On Wed, Apr 02, 2025 at 12:05:56AM +0000, Benno Lossin wrote: > On Tue Apr 1, 2025 at 3:51 PM CEST, Danilo Krummrich wrote: > > On Mon, Mar 24, 2025 at 06:32:53PM +0000, Benno Lossin wrote: > >> On Mon Mar 24, 2025 at 7:13 PM CET, Danilo Krummrich wrote: > >> > On Mon, Mar 24, 2025 at 05:36:45PM +0000, Benno Lossin wrote: > >> >> On Mon Mar 24, 2025 at 5:49 PM CET, Danilo Krummrich wrote: > >> >> > On Mon, Mar 24, 2025 at 04:39:25PM +0000, Benno Lossin wrote: > >> >> >> On Sun Mar 23, 2025 at 11:10 PM CET, Danilo Krummrich wrote: > >> >> >> > On Sat, Mar 22, 2025 at 11:10:57AM +0100, Danilo Krummrich wrote: > >> >> >> >> On Fri, Mar 21, 2025 at 08:25:07PM -0700, Greg KH wrote: > >> >> >> >> > Along these lines, if you can convince me that this is something that we > >> >> >> >> > really should be doing, in that we should always be checking every time > >> >> >> >> > someone would want to call to_pci_dev(), that the return value is > >> >> >> >> > checked, then why don't we also do this in C if it's going to be > >> >> >> >> > something to assure people it is going to be correct? I don't want to > >> >> >> >> > see the rust and C sides get "out of sync" here for things that can be > >> >> >> >> > kept in sync, as that reduces the mental load of all of us as we travers > >> >> >> >> > across the boundry for the next 20+ years. > >> >> >> >> > >> >> >> >> I think in this case it is good when the C and Rust side get a bit > >> >> >> >> "out of sync": > >> >> >> > > >> >> >> > A bit more clarification on this: > >> >> >> > > >> >> >> > What I want to say with this is, since we can cover a lot of the common cases > >> >> >> > through abstractions and the type system, we're left with the not so common > >> >> >> > ones, where the "upcasts" are not made in the context of common and well > >> >> >> > established patterns, but, for instance, depend on the semantics of the driver; > >> >> >> > those should not be unsafe IMHO. > >> >> >> > >> >> >> I don't think that we should use `TryFrom` for stuff that should only be > >> >> >> used seldomly. A function that we can document properly is a much better > >> >> >> fit, since we can point users to the "correct" API. > >> >> > > >> >> > Most of the cases where drivers would do this conversion should be covered by > >> >> > the abstraction to already provide that actual bus specific device, rather than > >> >> > a generic one or some priv pointer, etc. > >> >> > > >> >> > So, the point is that the APIs we design won't leave drivers with a reason to > >> >> > make this conversion in the first place. For the cases where they have to > >> >> > (which should be rare), it's the right thing to do. There is not an alternative > >> >> > API to point to. > >> >> > >> >> Yes, but for such a case, I wouldn't want to use `TryFrom`, since that > >> >> trait to me is a sign of a canonical way to convert a value. > >> > > >> > Well, it is the canonical way to convert, it's just that by the design of other > >> > abstractions drivers should very rarely get in the situation of needing it in > >> > the first place. > >> > >> I'd still prefer it though, since one can spot a > >> > >> let dev = CustomDevice::checked_from(dev)? > >> > >> much better in review than the `try_from` conversion. It also prevents > >> one from giving it to a generic interface expecting the `TryFrom` trait. > > > > (I plan to rebase this on my series introducing the Bound device context [1].) > > > > I thought about this for a while and I still think TryFrom is fine here. > > What reasoning do you have? The concern in terms of abuse is that one could try to randomly guess the "outer" device type (if any), which obiously indicates a fundamental design issue. But that's not specific to devices; it is a common anti-pattern in OOP to randomly guess the subclass type of an object instance. So, I don't think the situation here is really that special such that it needs an extra highlight. > > At some point I want to replace this implementation with a macro, since the code > > is pretty similar for bus specific devices. I think that's a bit cleaner with > > TryFrom compared to with a custom method, since we'd need the bus specific > > device to call the macro from the generic impl, i.e. > > > > impl<Ctx: DeviceContext> Device<Ctx> > > > > rather than a specific one, which we can't control. We can control it for > > TryFrom though. > > We could have our own trait for that. I don't think we should have a trait specific for devices for this. If we really think the above anti-pattern deserves special attention, then we should have a generic trait (e.g. FromSuper<T>) instead. But I'm not sure that we really need to put special attention on that.
