linux-kernel - Re: [PATCH] rust: driver: let probe() return impl PinInit<Self, Error>

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Message-Id: <DDKT6WXI1S4I.30CBHFLJ9Q6CY@kernel.org>
Date: Fri, 17 Oct 2025 20:29:22 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: <gregkh@...uxfoundation.org>, <rafael@...nel.org>,
 <viresh.kumar@...aro.org>, <acourbot@...dia.com>, <ira.weiny@...el.com>,
 <leon@...nel.org>, <daniel.almeida@...labora.com>, <bhelgaas@...gle.com>,
 <kwilczynski@...nel.org>, <abdiel.janulgue@...il.com>,
 <robin.murphy@....com>, <ojeda@...nel.org>, <alex.gaynor@...il.com>,
 <boqun.feng@...il.com>, <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>,
 <lossin@...nel.org>, <a.hindborg@...nel.org>, <tmgross@...ch.edu>,
 <rust-for-linux@...r.kernel.org>, <linux-pci@...r.kernel.org>,
 <linux-pm@...r.kernel.org>, <nouveau@...ts.freedesktop.org>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rust: driver: let probe() return impl PinInit<Self,
 Error>

On Fri Oct 17, 2025 at 3:04 PM CEST, Alice Ryhl wrote:
> On Thu, Oct 16, 2025 at 02:55:28PM +0200, Danilo Krummrich wrote:
>> The driver model defines the lifetime of the private data stored in (and
>> owned by) a bus device to be valid from when the driver is bound to a
>> device (i.e. from successful probe()) until the driver is unbound from
>> the device.
>> 
>> This is already taken care of by the Rust implementation of the driver
>> model. However, we still ask drivers to return a Result<Pin<KBox<Self>>>
>> from probe().
>> 
>> Unlike in C, where we do not have the concept of initializers, but
>> rather deal with uninitialized memory, drivers can just return an
>> impl PinInit<Self, Error> instead.
>> 
>> This contributed to more clarity to the fact that a driver returns it's
>> device private data in probe() and the Rust driver model owns the data,
>> manages the lifetime and - considering the lifetime - provides (safe)
>> accessors for the driver.
>> 
>> Hence, let probe() functions return an impl PinInit<Self, Error> instead
>> of Result<Pin<KBox<Self>>>.
>> 
>> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
>> ---
>> Depends on a minor pin-init patch [1] (Benno will send it to the list
>> soon). A branch with this patch and the pin-init dependency is available
>> in [2].
>> 
>> [1] https://github.com/Rust-for-Linux/pin-init/pull/86/commits
>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/dakr/linux.git/log/?h=probe_return
>
> Overall LGTM.
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>

Thanks!

>>  impl Device<CoreInternal> {
>>      /// Store a pointer to the bound driver's private data.
>> -    pub fn set_drvdata(&self, data: impl ForeignOwnable) {
>> +    pub fn set_drvdata<T: 'static>(&self, data: impl PinInit<T, Error>) -> Result {
>> +        let data = KBox::pin_init(data, GFP_KERNEL)?;
>
> Perhaps the gfp flags should be an argument set_drvdata?

There shouldn't be a need, so I'd rather do that should we find a valid case.