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 4335BC433DF for ; Thu, 15 Oct 2020 18:03:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DC9C322240 for ; Thu, 15 Oct 2020 18:03:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730660AbgJOSDs (ORCPT ); Thu, 15 Oct 2020 14:03:48 -0400 Received: from mail.kernel.org ([198.145.29.99]:52066 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728460AbgJOSDs (ORCPT ); Thu, 15 Oct 2020 14:03:48 -0400 Received: from gaia (unknown [95.149.105.49]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id DCEC820797; Thu, 15 Oct 2020 18:03:43 +0000 (UTC) Date: Thu, 15 Oct 2020 19:03:41 +0100 From: Catalin Marinas To: Hanjun Guo Cc: Nicolas Saenz Julienne , robh+dt@kernel.org, hch@lst.de, ardb@kernel.org, linux-kernel@vger.kernel.org, Lorenzo Pieralisi , Sudeep Holla , 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 Subject: Re: [PATCH v3 7/8] arm64: mm: Set ZONE_DMA size based on early IORT scan Message-ID: <20201015180340.GB2624@gaia> References: <20201014191211.27029-1-nsaenzjulienne@suse.de> <20201014191211.27029-8-nsaenzjulienne@suse.de> <1a3df60a-4568-cb72-db62-36127d0ffb7e@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1a3df60a-4568-cb72-db62-36127d0ffb7e@huawei.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org On Thu, Oct 15, 2020 at 10:26:18PM +0800, Hanjun Guo wrote: > On 2020/10/15 3:12, Nicolas Saenz Julienne wrote: > > From: Ard Biesheuvel > > > > 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) > > > > 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. > > > > 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. > > > > 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. > > Sorry, I'm still a little bit confused. With this patch, if we have > a device which set the right _DMA method (DMA size >= 32), but with the > wrong DMA size in IORT, we still have the ZONE_DMA created which > is actually not needed? With the current kernel, we get a ZONE_DMA already with an arbitrary size of 1GB that matches what RPi4 needs. We are trying to eliminate such unnecessary ZONE_DMA based on some heuristics (well, something that looks "better" than a OEM ID based quirk). Now, if we learn that IORT for platforms in the field is that broken as to describe few bits-wide DMA masks, we may have to go back to the OEM ID quirk. -- Catalin