Messages in this thread | | | Date | Fri, 24 Feb 2023 15:14:11 +0000 | Subject | Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait | From | Robin Murphy <> |
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On 2023-02-24 14:48, Asahi Lina wrote: > > > On 2023/02/24 23:32, Robin Murphy wrote: >> On 2023-02-24 14:11, Greg Kroah-Hartman wrote: >>> Thanks for the detailed rust explainations, I'd like to just highlight >>> one thing: >>> >>> On Fri, Feb 24, 2023 at 10:15:12PM +0900, Asahi Lina wrote: >>>> On 24/02/2023 20.23, Greg Kroah-Hartman wrote: >>>>> And again, why are bindings needed for a "raw" struct device at all? >>>>> Shouldn't the bus-specific wrappings work better? >>>> >>>> Because lots of kernel subsystems need to be able to accept "any" device >>>> and don't care about the bus! That's what this is for. >>> >>> That's great, but: >>> >>>> All the bus >>>> wrappers would implement this so they can be used as an argument for all >>>> those subsystems (plus a generic one when you just need to pass around >>>> an actual owned generic reference and no longer need bus-specific >>>> operations - you can materialize that out of a RawDevice impl, which is >>>> when get_device() would be called). That's why I'm introducing this now, >>>> because both io_pgtable and rtkit need to take `struct device` pointers >>>> on the C side so we need some "generic struct device" view on the >>>> Rust side. >>> >>> In looking at both ftkit and io_pgtable, those seem to be good examples >>> of how "not to use a struct device", so trying to make safe bindings >>> from Rust to these frameworks is very ironic :) >>> >>> rtkit takes a struct device pointer and then never increments it, >>> despite saving it off, which is unsafe. It then only uses it to print >>> out messages if things go wrong (or right in some cases), which is odd. >>> So it can get away from using a device pointer entirely, except for the >>> devm_apple_rtkit_init() call, which I doubt you want to call from rust >>> code, right? >>> >>> for io_pgtable, that's a bit messier, you want to pass in a device that >>> io_pgtable treats as a "device" but again, it is NEVER properly >>> reference counted, AND, it is only needed to try to figure out the bus >>> operations that dma memory should be allocated from for this device. So >>> what would be better to save off there would be a pointer to the bus, >>> which is constant and soon will be read-only so there are no lifetime >>> rules needed at all (see the major struct bus_type changes going into >>> 6.3-rc1 that will enable that to happen). >> >> FWIW the DMA API *has* to know which specific device it's operating >> with, since the relevant properties can and do vary even between >> different devices within a single bus_type (e.g. DMA masks). >> >> In the case of io-pgtable at least, there's no explicit refcounting >> since the struct device must be the one representing the physical >> platform/PCI/etc. device consuming the pagetable, so if that were to >> disappear from underneath its driver while the pagetable is still in >> use, things would already have gone very very wrong indeed :) > > There's no terribly good way to encode this relationship in safe Rust as > far as I know. So although it might be "obvious" (and I think my driver > can never violate it as it is currently designed), this means the Rust > abstraction will have to take the device reference if the C side does > not, because safe rust abstractions have to actually make these bugs > impossible and nothing stops a Rust driver from, say, stashing an > io_pgtable reference into a global and letting the device go away.
If someone did that, then simply holding a struct device reference wouldn't guarantee much, since it only prevents the pointer itself from becoming invalid - it still doesn't say any of the data *in* the structure is still valid and "safe" for what a DMA API call might do with it.
At the very least you'd probably have to somehow also guarantee that the device has a driver bound (which is the closest thing to a general indication of valid DMA ops across all architectures) and block it from unbinding for the lifetime of the reference, but that would then mean a simple driver which expects to tear down its io-pgtable from its .remove callback could never be unbound due to the circular dependency :/
Cheers, Robin.
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