From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joerg Roedel Subject: Re: dma_alloc_coherent - cma - and IOMMU question Date: Wed, 4 Feb 2015 15:15:20 +0100 Message-ID: <20150204141519.GL3702@8bytes.org> References: <1422648686.22865.258.camel@redhat.com> <54CBF290.7050707@compro.net> <1422654680.22865.277.camel@redhat.com> <54CF9F73.5080803@compro.net> <1422898536.22865.382.camel@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1422898536.22865.382.camel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Alex Williamson Cc: markh-n2QNKt385d+sTnJN9+BGXg@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org List-Id: iommu@lists.linux-foundation.org On Mon, Feb 02, 2015 at 10:35:36AM -0700, Alex Williamson wrote: > [cc +joerg] > > On Mon, 2015-02-02 at 11:01 -0500, Mark Hounschell wrote: > > From what I understand of CMA, and it seems provable to me, is that > > dma_alloc_coherent allocates my 8MB buffer from CMA defined on the > > cmdline. Without CMA specified on the cmdline, dma_alloc_coherent > > definitely fails to allocate an 8MB contiguous buffer. From what I've > > read about it, it is supposed to transparently "just work" when > > dma_alloc_coherent is used? > > Yes, if you're running with the software iotlb (aka. bounce buffers), > then dma_ops->alloc is x86_swiotlb_alloc_coherent(), which calls > dma_generic_alloc_coherent(), which attempts to use CMA via > dma_alloc_from_contiguous(). > > If you look at the same path with AMD-Vi, dma_ops->alloc is > amd_iommu.c:alloc_coherent(), which simply uses __get_free_pages() to > allocate the buffer. I don't see any CMA integration along that path. > If you were using Intel VT-d, then the buffer is again allocated with > dma_alloc_from_contiguous() and should use CMA. This was added in > kernel v3.16, but no corresponding AMD-Vi change was added. Joerg, this > might be an easily fixed oversight. Right, CMA is not yet integrated in the AMD IOMMU driver, nobody asked fot it until now :) I will add that fall-back there for big allocations. Joerg