I/O "sub cycles" generated as a result of byte assembly or disassembly. This register defaults to 8 and 16-bit recovery enabled with two clocks added to the standard I/O recovery." where it won't be causing unnecessary overhead for native PCI devices or indeed excessive one for ISA devices. It might be interesting to note that later SIO versions like the 82378ZB increased the minimum to five SYSCLKs, so maybe a missing SYSCLK (that can still be inserted by suitably programming the ICRT) is the source of the problem? References: [1] "82378IB System I/O (SIO)", April 1993, Intel Corporation, Order Number: 290473-002, Section 4.1.17 "ICRT -- ISA Controller Recovery Timer Register" Maciej