linux-kernel - Re: [RFC PATCH 0/3] gpu: nova-core: add basic timer subdevice implementation

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Message-ID: <Z75riltJo0WvOsS5@cassiopeiae>
Date: Wed, 26 Feb 2025 02:16:58 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Joel Fernandes <joelagnelf@...dia.com>,
	Alexandre Courbot <acourbot@...dia.com>,
	Dave Airlie <airlied@...il.com>, Gary Guo <gary@...yguo.net>,
	Joel Fernandes <joel@...lfernandes.org>,
	Boqun Feng <boqun.feng@...il.com>,
	John Hubbard <jhubbard@...dia.com>, Ben Skeggs <bskeggs@...dia.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	paulmck@...nel.org
Subject: Re: [RFC PATCH 0/3] gpu: nova-core: add basic timer subdevice
 implementation

On Tue, Feb 25, 2025 at 08:49:16PM -0400, Jason Gunthorpe wrote:
> I'm pointing out the fundamental different in approachs. The typical
> widely used pattern results in __device_release_driver() completing
> with no concurrent driver code running.

Typically yes, but there are exceptions, such as DRM.

> 
> DRM achieves this, in part, by using drm_dev_unplug().

No, DRM can have concurrent driver code running, which is why drm_dev_enter()
returns whether the device is unplugged already, such that subsequent
operations, (e.g. I/O) can be omitted.

> 
> The Rust approach ends up with __device_release_driver() completing
> and leaving driver code still running in other threads.

No, this has nothing to do with Rust device / driver or I/O abstractions.

It entirely depends on the driver you implement. If you register a DRM device,
then yes, there may be concurrent driver code running after
__device_release_driver() completes. But this is specific to the DRM
implementation, *not* to Rust.

Again, the reason a pci::Bar needs to be revocable in Rust is that we can't have
the driver potentially keep the pci::Bar alive (or even access it) after the
device is unbound.

A driver can also be unbound without the module being removed, and if the driver
would be able to keep the pci::Bar alive, it would mean that the resource region
is not freed and the MMIO mapping is not unmapped, because the resource region
and the MMIO mapping is bound to the lifetime of the pci::Bar object. This would
not be acceptable for a Rust driver.

That this also comes in handy for subsystems like DRM, where we could have
attempts to access to the pci::Bar object after the device is unbound by design,
can be seen as a nice side effect.