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 6ED13C4361B for ; Wed, 9 Dec 2020 13:08:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 368DB23BC7 for ; Wed, 9 Dec 2020 13:08:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732172AbgLINHq (ORCPT ); Wed, 9 Dec 2020 08:07:46 -0500 Received: from foss.arm.com ([217.140.110.172]:34310 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732116AbgLINGH (ORCPT ); Wed, 9 Dec 2020 08:06:07 -0500 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 9D4AC31B; Wed, 9 Dec 2020 05:05:16 -0800 (PST) Received: from [10.57.61.6] (unknown [10.57.61.6]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BB3F73F66B; Wed, 9 Dec 2020 05:05:14 -0800 (PST) Subject: Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API To: Christoph Hellwig , Tomasz Figa Cc: Sergey Senozhatsky , Ricardo Ribalda , Marek Szyprowski , Mauro Carvalho Chehab , IOMMU DRIVERS , Joerg Roedel , Linux Doc Mailing List , Linux Kernel Mailing List , Linux Media Mailing List , Sergey Senozhatsky References: <20201125221917.150463-1-ribalda@chromium.org> <20201130083410.GD32234@lst.de> <20201201033658.GE3723071@google.com> <20201201144916.GA14682@lst.de> <20201209111235.GA22806@lst.de> From: Robin Murphy Message-ID: <6b34596d-34c4-bd99-c5a7-5a4742c5b886@arm.com> Date: Wed, 9 Dec 2020 13:05:11 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <20201209111235.GA22806@lst.de> 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 2020-12-09 11:12, Christoph Hellwig wrote: > On Tue, Dec 08, 2020 at 01:54:00PM +0900, Tomasz Figa wrote: >> >From the media perspective, it would be good to have the vmap >> optional, similarly to the DMA_ATTR_NO_KERNEL_MAPPING attribute for >> coherent allocations. Actually, in the media drivers, the need to have >> a kernel mapping of the DMA buffers corresponds to a minority of the >> drivers. Most of them only need to map them to the userspace. >> >> Nevertheless, that minority actually happens to be quite widely used, >> e.g. the uvcvideo driver, so we can't go to the other extreme and just >> drop the vmap at all. > > My main problem is that the DMA_ATTR_NO_KERNEL_MAPPING makes a mess > of an API. I'd much rather have low-level API that returns the > discontiguous allocations and another one that vmaps them rather > than starting to overload arguments like in dma_alloc_attrs with > DMA_ATTR_NO_KERNEL_MAPPING. Agreed - if iommu-dma's dma_alloc_coherent() ends up as little more than a thin wrapper around those two functions I think that would be a good sign. It also seems like it might be a good idea for this API to use scatterlists rather than page arrays as it's fundamental format, to help reduce impedance with dma-buf - if we can end up with a wider redesign that also gets rid of dma_get_sgtable(), all the better! Robin.