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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 55AD6C3DA49 for ; Tue, 30 Jul 2024 15:53:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=2RD9lzL7XYOhGbtGyYhZuS6o/ijEK3hJe15DzxZ9A/E=; b=cVkwF+ZKbpA1H+6hd8xhGXIeMD fQCHNUL1YbxVD7MM+xhF/tyW48vPI9TPoOKPJYbRw1lsVpNq3+ITKCvqJNJsvQ85LYz3wqroSo0rQ 3/Y9h6rhnCFETkkDR05X8NWwYcc3BAs+RUxtJDwP7wTir98vukxDP1FqoqO8cm805ilZBDtlLw8Wy 1qheqZjxMs67YPViq+1MwSL0nNsk/xwYhgkWWihDKFZbn+2m6F8BIPtcjyi5ZzMNGa1j23XRugr9z mFm6WO3J01hljwdnykMBsBtODo1agByjWjxOOY1CT8dp2tboYAyUTKLKgUDMGHBUkW9bD1RRKIs5i LsEdP2gw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1sYp9i-0000000FjE8-066U; Tue, 30 Jul 2024 15:52:54 +0000 Received: from dfw.source.kernel.org ([139.178.84.217]) by bombadil.infradead.org with esmtps (Exim 4.97.1 #2 (Red Hat Linux)) id 1sYp9E-0000000Fj6b-2VI2 for linux-arm-kernel@lists.infradead.org; Tue, 30 Jul 2024 15:52:25 +0000 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 Cc: Christoph Hellwig , Marek Szyprowski , Rob Herring , Saravana Kannan , Will Deacon , Robin Murphy , iommu@lists.linux.dev, devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, Petr =?utf-8?B?VGVzYcWZw61r?= , Ramon Fried , Elad Nachman 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-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240730_085224_736537_32A3ACAE X-CRM114-Status: GOOD ( 22.97 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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