RCC.constrain eats the RCC

[diondokter]

When calling the constrain function on the RCC, it eats the register. This is highly inconvenient as I want to later enable and disable different peripheral clocks and can't access the register. Now I need to resort to getting the register unsafely which I want to minimize.

Please consider to either give it back when freeze is called or implement the turning on and off of peripheral clocks in the Rcc type.

[therealprof]

When calling the constrain function on the RCC, it eats the register.

Yes, the register is moved into the the Rcc type. And yes, that's on purpose for safety reasons. If you want to enable peripheral clocks, we could add methods on the Rcc to allow doing that.

[maxekman]

See this blog post by @japaric for more info about how and why: https://blog.japaric.io/brave-new-io/#freezing-the-clock-configuration

[diondokter]

See this blog post by @japaric for more info about how and why: https://blog.japaric.io/brave-new-io/#freezing-the-clock-configuration

I agree with the principle, but not all the peripherals are there. When I made this issue, I had a need of turning the CAN clock on. When all the options are accounted for, then it's a great idea!

Also, something else to consider:
When designing a low power battery device, you may want to turn off some clocks to save power. That also isn't possible right now without major work.

[maxekman]

I share the same situation as you; in my case I’m working on enabling the I2S clocks for a HAL proposal. My current experimentation with this crate is similar to a combination of how the peripheral clocks and 48MHz (USB I think) clock is configured. I have added a function called plli2sclk(freq/audio sampling rate). I think that is a pretty decent way to go for now, with a balance of safety and configurability.

[muttering-oldman]

I have the same difficulties as the @diondokter. I need to enable RTC, but "RTC.constrains" eats the register.
I'm new to rust embedded and I do a lot of things by examples. Unfortunately, I have not found the RTC example.
I tried to do it by datasheet but came across that the register doesn't live up to the moment I need it.
I have to get to the bdcr register to enable lse.
It would be great to add work with RTC like in stm32f1xx-hal.

[birkenfeld]

@muttering-oldman note that you can always bypass the HAL by using e.g. unsafe { *RTC::ptr() }. This can be necessary in cases where the HAL doesn't expose the functionality you need.

[muttering-oldman]

Thank you @birkenfeld.
I will try to bypass HAL :)
But still it would be nice to have this feature out of the box.

[ryan-summers]

In the STM32H7xx-hal, we get around this by having the RCC constrain function return a PREC structure, which essentially provides a proxy into the RCC to enable and reset various peripherals.

Reference code: https://github.com/stm32-rs/stm32h7xx-hal/blob/master/src/rcc/rec.rs

[therealprof]

I agree that having some proxy is probably the way to go.