On 7/24/22 02:38, Marc Zyngier wrote: > On Fri, 22 Jul 2022 18:17:06 +0100, > Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: >> >> On Fri, Jul 22, 2022 at 06:06:07PM +0100, Marc Zyngier wrote: >>> On Fri, 22 Jul 2022 15:39:05 +0100, >>> Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: >>>> >>>> [+cc Marc, can you clarify when we need irq_dispose_mapping()?] >>> >>> In general, interrupt controllers should not have to discard mappings >>> themselves, just like they rarely create mappings themselves. That's >>> usually a different layer that has created it (DT, for example). >>> >>> The problem is that these mappings persist even if the interrupt has >>> been released by the driver (it called free_irq()), and the IRQ number >>> can be further reused. The client driver could dispose of the mapping >>> after having released the IRQ, but nobody does that in practice. >>> >>> From the point of view of the controller, there is no simple way to >>> tell when an interrupt is "unused". And even if a driver was >>> overzealous and called irq_dispose_mapping() on all the possible >>> mappings (and made sure no mapping could be created in parallel), this >>> could result in a bunch of dangling pointers should a client driver >>> still have the interrupt requested. >>> >>> Fixing this is pretty hard, as IRQ descriptors are leaky (you can >>> either have a pointer to one, or just an IRQ number -- they are >>> strictly equivalent). So in general, being able to remove an interrupt >>> controller driver is at best fragile, and I'm trying not to get more >>> of this in the tree. >> >> Thank you! >> >> How do we identify an interrupt controller driver? Apparently some of >> these PCIe controller drivers also include an interrupt controller >> driver, but I don't know what to look for to find them. > > If you see a struct irq_chip somewhere, this is an interrupt > controller. And yes, most of the PCIe RC drivers will have some sort > of interrupt controller driver for INTx support, as well as MSI when > the RC doesn't/cannot rely on the platform providing one. > > It means that these PCIe RC drivers probably shouldn't be removable if > built as modules. Which I don't think is a big problem. You want > modularity to reduce the size of the kernel image and only load the > drivers the platform actually requires, saving memory in the process. > And for something as fundamental as an interrupt controller (and PCIe > in general), you probably want to keep it around for the lifetime of > the machine. No disagreement, however being able to fully remove and load the module again ensures that you bring the hardware and software in a sane state every time, i.e.: it does help find actual bugs in either implementations. It's also a faster turn around time if you are working on that specific subsystem in avoid rebooting the kernel needlessly (that puts a lot of faith into not crashing the kernel, still). -- Florian
