Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753339AbdDKNg6 (ORCPT ); Tue, 11 Apr 2017 09:36:58 -0400 Received: from foss.arm.com ([217.140.101.70]:60898 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752151AbdDKNgz (ORCPT ); Tue, 11 Apr 2017 09:36:55 -0400 Date: Tue, 11 Apr 2017 14:36:50 +0100 From: Catalin Marinas To: Julien Grall Cc: Bart Van Assche , Stefano Stabellini , "boris.ostrovsky@oracle.com" , "jgross@suse.com" , "benh@kernel.crashing.org" , "dwmw2@infradead.org" , "hpa@zytor.com" , "mingo@redhat.com" , "linux@armlinux.org.uk" , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "x86@kernel.org" , linux-arm-kernel@lists.infradead.org Subject: Re: "Consolidate get_dma_ops" breaks Xen on ARM Message-ID: <20170411133649.GB27857@e104818-lin.cambridge.arm.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2830 Lines: 64 On Tue, Apr 11, 2017 at 01:43:28PM +0100, Julien Grall wrote: > On 11/04/17 02:14, Bart Van Assche wrote: > > On 04/10/17 17:31, Stefano Stabellini wrote: > >> I think the reason is that, as you can see, if (dev && dev->dma_ops), > >> dev->dma_ops is returned, while before this changes, xen_dma_ops was > >> returned on Xen on ARM. > >> > >> Unfortunately DMA cannot work properly without using the appropriate > >> xen_dma_ops. See drivers/xen/swiotlb-xen.c and arch/arm/xen/mm.c for > >> more details. (The problem is easy to spot, but I wasn't CC'ed on the > >> patch.) > >> > >> I don't know how to solve this problem without introducing some sort of > >> if (xen()) in include/linux/dma-mapping.h. > > > > Sorry but I don't have access to an ARM development system. Does your > > comment apply to dev == NULL only, dev != NULL only or perhaps to both? > > If your comment applies to dev != NULL only, can you check whether > > adding something like set_dma_ops(dev, get_arch_dma_ops(NULL)) to the > > appropriate ARM arch_setup_dma_ops() function is sufficient? > > If I understand correctly, set_dma_ops will replace dev->dma_ops with > Xen DMA ops. > > However, Xen DMA ops will need in some places to call the device > specific DMA ops (see __generic_dma_ops(...)). So I think replacing > dev->dma_ops is not a solution here. > > The hackish patch below is fixing the problem for both ARM64 and ARM32. > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h > index 0977317c6835..43a73ddeec7a 100644 > --- a/include/linux/dma-mapping.h > +++ b/include/linux/dma-mapping.h > @@ -174,6 +174,8 @@ int dma_mmap_from_coherent(struct device *dev, struct vm_area_struct *vma, > #include > static inline const struct dma_map_ops *get_dma_ops(struct device *dev) > { > + if (xen_initial_domain()) > + return xen_dma_ops; > if (dev && dev->dma_ops) > return dev->dma_ops; > return get_arch_dma_ops(dev ? dev->bus : NULL); If we do this, I guess there is no need to check for xen_initial_domain() in the get_arch_dma_ops() function. Anyway, this hunk would break the other architectures since xen_dma_ops is only defined for arm and arm64. > It is not nice as this is common code, but I can't find a better solution > so far. Any opinions? A different hack would be to avoid the generic get_dma_ops implementation on arm with some #ifdef hacks above. Yet another way would be for dom0 to always set dev->dma_ops to xen_dma_ops and preserve the real dma_ops somewhere under dev->archdata. You could intercept the arch_setup_dma_ops() function for this or use bus_register_notifier() (though I think the former is easier). The Xen code making use of the real dma_ops would have to dig them out from dev->archdata. -- Catalin