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 E7182C4363A for ; Mon, 5 Oct 2020 13:08:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9BC54207BC for ; Mon, 5 Oct 2020 13:08:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726070AbgJENI4 (ORCPT ); Mon, 5 Oct 2020 09:08:56 -0400 Received: from verein.lst.de ([213.95.11.211]:59012 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725931AbgJENI4 (ORCPT ); Mon, 5 Oct 2020 09:08:56 -0400 Received: by verein.lst.de (Postfix, from userid 2407) id 7EDB367373; Mon, 5 Oct 2020 15:08:52 +0200 (CEST) Date: Mon, 5 Oct 2020 15:08:52 +0200 From: Christoph Hellwig To: Lorenzo Pieralisi Cc: Alex Williamson , Auger Eric , joro@8bytes.org, iommu@lists.linux-foundation.org, robin.murphy@arm.com, dwmw2@infradead.org, eric.auger.pro@gmail.com, linux-kernel@vger.kernel.org, Will Deacon , Lu Baolu , shameerali.kolothum.thodi@huawei.com, Jean-Philippe Brucker , hch@lst.de Subject: Re: [RFC 0/3] iommu: Reserved regions for IOVAs beyond dma_mask and iommu aperture Message-ID: <20201005130852.GB2163@lst.de> References: <20200928195037.22654-1-eric.auger@redhat.com> <20200928164224.12350d84@w520.home> <1cbaf3e7-cf88-77f6-4cc4-46dcd60eb649@redhat.com> <20200929121849.455af184@w520.home> <20201005104410.GA12138@e121166-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201005104410.GA12138@e121166-lin.cambridge.arm.com> User-Agent: Mutt/1.5.17 (2007-11-01) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 05, 2020 at 11:44:10AM +0100, Lorenzo Pieralisi wrote: > > I see that there are both OF and ACPI hooks in pci_dma_configure() and > > both modify dev->dma_mask, which is what pci-sysfs is exposing here, > > but I'm not convinced this even does what it's intended to do. The > > driver core calls this via the bus->dma_configure callback before > > probing a driver, but then what happens when the driver calls > > pci_set_dma_mask()? This is just a wrapper for dma_set_mask() and I > > don't see anywhere that would take into account the existing > > dev->dma_mask. It seems for example that pci_dma_configure() could > > produce a 42 bit mask as we have here, then the driver could override > > that with anything that the dma_ops.dma_supported() callback finds > > acceptable, and I don't see any instances where the current > > dev->dma_mask is considered. Am I overlooking something? > > I don't think so but Christoph and Robin can provide more input on > this - it is a long story. > > ACPI and OF bindings set a default dma_mask (and dev->bus_dma_limit), > this does not prevent a driver from overriding the dev->dma_mask but DMA > mapping code still takes into account the dev->bus_dma_limit. > > This may help: > > git log -p 03bfdc31176c This is at best a historic artefact. Bus drivers have no business messing with the DMA mask, dev->bus_dma_limit is the way to communicate addressing limits on the bus (or another interconnect closer to the CPU).