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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable 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 01FDCC43387 for ; Tue, 15 Jan 2019 17:54:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CC18920859 for ; Tue, 15 Jan 2019 17:54:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388581AbfAORyU (ORCPT ); Tue, 15 Jan 2019 12:54:20 -0500 Received: from verein.lst.de ([213.95.11.211]:54554 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728073AbfAORyU (ORCPT ); Tue, 15 Jan 2019 12:54:20 -0500 Received: by newverein.lst.de (Postfix, from userid 2407) id 84A3467358; Tue, 15 Jan 2019 18:54:18 +0100 (CET) Date: Tue, 15 Jan 2019 18:54:18 +0100 From: Christoph Hellwig To: Joerg Roedel Cc: Christoph Hellwig , "Michael S . Tsirkin" , Jason Wang , Konrad Rzeszutek Wilk , Jens Axboe , virtualization@lists.linux-foundation.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, jfehlig@suse.com, jon.grimm@amd.com, brijesh.singh@amd.com, jroedel@suse.de Subject: Re: [PATCH 2/3] dma: Introduce dma_max_mapping_size() Message-ID: <20190115175418.GA11402@lst.de> References: <20190115132257.6426-1-joro@8bytes.org> <20190115132257.6426-3-joro@8bytes.org> <20190115133754.GB29225@lst.de> <20190115162322.GA4681@8bytes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190115162322.GA4681@8bytes.org> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 15, 2019 at 05:23:22PM +0100, Joerg Roedel wrote: > Right, I thought about that too, but didn't find a generic way to check > for all the cases. There are various checks that could be done: > > 1) Check if SWIOTLB is initialized at all, if not, return > SIZE_MAX as the limit. This can't be checked from dma-direct > code right now, but could be easily implemented. Yes, this is the low hanging fruit. > 2) Check for swiotlb=force needs to be done. > > 3) Check whether the device can access all of available RAM. I > have no idea how to check that in an architecture independent > way. It also has to take memory hotplug into account as well > as the DMA mask of the device. > > An easy approximation could be to omit the limit if the > dma-mask covers all of the physical address bits available > on the platform. It would require to pass the dma-mask as an > additional parameter like it is done in dma_supported(). > > Any better ideas for how to implement 3)? And yeah, this is hard. So I'd just go for the low hanging fruit for now and only implement 1) with a comment mentioning that we are a little pessimistic.