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=-3.8 required=3.0 tests=BAYES_00, 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 45BA2C43457 for ; Thu, 15 Oct 2020 15:15:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E41F922255 for ; Thu, 15 Oct 2020 15:15:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389160AbgJOPPM (ORCPT ); Thu, 15 Oct 2020 11:15:12 -0400 Received: from mx2.suse.de ([195.135.220.15]:52924 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388086AbgJOPPM (ORCPT ); Thu, 15 Oct 2020 11:15:12 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id EFBECAB0E; Thu, 15 Oct 2020 15:15:10 +0000 (UTC) Message-ID: <64bbecb547422272417feaff1aa6b3d8c3da1157.camel@suse.de> Subject: Re: [PATCH v3 7/8] arm64: mm: Set ZONE_DMA size based on early IORT scan From: Nicolas Saenz Julienne To: Hanjun Guo , robh+dt@kernel.org, catalin.marinas@arm.com, hch@lst.de, ardb@kernel.org, linux-kernel@vger.kernel.org, Lorenzo Pieralisi , Sudeep Holla Cc: robin.murphy@arm.com, linux-arm-kernel@lists.infradead.org, linux-rpi-kernel@lists.infradead.org, jeremy.linton@arm.com, iommu@lists.linux-foundation.org, devicetree@vger.kernel.org, Anshuman Khandual , Will Deacon , "Rafael J. Wysocki" , Len Brown , linux-acpi@vger.kernel.org Date: Thu, 15 Oct 2020 17:15:09 +0200 In-Reply-To: <1a3df60a-4568-cb72-db62-36127d0ffb7e@huawei.com> References: <20201014191211.27029-1-nsaenzjulienne@suse.de> <20201014191211.27029-8-nsaenzjulienne@suse.de> <1a3df60a-4568-cb72-db62-36127d0ffb7e@huawei.com> Content-Type: multipart/signed; micalg="pgp-sha256"; protocol="application/pgp-signature"; boundary="=-rA4ctgOOrD1oldFe6ryS" User-Agent: Evolution 3.36.5 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org --=-rA4ctgOOrD1oldFe6ryS Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2020-10-15 at 22:26 +0800, Hanjun Guo wrote: > On 2020/10/15 3:12, Nicolas Saenz Julienne wrote: > > From: Ard Biesheuvel > >=20 > > We recently introduced a 1 GB sized ZONE_DMA to cater for platforms > > incorporating masters that can address less than 32 bits of DMA, in > > particular the Raspberry Pi 4, which has 4 or 8 GB of DRAM, but has > > peripherals that can only address up to 1 GB (and its PCIe host > > bridge can only access the bottom 3 GB) > >=20 > > Instructing the DMA layer about these limitations is straight-forward, > > even though we had to fix some issues regarding memory limits set in > > the IORT for named components, and regarding the handling of ACPI _DMA > > methods. However, the DMA layer also needs to be able to allocate > > memory that is guaranteed to meet those DMA constraints, for bounce > > buffering as well as allocating the backing for consistent mappings. > >=20 > > This is why the 1 GB ZONE_DMA was introduced recently. Unfortunately, > > it turns out the having a 1 GB ZONE_DMA as well as a ZONE_DMA32 causes > > problems with kdump, and potentially in other places where allocations > > cannot cross zone boundaries. Therefore, we should avoid having two > > separate DMA zones when possible. > >=20 > > So let's do an early scan of the IORT, and only create the ZONE_DMA > > if we encounter any devices that need it. This puts the burden on > > the firmware to describe such limitations in the IORT, which may be > > redundant (and less precise) if _DMA methods are also being provided. > > However, it should be noted that this situation is highly unusual for > > arm64 ACPI machines. Also, the DMA subsystem still gives precedence to > > the _DMA method if implemented, and so we will not lose the ability to > > perform streaming DMA outside the ZONE_DMA if the _DMA method permits > > it. >=20 > Sorry, I'm still a little bit confused. With this patch, if we have > a device which set the right _DMA method (DMA size >=3D 32), but with the > wrong DMA size in IORT, we still have the ZONE_DMA created which > is actually not needed? Yes, that would be the case. 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