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,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 E5D66C4727D for ; Mon, 5 Oct 2020 08:26:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A359420644 for ; Mon, 5 Oct 2020 08:26:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726073AbgJEI0c (ORCPT ); Mon, 5 Oct 2020 04:26:32 -0400 Received: from verein.lst.de ([213.95.11.211]:58080 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725885AbgJEI0c (ORCPT ); Mon, 5 Oct 2020 04:26:32 -0400 Received: by verein.lst.de (Postfix, from userid 2407) id 5D8DD67373; Mon, 5 Oct 2020 10:26:29 +0200 (CEST) Date: Mon, 5 Oct 2020 10:26:29 +0200 From: Christoph Hellwig To: Tomasz Figa Cc: Christoph Hellwig , Mauro Carvalho Chehab , Marek Szyprowski , iommu@lists.linux-foundation.org, Robin Murphy , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org Subject: Re: [PATCH 8/8] WIP: add a dma_alloc_contiguous API Message-ID: <20201005082629.GA13850@lst.de> References: <20200930160917.1234225-1-hch@lst.de> <20200930160917.1234225-9-hch@lst.de> <20201002175040.GA1131147@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201002175040.GA1131147@chromium.org> User-Agent: Mutt/1.5.17 (2007-11-01) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 02, 2020 at 05:50:40PM +0000, Tomasz Figa wrote: > Hi Christoph, > > On Wed, Sep 30, 2020 at 06:09:17PM +0200, Christoph Hellwig wrote: > > Add a new API that returns a virtually non-contigous array of pages > > and dma address. This API is only implemented for dma-iommu and will > > not be implemented for non-iommu DMA API instances that have to allocate > > contiguous memory. It is up to the caller to check if the API is > > available. > > Would you mind scheding some more light on what made the previous attempt > not work well? I liked the previous API because it was more consistent with > the regular dma_alloc_coherent(). The problem is that with a dma_alloc_noncoherent that can return pages not in the kernel mapping we can't just use virt_to_page to fill in scatterlists or mmap the buffer to userspace, but would need new helpers and another two methods. > > - no kernel mapping or only temporary kernel mappings are required. > > That is as a better replacement for DMA_ATTR_NO_KERNEL_MAPPING > > - a kernel mapping is required for cached and DMA mapped pages, but > > the driver also needs the pages to e.g. map them to userspace. > > In that sense it is a replacement for some aspects of the recently > > removed and never fully implemented DMA_ATTR_NON_CONSISTENT > > What's the expected allocation and mapping flow with the latter? Would that be > > pages = dma_alloc_noncoherent(...) > vaddr = vmap(pages, ...); > > ? Yes. Witht the vmap step optional for replacements of DMA_ATTR_NO_KERNEL_MAPPING, which is another nightmare to deal with. > Would one just use the usual dma_sync_for_{cpu,device}() for cache > invallidate/clean, while keeping the mapping in place? Yes. And make sure the API isn't implemented when VIVT caches are used, but that isn't really different from the current interface.