Thierry Reding <thierry.reding@xxxxxxxxx> wrote @ Fri, 25 Oct 2013 11:11:05 +0200: > > > This is actually "the other problem that I'm aware of that could benefit > > > from [interrupt resolution at probe time]". My idea was that once we had > > > a way within the driver core to resolve interrupt references at probe > > > time it could be used for potentially many other resources as well. Some > > > of the resources like GPIOs and regulators are obviously not something > > > that the core can or should be requesting, but mostly resources that you > > > don't actually need to control after probing (such as interrupts) would > > > be a good fit because otherwise people would write the same boilerplate > > > over and over again. > > > > > > IOMMUs seem to me to be in that same category. As far as I can tell, an > > > IOMMU driver registers the IOMMU for a given bus, upon which every > > > device can simply be used (mostly transparently) with that IOMMU. While > > > I haven't figured out how exactly, I'm pretty sure we can take advantage > > > of the resolution of resources at probe time within the core to both > > > keep drivers from having to do anything in particular and at the same > > > time have code to determine if the IOMMU driver hasn't been probed yet > > > and return -EPROBE_DEFER appropriately. > > > > Can you explain the above a bit more? > > > > Originally I thought that what Grant suggested would work ok with this > > patch. > > I think the objection to these patches is that they special case the > instantiation of some devices. It's not a proper solution because it > implies various things. For example merely instantiating the IOMMU > device earlier than others is only going to work *if* the driver is > actually probed before the drivers of other devices. If you want to > build the driver as a module for example, probe order becomes entirely > non-deterministic. I understand the above limitation. What I thought for the solution is that I can make use of iommu_bus even before IOMMU is instanciated. iommu_bus has its notifier which calls iommu_ops()->add_device(). This could return -EPROBE_DEFER when IOMMU isn't ready. Only the problem is the current "bus_notifier" doesn't have the ability to return error. I'll see if it can be modified. Even with this, at least IOMMU *driver* needs to be init'ed enough earlier to prevent devices from being registered without IOMMU. > Instead of handling such dependencies implicitly by making sure all > resource providers are probed earlier than any of their consumers, the > dependencies are handled more explicitly, which turns out to simplify > things a lot. There's some additional work required in the core, but if > done consistently no driver needs to care about the dependencies and it > no longer matters where the resources come from. The problem is reduced > to essentially this: > > while (!resource_available()) > load_more_drivers(); > > So my proposed solution for the IOMMU case is to treat it the same as > any other resources. Perhaps resource isn't the right word, but at the > core the issue is the same. A device requires the services of an IOMMU > so that it can be put into the correct address space. If the IOMMU is > not available yet it cannot do that, so we simply return -EPROBE_DEFER > and cause the probe to be retried later. This looks somewhat similar to the above iommu_bus notifier. Is there any way to implement the same mechanism rather than using bus? -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
