Hi Will, On Monday 19 January 2015 11:12:02 Will Deacon wrote: > On Sun, Jan 18, 2015 at 11:18:51AM +0000, Laurent Pinchart wrote: > > On Sunday 18 January 2015 15:54:34 Alexandre Courbot wrote: > >> On 01/16/2015 08:18 AM, Laurent Pinchart wrote: > >>> On Thursday 15 January 2015 11:12:17 Will Deacon wrote: > >>>> On Thu, Jan 15, 2015 at 08:28:44AM +0000, Thierry Reding wrote: > >>>>> On Wed, Jan 14, 2015 at 10:46:10AM +0000, Will Deacon wrote: > >>>>>> On Wed, Jan 14, 2015 at 09:00:24AM +0000, Alexandre Courbot wrote: > >>>>> [...] > >>>>> > >>>>>>> 2) Say you want to use the IOMMU API in your driver, and have an > >>>>>>> iommu property in your device's DT node. If by chance your IOMMU > >>>>>>> is registered early, you will already have a mapping automatically > >>>>>>> created even before your probe function is called. Can this be > >>>>>>> avoided? Is it even safe? > >>>>>> > >>>>>> Currently, I think you have to either teardown the ops manually or > >>>>>> return an error from of_xlate. Thierry was also looking at this > >>>>>> sort of thing, so it might be worth talking to him. > >>>>> > >>>>> I already explained in earlier threads why I think this is a bad > >>>>> idea. It's completely unnatural for any driver to manually tear down > >>>>> something that it didn't want set up in the first place. It also > >>>>> means that you have to carefully audit any users of these IOMMU APIs > >>>>> to make sure that they do tear down. That doesn't sound like a good > >>>>> incremental approach, as evidenced by the breakage that Alex and > >>>>> Heiko have encountered. > >>>> > >>>> Well, perhaps we hide that behind a get_iommu API or something. We > >>>> *do* need this manual teardown step to support things like VFIO, so > >>>> it makes sense to reuse it for other users too imo. > >>>> > >>>>> The solution for me has been to completely side-step the issue and > >>>>> not register the IOMMU with the new mechanism at all. That is, > >>>>> there's no .of_xlate() implementation, which means that the ARM DMA > >>>>> API glue won't try to be smart and use the IOMMU in ways it's not > >>>>> meant to be used. > >>> > >>> That will break when someone will want to use the same IOMMU type for > >>> devices that use the DMA mapping API to hide the IOMMU. That might not > >>> be the case for your IOMMU today, but it's pretty fragile, we need to > >>> fix it. > >>> > >>>>> This has several advantages, such as that I can also use the regular > >>>>> driver model for suspend/resume of the IOMMU, and I get to enjoy the > >>>>> benefits of devres in the IOMMU driver. Probe ordering is still a > >>>>> tiny issue, but we can easily solve that using explicit initcall > >>>>> ordering (which really isn't any worse than IOMMU_OF_DECLARE()). > >>>> > >>>> That's a pity. I'd much rather extend what we currently have to > >>>> satisfy your use-case. Ho-hum. > >>> > >>> Assuming we want the IOMMU to be handled transparently for the > >>> majority of devices I only see two ways to fix this, > >>> > >>> The first way is to create a default DMA mapping unconditionally and > >>> let drivers that can't live with it tear it down. That's what is > >>> implemented today. > >> > >> I strongly support Thierry's point that drivers should not have to tear > >> down things they don't need. The issue we are facing today is a very > >> good illustration of why one should not have to do this. > >> > >> Everybody hates to receive unsollicited email with a link that says "to > >> unsubscribe, click here". Let's not import that unpleasant culture into > >> the kernel. > >> > >> I am arriving late in this discussion, but what is wrong with asking > >> drivers to explicitly state that they want the DMA API to be backed by > >> the IOMMU instead of forcibly making it work that way? > > > > The vast majority of the drivers are not IOMMU-aware. We would thus need > > to add a call at the beginning of the probe function of nearly every > > driver that can perform DMA to state that the driver doesn't need to > > handle any IOMMU that might be present in the system itself. I don't think > > that's a better solution. > > > > Explicitly tearing down mappings in drivers that want to manage IOMMUs > > isn't a solution I like either. A possibly better solution would be to > > call a function to state that the DMA mapping API shouldn't not handle > > IOMMUs. Something like > > > > dma_mapping_ignore_iommu(dev); > > > > at the beginning of the probe function of such drivers could do. The > > function would perform behind the scene all operations needed to tear > > down everything that shouldn't have been set up. > > An alternative would be to add a flag to platform_driver, like we have for > "prevent_deferred_probe" which is something like "prevent_dma_configure".
That's a solution I have proposed (albeit as a struct device_driver field, but that's a small detail), so I'm fine with it :-) > For the moment, that would actually teardown the DMA configuration in > platform_drv_probe, but if things are reordering in future then we can avoid > setting up the ops altogether without an API change. -- Regards, Laurent Pinchart _______________________________________________ iommu mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/iommu
