Re: [PATCH RFC 1/4] serdev: Export internal is_serdev_device() for drivers

["Danilo Krummrich" <dakr@kernel.org>]

On Sun Dec 21, 2025 at 5:28 PM CET, Markus Probst wrote:
> On Sun, 2025-12-21 at 17:10 +0100, Greg Kroah-Hartman wrote:
>> 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.
>
> At least in the pwm example, this could/should be circumvented with the
> `AsBusDevice` trait [2].

Yes, class device implementations should start using AsBusDevice, so you don't
need the TryFrom upcast.

> But this technically also applies to all other bus device abstractions
> and they also have a `TryFrom<&device::Device<Ctx>>` implementation, so
> with this it would be consistent across all rust bus device
> abstractions.

It should only be PCI and platform and they have it for a reason ()which is the
one Greg already mentioned, i.e. when you potentially have multiple device types
on a bus).

The concrete use-case that motivated adding those for PCI and platform is when
they interact with the auxiliary bus: Imagine a driver that supports devices
from the platform bus and the PCI bus and exports an auxiliary device.

In this case, when you call back into the parent driver from the auxiliary
device the parent driver has to upcast.

This exact constellation is what is implemented by nova-core and nova-drm.

However, outside of such a constellation we should avoid doing upcasts with
TryFrom and make the correct type available in the first place.

- Danilo