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=-2.2 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 A30E8C432C3 for ; Thu, 14 Nov 2019 07:43:28 +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 635E5206DC for ; Thu, 14 Nov 2019 07:43:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 635E5206DC 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 47DD4G1KMPzF4vg for ; Thu, 14 Nov 2019 18:43:26 +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 47DD1f2jKXzDq6K for ; Thu, 14 Nov 2019 18:41:10 +1100 (AEDT) Received: by verein.lst.de (Postfix, from userid 2407) id EF39468AFE; Thu, 14 Nov 2019 08:41:05 +0100 (CET) Date: Thu, 14 Nov 2019 08:41:05 +0100 From: Christoph Hellwig To: Robin Murphy Subject: Re: generic DMA bypass flag Message-ID: <20191114074105.GC26546@lst.de> References: <20191113133731.20870-1-hch@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 , linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org, Christoph Hellwig Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Wed, Nov 13, 2019 at 02:45:15PM +0000, Robin Murphy wrote: > In all honesty, this seems silly. If we can set a per-device flag to say > "oh, bypass these ops and use some other ops instead", then we can just as > easily simply give the device the appropriate ops in the first place. > Because, y'know, the name of the game is "per-device ops". Except that we can't do it _that_ easily. The problem is that for both the powerpc and intel case the selection is dynamic. If a device is in the identify domain with intel-iommu (or the equivalent on powerpc which doesn't use the normal iommu framework), we still want to use the iommu to be able to map memory for devices with a too small dma mask using the iommu instead of using swiotlb bouncing. So to "just" use the per-device dma ops we'd need: a) a hook in dma_direct_supported to pick another set of ops for small dma masks b) a hook in the IOMMU ops to propagate to the direct ops for full 64-bit masks I looked into that for powerpc a while ago and it wasn't pretty at all. Compared to that just checking another flag for the DMA direct calls is relatively clean and trivial as seens in the diffstat for this series alone. > I don't see a great benefit to pulling legacy cruft out into common code > instead of just working to get rid of it in-place, when said cruft pulls in > the opposite direction to where we're taking the common code (i.e. it's > inherently based on the premise of global ops). I'm not sure what legacy cruft it pull in. I think it actually fits very much into a mental model of "direct mapping is the default, to be overriden if needed" which is pretty close to what we have at the moment. Just with a slightly more complicated processing of the override.