From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH 2/3] dma: Introduce dma_max_mapping_size() Date: Tue, 15 Jan 2019 18:54:18 +0100 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 Return-path: Content-Disposition: inline In-Reply-To: <20190115162322.GA4681@8bytes.org> Sender: linux-kernel-owner@vger.kernel.org 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 List-Id: iommu@lists.linux-foundation.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.