Re: [RFC PATCH v3 1/5] rtc: add device selector for rtc_class_ops callbacks

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

On 24/02/2026 01:12:32+0100, Danilo Krummrich wrote:
> Let's have a look at how probe() looks like for the example below, which is what
> we do in other subsystems, such as DRM or PWM.
> 
> > Example 3:
> >
> >	struct SampleIrqData {
> >	    rtc: ARef<rtc::Device>,
> >	};
> >
> > 	// The bus device private data.
> > 	#[pin_data]
> > 	struct SampleDriver {
> > 	    #[pin]
> > 	    irq: irq::Registration<SampleIrqHandler>,
> > 	    rtc: ARef<rtc::Device<SampleRtcData>>,
> > 	}
> >
> > 	// The class device private data.
> > 	struct SampleRtcData {
> > 	    io: Devres<IoMem<PL031_REG_SIZE>>,
> > 	    hw_variant: VendorVariant,
> > 	}
> >
> > 	impl rtc::Ops for MyRtcOps {
> > 	    type BusDeviceType = platform::Device<Bound>;
> >
> > 	    fn read_time(
> > 	        rtc: &rtc::Device<SampleRtcData>
> > 	        parent: &platform::Device<Bound>,
> > 	        time: &mut rtc::Time,
> > 	    ) -> Result {
> > 	        let io = rtc.io.access(parent)?;
> >
> > 	        match rtc.hw_variant {
> > 	            VendorVariant::Arm | VendorVariant::StV1 => {
> > 	                let my_time = io.read(...);
> >
> > 	                my_time.write_into(time);
> > 	            },
> > 	            VendorVariant::StV2 => { ... },
> > 	        }
> > 	    }
> > 	}
> >
> 
> 	impl pci::Driver for SampleDriver {
> 	    fn probe(pdev: &pci::Device<Core>, info: &Self::IdInfo) -> impl PinInit<Self, Error> {
> 	        let dev = pdev.as_ref();
> 
> 	        let rtc_data = impl_pin_init!(SampleRtcData {
> 	            io: iomap_region_sized::<BAR0_SIZE>(0, c"my_rtc/bar0")?,
> 	            hw_variant: VendorVariant::StV1,
> 	        });
> 
> 	        let rtc = rtc::Device::new(dev, rtc_data)?;
> 
> 	        // Internally calls `devres::register(rtc::Registration::new())`.
> 	        rtc::Registration::register(rtc)?;
> 
> 	        Ok(impl_pin_init!(Self {
> 	            // Give the IRQ handler a reference count of the `rtc::Device`.
> 	            irq <- irq::Registration::new(..., rtc.clone()),
> 	            rtc,
> 	        })

I can't really read rust yet but this seems to open a race condition
with userspace if irq::Registration::new(...) fails, there is an
ordering constraint you missed.

> 	    }
> 	}
> 
> With this there are no (subtle) ordering constraints the driver has to get
> right; ownership and lifetimes are well defined.
> 
> (I.e. whatever order a driver picks, it either works properly or it does not
> compile in the first place, which is a huge improvement over the situation we
> have in C.)

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com