Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait

[Asahi Lina <lina@asahilina.net>]

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.

~~ Lina