linux-kernel - Re: [PATCH v2 1/4] rust: device: implement Device::parent()

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Message-ID: <Z90rlKC_S5WQzO8P@pollux>
Date: Fri, 21 Mar 2025 10:04:20 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: bhelgaas@...gle.com, rafael@...nel.org, ojeda@...nel.org,
	alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net,
	bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] rust: device: implement Device::parent()

On Thu, Mar 20, 2025 at 06:40:56PM -0700, Greg KH wrote:
> On Thu, Mar 20, 2025 at 11:27:43PM +0100, Danilo Krummrich wrote:
> > Device::parent() returns a reference to the device' parent device, if
> > any.
> 
> Ok, but why?  You don't use it in this series, or did I miss it?

Indeed, it should rather be at the auxbus series.

> A driver shouldn't care about the parent of a device, as it shouldn't
> really know what it is.  So what is this needed for?

Generally, that's true. I use in the auxbus example and in nova-drm [1] (which
is connected through the auxbus to nova-core) to gather some information about
the device managed by nova-core.

Later on, since that's surely not enough, we'll have an interface to nova-core
that takes the corresponding auxiliary device. nova-core can then check whether
the given device originates from nova-core, and through the parent find its own
device.

[1] https://gitlab.freedesktop.org/drm/nova/-/blob/staging/nova-drm/drivers/gpu/drm/nova/driver.rs