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=-0.7 required=3.0 tests=DKIM_ADSP_CUSTOM_MED, FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM,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 84107C433E0 for ; Tue, 7 Jul 2020 15:11:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6CE13207E8 for ; Tue, 7 Jul 2020 15:11:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728670AbgGGPLU (ORCPT ); Tue, 7 Jul 2020 11:11:20 -0400 Received: from smtp8.emailarray.com ([65.39.216.67]:34497 "EHLO smtp8.emailarray.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728651AbgGGPLS (ORCPT ); Tue, 7 Jul 2020 11:11:18 -0400 Received: (qmail 99413 invoked by uid 89); 7 Jul 2020 15:11:12 -0000 Received: from unknown (HELO localhost) (amxlbW9uQGZsdWdzdmFtcC5jb21AMTYzLjExNC4xMzIuMw==) (POLARISLOCAL) by smtp8.emailarray.com with SMTP; 7 Jul 2020 15:11:12 -0000 Date: Tue, 7 Jul 2020 08:11:09 -0700 From: Jonathan Lemon To: Christoph Hellwig Cc: =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= , Magnus Karlsson , iommu@lists.linux-foundation.org, netdev@vger.kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/4] dma-mapping: add a new dma_need_sync API Message-ID: <20200707151109.qui5uzzzq4dihfie@bsd-mbp> References: <20200629130359.2690853-1-hch@lst.de> <20200629130359.2690853-2-hch@lst.de> <20200706194227.vfhv5o4lporxjxmq@bsd-mbp.dhcp.thefacebook.com> <20200707064730.GA23602@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200707064730.GA23602@lst.de> Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Tue, Jul 07, 2020 at 08:47:30AM +0200, Christoph Hellwig wrote: > On Mon, Jul 06, 2020 at 12:42:27PM -0700, Jonathan Lemon wrote: > > On Mon, Jun 29, 2020 at 03:03:56PM +0200, Christoph Hellwig wrote: > > > Add a new API to check if calls to dma_sync_single_for_{device,cpu} are > > > required for a given DMA streaming mapping. > > > > > > +:: > > > + > > > + bool > > > + dma_need_sync(struct device *dev, dma_addr_t dma_addr); > > > + > > > +Returns %true if dma_sync_single_for_{device,cpu} calls are required to > > > +transfer memory ownership. Returns %false if those calls can be skipped. > > > > Hi Christoph - > > > > Thie call above is for a specific dma_addr. For correctness, would I > > need to check every addr, or can I assume that for a specific memory > > type (pages returned from malloc), that the answer would be identical? > > You need to check every mapping. E.g. this API pairs with a > dma_map_single/page call. For S/G mappings you'd need to call it for > each entry, although if you have a use case for that we really should > add a dma_sg_need_sync helper instea of open coding the scatterlist walk. My use case is setting up a pinned memory area, and caching the dma mappings. I'd like to bypass storing the DMA addresses if they aren't needed. For example: setup() { if (dma_need_sync(dev, addr, len)) { kvmalloc_array(...) cache_dma_mappings(...) } dev_get_dma(page) { if (!cache) return page_to_phys(page) return dma_cache_lookup(...) The reason for doing it this way is that the page in question may be backed by either system memory, or device memory such as a GPU. For the latter, the GPU provides a table of DMA addresses where data may be accessed, so I'm unable to use the dma_map_page() API. -- Jonathan