Messages in this thread | | | Date | Fri, 24 Feb 2023 14:32:47 +0000 | Subject | Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait | From | Robin Murphy <> |
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On 2023-02-24 14:11, 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 :)
Cheers, Robin.
> So the two subsystems you want to call from rust code don't properly > handle the reference count of the object you are going to pass into it, > and only need it for debugging and iommu stuff, which is really only the > bus that the device is on, not good examples to start out with :) > > Yeah, this is yack-shaving, sorry, but it's how we clean up core > subsystems for apis and implementations that are not really correct and > were not noticed at the time. > > Can we see some users of this code posted so I can see how struct device > is going to work in a rust driver? That's the thing I worry most about > the rust/C interaction here as we have two different ways of thinking > about reference counts from the two worlds and putting them together is > going to be "interesting", as can be seen here already. > > thanks, > > greg k-h
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