Hi Thierry, On Mon, Jul 14, 2014 at 07:44:53AM +0100, Thierry Reding wrote: > On Sun, Jul 13, 2014 at 10:43:41AM +0100, Will Deacon wrote: > > My plan for the ARM SMMU driver is: > > > > (1) Change ->probe() to walk the device-tree looking for all masters with > > phandles back to the SMMU instance being probed > > You and Rob mentioned this several times and I don't understand why the > SMMU needs to know all masters up front. Is this necessary because it > needs to program all registers at .probe() time and they can't be > reprogrammed subsequently? Or is this just some kind of optimization? It's an optimization to reduce resource usage in the IOMMU, but one that is *required* by certain platforms (e.g. Olav mentioned his 43-ID master and Calxeda had something similar). Basically, in order to perform an ->attach() for a master to a domain, we need complete knowledge of the system so that we can avoid accidentally attaching other masters to the same domain. The programming is done using a form of StreamID wildcarding, so at this point we would need to have parsed the entire DT to ensure our wildcard doesn't match other masters. > > (2) For each master, extract the Stream IDs and add them to the internal > > SMMU driver data structures (an rbtree per SMMU instance). For > > hotpluggable buses, we'll need a way for the bus controller to > > reserve a range of IDs -- this will likely be a later extension to > > the binding. > > > > (3) When we get an ->add() call, warn if it's a device we haven't seen > > and reject the addition. > > It seems to me like this would be the logical place to parse stream IDs. We could do that only if we were guaranteed to have an ->add() call for *every* master before an ->attach() call for *any* master. I don't think that is necessarily true. > You could for example have a case where some device tree contains a node > for which no driver will ever be loaded (for example because it hasn't > been built-in, or the device is never used and the module is therefore > never loaded). That's a situation that you cannot determine by simply > walking the device tree in the IOMMU's .probe(). Why not? If we're simply searching for phandles to the IOMMU, why does it matter whether a driver is bound to the master? > I've always thought about IOMMU masters much in the same way as other > types of resources, such as memory or interrupts. In the rest of the > kernel we do carefully try to postpone allocation of these resources > until they are required, specifically so we don't waste resources when > they're unused. See above, they are all required the moment anybody tries an ->attach(). > That's also one of the reasons why I think associating an IOMMU with the > bus type is bad. Currently if an IOMMU driver thinks it should enable > translation for a given device, then there's no way for that device's > driver to opt out again. There may be reasons (performance, hardware > bugs, ...) for the driver to decide against using the IOMMU for > translation, but there's currently no way to do that if the IOMMU driver > disagrees. Yes, we need a way to associate an IOMMU with a bus instance, but I think that's a separate topic, no? Will -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
