Re: [RFC PATCH v1 4/4] rust: add PL031 RTC driver

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

On Tue Jan 6, 2026 at 3:44 PM CET, Greg Kroah-Hartman wrote:
> On Tue, Jan 06, 2026 at 02:32:56PM +0100, Danilo Krummrich wrote:
>> (Cc: Greg, Rafael)
>> 
>> On Tue Jan 6, 2026 at 3:51 AM CET, Ke Sun wrote:
>> > Following the platform driver implementation, the AMBA driver stores its 
>> > drvdata in amba_device->dev. However,
>> > the RTC driver also stores its drvdata in the parent device (which is 
>> > also amba_device->dev), causing a conflict.
>
> Wait, what?  That's broken in the C implementation, please, let's fix
> that up now first.  drvdata is ONLY for the specific device that driver
> is attached to, it can not be used by anyone else.

The C RTC class device implementation passes its parent device to all its
callbacks, so the driver can obtain the device private data of the parent (bus)
device.

The RTC core does only provide devres guarded registration functions and does
some synchronization in devm_rtc_unregister_device() [1], such that no RTC class
device callbacks can happen after the parent (bus) device is unbound (which is
great).

So, technically it doesn't seem to be broken, but given the rationale in my
previous reply, I dislike that there is no separation of ownership and lifetime
due to not separating bus and class device private data.

[1] https://elixir.bootlin.com/linux/v6.19-rc4/source/drivers/rtc/class.c#L340