linux-kernel - Re: [PATCH v4 2/3] rust: pci: impl TryFrom<&Device> for &pci::Device

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Message-ID: <Z-z-M3g3FFz2HbrR@pollux>
Date: Wed, 2 Apr 2025 11:06:59 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Greg KH <gregkh@...uxfoundation.org>, bhelgaas@...gle.com,
	rafael@...nel.org, ojeda@...nel.org, alex.gaynor@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] rust: pci: impl TryFrom<&Device> for &pci::Device

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.