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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 AFF48C12002 for ; Fri, 16 Jul 2021 15:33:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9718F613F6 for ; Fri, 16 Jul 2021 15:33:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240928AbhGPPf7 (ORCPT ); Fri, 16 Jul 2021 11:35:59 -0400 Received: from foss.arm.com ([217.140.110.172]:40190 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240845AbhGPPfz (ORCPT ); Fri, 16 Jul 2021 11:35:55 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 74B81D6E; Fri, 16 Jul 2021 08:32:59 -0700 (PDT) Received: from [10.57.36.240] (unknown [10.57.36.240]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A607E3F774; Fri, 16 Jul 2021 08:32:56 -0700 (PDT) Subject: Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver To: Christoph Hellwig Cc: Sven Peter , Will Deacon , Joerg Roedel , Arnd Bergmann , devicetree@vger.kernel.org, Hector Martin , linux-kernel@vger.kernel.org, Marc Zyngier , Mohamed Mediouni , Stan Skowronek , linux-arm-kernel@lists.infradead.org, Mark Kettenis , iommu@lists.linux-foundation.org, Alexander Graf , Alyssa Rosenzweig , Rob Herring , r.czerwinski@pengutronix.de, Mauro Carvalho Chehab , linux-media@vger.kernel.org References: <20210627143405.77298-1-sven@svenpeter.dev> <7261df01-34a9-4e53-37cd-ae1aa15b1fb4@arm.com> From: Robin Murphy Message-ID: <0d20bd6b-d0a1-019c-6398-b12f83f4fdf7@arm.com> Date: Fri, 16 Jul 2021 16:32:50 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-07-16 07:24, Christoph Hellwig wrote: > On Wed, Jul 14, 2021 at 07:19:50PM +0100, Robin Murphy wrote: >> Even at the DMA API level you could hide *some* of it (at the cost of >> effectively only having 1/4 of the usable address space), but there are >> still cases like where v4l2 has a hard requirement that a page-aligned >> scatterlist can be mapped into a contiguous region of DMA addresses. > > Where does v4l2 make that broken assumption? Plenty of dma mapping > implementations including dma-direct do not support that. See vb2_dc_get_contiguous_size() and its callers. I still remember spending an entire work day on writing one email at the culmination of this discussion: https://lore.kernel.org/linux-iommu/56409B6D.5090903@arm.com/ 809eac54cdd6 was framed as an efficiency improvement because it technically was one (and something I had wanted to implement anyway), but it was also very much to save myself from any further email debates or customer calls about "regressing" code ported from 32-bit platforms... Robin.