linux-kernel - Re: [PATCH 3/4] rust: drm: remove pin annotations from drm::Device

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Message-Id: <DBQY1S7V8IIS.3NCGOZ8ALU7QG@kernel.org>
Date: Fri, 01 Aug 2025 10:21:49 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Benno Lossin" <lossin@...nel.org>
Cc: <lorenzo.stoakes@...cle.com>, <vbabka@...e.cz>,
 <Liam.Howlett@...cle.com>, <urezki@...il.com>, <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>,
 <maarten.lankhorst@...ux.intel.com>, <mripard@...nel.org>,
 <tzimmermann@...e.de>, <airlied@...il.com>, <simona@...ll.ch>,
 <rust-for-linux@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] rust: drm: remove pin annotations from drm::Device

On Thu Jul 31, 2025 at 8:54 PM CEST, Benno Lossin wrote:
> On Thu Jul 31, 2025 at 5:48 PM CEST, Danilo Krummrich wrote:
>> The #[pin_data] and #[pin] annotations are not necessary for
>> drm::Device, since we don't use any pin-init macros, but only
>> __pinned_init() on the impl PinInit<T::Data, Error> argument of
>> drm::Device::new().
>
> But you're still pinning `Device`, right?

A drm::Device instance never exists other than as ARef<drm::Device>.

>> Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
>> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
>> ---
>>  rust/kernel/drm/device.rs | 2 --
>>  1 file changed, 2 deletions(-)
>>
>> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
>> index d19410deaf6c..d0a9528121f1 100644
>> --- a/rust/kernel/drm/device.rs
>> +++ b/rust/kernel/drm/device.rs
>> @@ -54,10 +54,8 @@ macro_rules! drm_legacy_fields {
>>  ///
>>  /// `self.dev` is a valid instance of a `struct device`.
>>  #[repr(C)]
>> -#[pin_data]
>>  pub struct Device<T: drm::Driver> {
>>      dev: Opaque<bindings::drm_device>,
>> -    #[pin]
>>      data: T::Data,
>
> Looking at this code again, I also noticed that it was wrong before this
> patch: `Device<T>` implemented `Unpin` if `T::Data` did which is most
> likely wrong (or is `drm_device` not address sensitive?).

It is, but as mentioned above a drm::Device only ever exists as
ARef<drm::Device>.

So, in drm::Device::new() we allocate the drm::Device with __drm_dev_alloc(),
initialize data in-place within this allocated memory and create an
ARef<drm::Device> directly from the raw pointer returned by __drm_dev_alloc().

> So good to see that fixed, thanks!
>
> ---
> Cheers,
> Benno
>
>>  }
>>