From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH v6 1/8] iommu: provide early initialisation hook for IOMMU drivers Date: Fri, 5 Dec 2014 12:10:37 +0000 Message-ID: <20141205121037.GI1630@arm.com> References: <2579924.PPXuLn3o19@wuerfel> <54805312.6000402@arm.com> <54806504.20507@arm.com> <20141204144925.GB31464@ulmo.nvidia.com> <54809D09.2050406@arm.com> <5480B924.2010205@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <5480B924.2010205-5wv7dgnIgG8@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: Robin Murphy Cc: "jroedel-l3A5Bk7waGM@public.gmane.org" , Arnd Bergmann , Pantelis Antoniou , Linux IOMMU , Thierry Reding , Laurent Pinchart , "grant.likely-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org" , Varun Sethi , David Woodhouse , "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" List-Id: iommu@lists.linux-foundation.org On Thu, Dec 04, 2014 at 07:42:28PM +0000, Robin Murphy wrote: > On 04/12/14 17:58, Grant Likely wrote: > [...] > >>>> +struct of_iommu_node { > >>>> + struct list_head list; > >>>> + struct device_node *np; > >>>> + struct iommu_ops *ops; > >>> > >>> > >>> Why can't this be const? Why would anyone ever need to modify it? Also > >>> drivers do define their iommu_ops structures const these days, so you'll > >>> either still have to cast away at some point or the compiler will > >>> complain once you start calling this from drivers. > >>> > >> > >> ...whereas if we make everything const then the drivers that _do_ want to > >> use the private data introduced by this series and thus pass mutable > >> per-instance iommu_ops will be the ones having to cast. We have no users > >> either way until this series is merged, and the need to stash the > >> per-instance data somewhere other than np->data is in the way of getting it > >> merged - this is just a quick hack to address that. I think we've already > >> agreed that mutable per-instance iommu_ops holding private data aren't > >> particularly pleasant and will (hopefully) go away in the next iteration[1], > >> at which point this all changes anyway. > > > > Do you expect drivers to modify that *priv pointer after the ops > > structure is registered? I'd be very surprised if that was the use > > case. It's fine for the driver to register a non-const version, but > > once it is registered, the infrastructure can treat it as const from > > then on. > > Possibly not - certainly my current port of the ARM SMMU which makes use > of *priv is only ever reading it - although we did also wave around > reasons for mutable ops like dynamically changing the pgsize_bitmap and > possibly even swizzling individual ops for runtime reconfiguration. On > consideration though, I'd agree that things like that are mad enough to > stay well within individual drivers if they did ever happen, and > certainly shouldn't apply to this bit of the infrastructure at any rate. I certainly need to update the pgsize_bitmap at runtime because I don't know the supported page sizes until I've both (a) probed the hardware and (b) allocated page tables for a domain. We've already discussed moving the pgsize_bitmap out of the ops, but moving it somewhere where it remains const doesn't really help. Can I just take the patch that Grant acked, in the interest of getting something merged? As you say, there's plenty of planned changes in this area anyway. I plan to send Olof a pull request this afternoon. Will From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Fri, 5 Dec 2014 12:10:37 +0000 Subject: [PATCH v6 1/8] iommu: provide early initialisation hook for IOMMU drivers In-Reply-To: <5480B924.2010205@arm.com> References: <2579924.PPXuLn3o19@wuerfel> <54805312.6000402@arm.com> <54806504.20507@arm.com> <20141204144925.GB31464@ulmo.nvidia.com> <54809D09.2050406@arm.com> <5480B924.2010205@arm.com> Message-ID: <20141205121037.GI1630@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Dec 04, 2014 at 07:42:28PM +0000, Robin Murphy wrote: > On 04/12/14 17:58, Grant Likely wrote: > [...] > >>>> +struct of_iommu_node { > >>>> + struct list_head list; > >>>> + struct device_node *np; > >>>> + struct iommu_ops *ops; > >>> > >>> > >>> Why can't this be const? Why would anyone ever need to modify it? Also > >>> drivers do define their iommu_ops structures const these days, so you'll > >>> either still have to cast away at some point or the compiler will > >>> complain once you start calling this from drivers. > >>> > >> > >> ...whereas if we make everything const then the drivers that _do_ want to > >> use the private data introduced by this series and thus pass mutable > >> per-instance iommu_ops will be the ones having to cast. We have no users > >> either way until this series is merged, and the need to stash the > >> per-instance data somewhere other than np->data is in the way of getting it > >> merged - this is just a quick hack to address that. I think we've already > >> agreed that mutable per-instance iommu_ops holding private data aren't > >> particularly pleasant and will (hopefully) go away in the next iteration[1], > >> at which point this all changes anyway. > > > > Do you expect drivers to modify that *priv pointer after the ops > > structure is registered? I'd be very surprised if that was the use > > case. It's fine for the driver to register a non-const version, but > > once it is registered, the infrastructure can treat it as const from > > then on. > > Possibly not - certainly my current port of the ARM SMMU which makes use > of *priv is only ever reading it - although we did also wave around > reasons for mutable ops like dynamically changing the pgsize_bitmap and > possibly even swizzling individual ops for runtime reconfiguration. On > consideration though, I'd agree that things like that are mad enough to > stay well within individual drivers if they did ever happen, and > certainly shouldn't apply to this bit of the infrastructure at any rate. I certainly need to update the pgsize_bitmap at runtime because I don't know the supported page sizes until I've both (a) probed the hardware and (b) allocated page tables for a domain. We've already discussed moving the pgsize_bitmap out of the ops, but moving it somewhere where it remains const doesn't really help. Can I just take the patch that Grant acked, in the interest of getting something merged? As you say, there's plenty of planned changes in this area anyway. I plan to send Olof a pull request this afternoon. Will