linux-kernel - Re: [PATCH RFC 1/4] serdev: Export internal is_serdev

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Message-ID: <2025122145-from-monetize-28e2@gregkh>
Date: Sun, 21 Dec 2025 17:46:53 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Markus Probst <markus.probst@...teo.de>
Cc: Rob Herring <robh@...nel.org>, Jiri Slaby <jirislaby@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Kari Argillander <kari.argillander@...il.com>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH RFC 1/4] serdev: Export internal is_serdev_device() for
 drivers

On Sun, Dec 21, 2025 at 04:28:11PM +0000, Markus Probst wrote:
> On Sun, 2025-12-21 at 17:10 +0100, Greg Kroah-Hartman wrote:
> > On Sat, Dec 20, 2025 at 06:44:05PM +0000, Markus Probst wrote:
> > > From: Kari Argillander <kari.argillander@...il.com>
> > > 
> > > The serdev core has an internal is_serdev_device() helper, but it was
> > > not accessible to drivers. Make it public by declaring it in serdev.h
> > > and exporting the symbol so that modular serdev drivers can rely on it
> > > instead of duplicating type checks.
> > > 
> > > This allows example future Rust serdev abstraction to have
> > > 
> > >     TryFrom<&device::Device<Ctx>> for &serdev::Device<Ctx>
> > 
> > But why is that going to be needed?
> > 
> > > That way using bus is easy for other substystems. Also some other
> > > subsystems expose similar function:
> > > 
> > >   - bool is_usb_device(const struct device *dev)
> > >   - bool dev_is_pci(const struct device *dev)
> > 
> > Yes, and usually that's not a good idea, unless you have a bus with
> > multiple types of devices on it.  I don't think serdev has that, does
> > it?
> > 
> > Only under special circumstances should this be required, so I'm curious
> > as to why you would ever have a pointer to a struct device and not
> > "know" that it is of this type?  Who is passing that to you?
> For example, the pwm rust abstraction currently only provides a
> `device::Device<device::Bound>` reference in callbacks [1]. If we want
> to write data to the serial device in one of the pwm callbacks, we need
> to convert the `device::Device<device::Bound>` reference to
> `serdev::Device<device::Bound>`. The TryFrom implementation provides a
> *safe* abstraction.

While I like the feeling of *safe* you can mark it as *safe* as the
driver core can NOT give you a callback of a pointer that is not of that
type.

That's how the C code works today, and is why the C code does not need
this function exported.  The rust code should do the same thing.

thanks,

greg k-h