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=-2.3 required=3.0 tests=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 3E77CC4332B for ; Mon, 23 Mar 2020 12:59:39 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D23DF20658 for ; Mon, 23 Mar 2020 12:59:37 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D23DF20658 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=lst.de Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 48mDx16w5VzDqXr for ; Mon, 23 Mar 2020 23:59:33 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=none (no SPF record) smtp.mailfrom=lst.de (client-ip=213.95.11.211; helo=verein.lst.de; envelope-from=hch@lst.de; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=lst.de Received: from verein.lst.de (verein.lst.de [213.95.11.211]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 48mDrT2BNPzDqHb for ; Mon, 23 Mar 2020 23:55:36 +1100 (AEDT) Received: by verein.lst.de (Postfix, from userid 2407) id 4428E68BEB; Mon, 23 Mar 2020 13:55:30 +0100 (CET) Date: Mon, 23 Mar 2020 13:55:30 +0100 From: Christoph Hellwig To: Robin Murphy Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device Message-ID: <20200323125530.GA17038@lst.de> References: <20200320141640.366360-1-hch@lst.de> <20200320141640.366360-2-hch@lst.de> <0a6003e5-8003-4509-4014-4b286d5e8fe0@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0a6003e5-8003-4509-4014-4b286d5e8fe0@arm.com> User-Agent: Mutt/1.5.17 (2007-11-01) 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: Alexey Kardashevskiy , Greg Kroah-Hartman , Joerg Roedel , "linux-kernel@vger.kernel.org" , "iommu@lists.linux-foundation.org" , "linuxppc-dev@lists.ozlabs.org" , Christoph Hellwig , Lu Baolu Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Mon, Mar 23, 2020 at 12:14:08PM +0000, Robin Murphy wrote: > On 2020-03-20 2:16 pm, Christoph Hellwig wrote: >> Several IOMMU drivers have a bypass mode where they can use a direct >> mapping if the devices DMA mask is large enough. Add generic support >> to the core dma-mapping code to do that to switch those drivers to >> a common solution. > > Hmm, this is _almost_, but not quite the same as the case where drivers are > managing their own IOMMU mappings, but still need to use streaming DMA for > cache maintenance on the underlying pages. In that case they should simply not call the DMA API at all. We'll just need versions of the cache maintainance APIs that tie in with the raw IOMMU API. > For that we need the ops bypass > to be a "true" bypass and also avoid SWIOTLB regardless of the device's DMA > mask. That behaviour should in fact be fine for the opportunistic bypass > case here as well, since the mask being "big enough" implies by definition > that this should never need to bounce either. In practice it does. But that means adding yet another code path vs the simple direct call to dma_direct_* vs calling the DMA ops which I'd rather avoid.