From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings Date: Thu, 10 Jul 2014 11:23:34 +0100 Message-ID: <20140710102334.GG2449@arm.com> References: <1404487757-18829-1-git-send-email-thierry.reding@gmail.com> <20140709134050.GN9485@arm.com> <20140709142125.GB3262@ulmo> <20140709181048.GX9485@arm.com> <20140710094909.GA21583@ulmo> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20140710094909.GA21583@ulmo> Sender: linux-tegra-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Thierry Reding Cc: Rob Herring , Pawel Moll , Mark Rutland , Ian Campbell , Kumar Gala , Stephen Warren , Arnd Bergmann , Joerg Roedel , Cho KyongHo , Grant Grundler , Dave P Martin , Marc Zyngier , Hiroshi Doyu , Olav Haugan , Varun Sethi , "devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org" , "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" , "linux-tegra-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" List-Id: devicetree@vger.kernel.org On Thu, Jul 10, 2014 at 10:49:10AM +0100, Thierry Reding wrote: > On Wed, Jul 09, 2014 at 07:10:48PM +0100, Will Deacon wrote: > > On Wed, Jul 09, 2014 at 03:21:27PM +0100, Thierry Reding wrote: > > > Anything beyond that (e.g. logical grouping of masters) isn't directly > > > within the scope of the binding (it doesn't describe hardware but some > > > policy pertaining to some specific use-case). > > > > This *is* for hardware. I can use PCI as an example, but this could equally > > apply to other types of bus. If you have a bunch of PCI master devices > > sitting being a non-transparent bridge, they can end up sharing the same > > master device ID (requester ID). This means that there is no way in the > > IOMMU to initialise a translation for one of these devices without also > > affecting the others. We currently have iommu_groups to deal with this, but > > it *is* a property of the hardware and we absolutely need a way to describe > > it. I'm happy to add it later, but we need to think about it now to avoid > > merging something that can't easily be extended. > > > > For PCI, the topology is probable but even then, we need this information to > > describe the resulting master device ID emitted by the bridge for the > > upstream group. One way to do this with your binding would be to treat all > > of the upstream masters as having the same device ID. > > Yes, I think that makes most sense. After all from the IOMMU's point of > view requests from all devices behind the bridge will originate from the > same ID. > > So technically it's not really correct to encode the master ID within > each of the devices, but rather they should be inheriting the ID from > the non-transparent bridge. Indeed. Is that possible with your binding, or would we just duplicate the IDs between the masters? > > With virtualisation, we may want to assign a group of devices to a guest but > > without emulating the bridge. This would need something the device-tree to > > describe that they are grouped together. > > But that's also a software decision, isn't it? Virtualization doesn't > have anything to do with the hardware description. Or am I missing > something? Of course I guess you could generate a DTB for the guest and > group device together, in which case you're pretty much free to do what > you want since you're essentially defining your own hardware. If you're doing device passthrough and you want to allow the guest to program the IOMMU, I think that virtualisation is directly related to the hardware description, since the guest will be bound by physical properties of the system. Will