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=-3.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 2D864C433E1 for ; Tue, 18 Aug 2020 08:37:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DF7F72067C for ; Tue, 18 Aug 2020 08:37:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="sChHbAxd" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726519AbgHRIhd (ORCPT ); Tue, 18 Aug 2020 04:37:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52666 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726336AbgHRIhd (ORCPT ); Tue, 18 Aug 2020 04:37:33 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7D345C061389 for ; Tue, 18 Aug 2020 01:37:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=z+RycezjV2WVCujs2S8itL6PhqDnVUaq7qp6xVdLAo4=; b=sChHbAxde70X2xuGN/HW+28KHz HI6YetzZL7SCINwQdD5+hHwkS6qWOP4voGDc8BYsONfhYhnkoQjB8JI7DJJ9WQFCYfhWEbCSaLAB/ omXVKQnx1SSatRgDqyYjcqU0OpvgaD2swot7bjhdFztT6gzNDrjEARqivLy9gDHU+rpvRWfZnA8TU EXesWd61vo9k2yjd6ObDCJVrP1fhwDN5MYWt/fXfGTXqq8r+QJLiuZzA0MqYq9MFwmi9U6wOVFmOZ 0288DMbrkGzzFf92AljB7ECFX3C4q0e5AVJDUpv6hEAlOSfHWemTogfSP8tf0Sw1RswoQai5g6knN CYOJR6QQ==; Received: from hch by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1k7x7Q-0002kj-2n; Tue, 18 Aug 2020 08:37:20 +0000 Date: Tue, 18 Aug 2020 09:37:20 +0100 From: Christoph Hellwig To: Will Deacon Cc: Cho KyongHo , joro@8bytes.org, catalin.marinas@arm.com, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, m.szyprowski@samsung.com, robin.murphy@arm.com, janghyuck.kim@samsung.com, hyesoo.yu@samsung.com Subject: Re: [PATCH 1/2] dma-mapping: introduce relaxed version of dma sync Message-ID: <20200818083720.GA9451@infradead.org> References: <1597736591-20457-1-git-send-email-pullip.cho@samsung.com> <20200818082852.GA15145@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200818082852.GA15145@willie-the-truck> X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 18, 2020 at 09:28:53AM +0100, Will Deacon wrote: > On Tue, Aug 18, 2020 at 04:43:10PM +0900, Cho KyongHo wrote: > > Cache maintenance operations in the most of CPU architectures needs > > memory barrier after the cache maintenance for the DMAs to view the > > region of the memory correctly. The problem is that memory barrier is > > very expensive and dma_[un]map_sg() and dma_sync_sg_for_{device|cpu}() > > involves the memory barrier per every single cache sg entry. In some > > CPU micro-architecture, a single memory barrier consumes more time than > > cache clean on 4KiB. It becomes more serious if the number of CPU cores > > are larger. > > Have you got higher-level performance data for this change? It's more likely > that the DSB is what actually forces the prior cache maintenance to > complete, so it's important to look at the bigger picture, not just the > apparent relative cost of these instructions. > > Also, it's a miracle that non-coherent DMA even works, so I'm not sure > that we should be complicating the implementation like this to try to > make it "fast". And without not just an important in-tree user but one that actually matters and can show how this is correct the whole proposal is complete nonstarter.