From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-10631.protonmail.ch (mail-10631.protonmail.ch [79.135.106.31]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EB7B5184; Wed, 2 Apr 2025 00:06:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=79.135.106.31 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743552371; cv=none; b=QcDahxYwj6GRE7Qr1f8grB2LXzBO+fMJVlelvZWbOT2czpbLBs075kcUpSJLHEZPv/uiBTLX4EbOqCWWbc8G2taKMP9MXUwgf2abOcmGkQXFl0OOddvztoc8jVLAX7BNmi5DB4ff988ldmFA0QRKNgqwXiUtk1hHseTu4hi4Ntk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743552371; c=relaxed/simple; bh=SKvTKbz6ni8FnmJ1GP4IVeexdsrRnOKxcqNOXAHpVHk=; h=Date:To:From:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=Bp3C+5Qvends5+zSb/IUUNONQsSCW+NPVjUVLycHNqYLlAECeYRU8GEhugKW5aLRLlfx23H1WtI9eB2ErER3LvQw8+d3FTdXmkFDkxyCkYWUQHbWB9NYFvpPqb0PHA2+Z4uTBUmLfj8l5fdftWeOIFx5X+6de1QVP4IhJFgT0+o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=proton.me; spf=pass smtp.mailfrom=proton.me; dkim=pass (2048-bit key) header.d=proton.me header.i=@proton.me header.b=TDpZp3n0; arc=none smtp.client-ip=79.135.106.31 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=proton.me Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=proton.me Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=proton.me header.i=@proton.me header.b="TDpZp3n0" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=proton.me; s=36zyj7lnbbclrifsrasst7ba2i.protonmail; t=1743552360; x=1743811560; bh=GG1TKNw1ad56XFapM4VnVdkZu9HjKmQ/qsMjqEKVOoA=; h=Date:To:From:Cc:Subject:Message-ID:In-Reply-To:References: Feedback-ID:From:To:Cc:Date:Subject:Reply-To:Feedback-ID: Message-ID:BIMI-Selector:List-Unsubscribe:List-Unsubscribe-Post; b=TDpZp3n0Ds4RNhIfHGagrDTveMMpe6uFvDjgRfJW4QFU2AFnVNvahozjVSxIgaVCj lZKY2rApK5LUIHTR8OJaZVDUIhMi3MC3DOLvT+fFmEkzDdLAGyQnuX004HiW7mJrmt CF9Rk8hYubo4zj9vMn7NwiiLGInLknCqMiUWsBT7ruznXUAudQ4YZ80aA6J64V8M8B EPbLuHUJt/pXPreRUg+zb4nBqbydtQfbertYomZltWa1YNHjAfXAama51ey5UZ6tjZ 8BmTdHfeTEzWQyp72D1Wx+TVP+W2gmU9W7OVO4OhrGRcdngCh1Y1hWw9AmmfP0EAhr 24A0eUfyPf8wg== Date: Wed, 02 Apr 2025 00:05:56 +0000 To: Danilo Krummrich From: Benno Lossin Cc: Greg KH , bhelgaas@google.com, rafael@kernel.org, ojeda@kernel.org, alex.gaynor@gmail.com, boqun.feng@gmail.com, gary@garyguo.net, bjorn3_gh@protonmail.com, a.hindborg@kernel.org, aliceryhl@google.com, tmgross@umich.edu, linux-pci@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 2/3] rust: pci: impl TryFrom<&Device> for &pci::Device Message-ID: In-Reply-To: References: <20250321214826.140946-1-dakr@kernel.org> Feedback-ID: 71780778:user:proton X-Pm-Message-ID: eaab470bc2e61cf080048f1271cbbfb5ec48b355 Precedence: bulk X-Mailing-List: linux-pci@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable 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 someth= ing 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 valu= e 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 th= at 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. >> >> >> >>=20 >> >> >> >> 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 no= t 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. >> >> >>=20 >> >> >> I don't think that we should use `TryFrom` for stuff that should o= nly 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 h= ave to >> >> > (which should be rare), it's the right thing to do. There is not an= alternative >> >> > API to point to. >> >>=20 >> >> Yes, but for such a case, I wouldn't want to use `TryFrom`, since tha= t >> >> 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 needin= g it in >> > the first place. >>=20 >> I'd still prefer it though, since one can spot a >>=20 >> let dev =3D CustomDevice::checked_from(dev)? >>=20 >> 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? > At some point I want to replace this implementation with a macro, since t= he 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 specifi= c > device to call the macro from the generic impl, i.e. > > =09impl Device > > 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. Also it's not as controllable as you think: anyone can implement `TryFrom<&device::Device> for &MyType`. > However, I also do not really object to your proposal, hence I'm willing = to make > the change. > > Do you want to make a proposal for the corresponding doc comment switchin= g to a > custom method? I think have too little context what `device::Device` and `pci::Device` are. But I can give it a try: /// Tries to converts a generic [`Device`](device::Device) into a [`pci= ::Device`]. /// /// Normally, one wouldn't need to call this function, because APIs sho= uld directly expose the /// concrete device type. Then I think another sentence about a valid use-case of this function would make a lot of sense, but I don't know any. --- Cheers, Benno