Message-Id: <C7E53051-738B-47CF-9A12-28D683FA0D4F@collabora.com>
Date: Thu, 22 Jan 2026 22:41:19 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Lyude Paul <lyude@...hat.com>
Cc: linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org,
 rust-for-linux@...r.kernel.org,
 Danilo Krummrich <dakr@...nel.org>,
 nouveau@...ts.freedesktop.org,
 Miguel Ojeda <ojeda@...nel.org>,
 Simona Vetter <simona@...ll.ch>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Shankari Anand <shankari.ak0208@...il.com>,
 David Airlie <airlied@...il.com>,
 Benno Lossin <lossin@...nel.org>,
 Asahi Lina <lina+kernel@...hilina.net>
Subject: Re: [PATCH v3 1/3] rust/drm: Introduce DeviceContext

Hi Lyude,

> On 22 Jan 2026, at 19:46, Lyude Paul <lyude@...hat.com> wrote:
> 
> One of the tricky things about DRM bindings in Rust is the fact that
> initialization of a DRM device is a multi-step process. It's quite normal
> for a device driver to start making use of its DRM device for tasks like
> creating GEM objects before userspace registration happens. This is an
> issue in rust though, since prior to userspace registration the device is
> only partly initialized. This means there's a plethora of DRM device
> operations we can't yet expose without opening up the door to UB if the DRM
> device in question isn't yet registered.
> 
> Additionally, this isn't something we can reliably check at runtime. And
> even if we could, performing an operation which requires the device be
> registered when the device isn't actually registered is a programmer bug,
> meaning there's no real way to gracefully handle such a mistake at runtime.
> And even if that wasn't the case, it would be horrendously annoying and
> noisy to have to check if a device is registered constantly throughout a
> driver.
> 
> In order to solve this, we first take inspiration from
> `kernel::device::DeviceContext` and introduce `kernel::drm::DeviceContext`.
> This provides us with a ZST type that we can generalize over to represent
> contexts where a device is known to have been registered with userspace at
> some point in time (`Registered`), along with contexts where we can't make
> such a guarantee (`Uninit`).
> 
> It's important to note we intentionally do not provide a `DeviceContext`
> which represents an unregistered device. This is because there's no
> reasonable way to guarantee that a device with long-living references to
> itself will not be registered eventually with userspace. Instead, we
> provide a new-type for this: `UnregisteredDevice` which can
> provide a guarantee that the `Device` has never been registered with
> userspace. To ensure this, we modify `Registration` so that creating a new
> `Registration` requires passing ownership of an `UnregisteredDevice`.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> 
> ---
> V2:
> * Make sure that `UnregisteredDevice` is not thread-safe (since DRM device
>  initialization is also not thread-safe)
> * Rename from AnyCtx to Uninit, I think this name actually makes a bit more
>  sense.
> * Change assume_registered() to assume_ctx()
>  Since it looks like in some situations, we'll want to update the
>  DeviceContext of a object to the latest DeviceContext we know the Device
>  to be in.
> * Rename Init to Uninit
>  When we eventually add KMS support, we're going to have 3 different
>  DeviceContexts - Uninit, Init, Registered. Additionally, aside from not
>  being registered there are a number of portions of the rest of the Device
>  which also aren't usable before at least the Init context - so the naming
>  of Uninit makes this a little clearer.
> * s/DeviceContext/DeviceContext/
>  For consistency with the rest of the kernel
> * Drop as_ref::<Device<T, Uninit>>() for now since I don't actually think
>  we need this quite yet
> V3:
> * Get rid of drm_dev_ctx!, as we don't actually need to implement Send or
>  Sync ourselves
> * Remove mention of C function in drm::device::Registration rustdoc
> * Add more documentation to the DeviceContext trait, go into detail about
>  the various setup phases and such.
> * Add missing period to comment in `UnregisteredDevice::new()`.
> 
> drivers/gpu/drm/nova/driver.rs |   8 +-
> drivers/gpu/drm/tyr/driver.rs  |  10 +-
> rust/kernel/drm/device.rs      | 179 +++++++++++++++++++++++++++------
> rust/kernel/drm/driver.rs      |  38 +++++--
> rust/kernel/drm/mod.rs         |   4 +
> 5 files changed, 193 insertions(+), 46 deletions(-)
> 

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>

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