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 Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5A620C3DA70 for ; Tue, 30 Jul 2024 15:53:13 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4WYKWW71Yjz3dVp for ; Wed, 31 Jul 2024 01:53:11 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=kernel.org (client-ip=2604:1380:4641:c500::1; helo=dfw.source.kernel.org; envelope-from=cmarinas@kernel.org; receiver=lists.ozlabs.org) Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4WYKVf1wkMz3dHR for ; Wed, 31 Jul 2024 01:52:26 +1000 (AEST) Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by dfw.source.kernel.org (Postfix) with ESMTP id C146961FE5; Tue, 30 Jul 2024 15:52:22 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B8B36C4AF0A; Tue, 30 Jul 2024 15:52:19 +0000 (UTC) Date: Tue, 30 Jul 2024 16:52:17 +0100 From: Catalin Marinas To: Baruch Siach Subject: Re: [PATCH RFC v2 2/5] of: get dma area lower limit Message-ID: References: <230ea13ef8e9f576df849e1b03406184ca890ba8.1712642324.git.baruch@tkos.co.il> <87cyn1k7yq.fsf@tarshish> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87cyn1k7yq.fsf@tarshish> X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Rob Herring , linux-s390@vger.kernel.org, Ramon Fried , Saravana Kannan , devicetree@vger.kernel.org, Petr =?utf-8?B?VGVzYcWZw61r?= , Will Deacon , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, iommu@lists.linux.dev, Elad Nachman , Robin Murphy , Christoph Hellwig , linux-arm-kernel@lists.infradead.org, Marek Szyprowski Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Thu, Jul 25, 2024 at 02:49:01PM +0300, Baruch Siach wrote: > Hi Catalin, > > On Tue, Jun 18 2024, Catalin Marinas wrote: > > On Tue, Apr 09, 2024 at 09:17:55AM +0300, Baruch Siach wrote: > >> of_dma_get_max_cpu_address() returns the highest CPU address that > >> devices can use for DMA. The implicit assumption is that all CPU > >> addresses below that limit are suitable for DMA. However the > >> 'dma-ranges' property this code uses also encodes a lower limit for DMA > >> that is potentially non zero. > >> > >> Rename to of_dma_get_cpu_limits(), and extend to retrieve also the lower > >> limit for the same 'dma-ranges' property describing the high limit. > > > > I don't understand the reason for the lower limit. The way the Linux > > zones work is that ZONE_DMA always starts from the start of the RAM. It > > doesn't matter whether it's 0 or not, you'd not allocate below the start > > of RAM anyway. If you have a device that cannot use the bottom of the > > RAM, it is pretty broken and not supported by Linux. > > I won't argue with that assertion. My target system RAM happens to start > at that the lower end of devices DMA zone, so I'm fine with skipping > this patch. > > Just curious. What is the inherent limitation that prevents Linux from > supporting DMA zone with lower limit above RAM start? It's the way the zone allocation fallback mechanism works. Let's say a ZONE_DMA32 allocation fails, it falls back to ZONE_DMA and it's supposed to be compatible with the GFP_DMA32 request. If you have some other zone below ZONE_DMA, it should also be compatible with GFP_DMA allocations. -- Catalin