Remove aliasing &mut references to DMA buffers (possible UB)

[ErikNatanael]

Getting the "warning: creating a mutable reference to mutable static is discouraged" warning prompted me to look into the code in audio.rs. Changing

let tx_buffer: &'static mut [u32; DMA_BUFFER_SIZE] = unsafe { &mut TX_BUFFER };

into

let tx_buffer: &'static mut [u32; DMA_BUFFER_SIZE] = unsafe { &mut *core::ptr::addr_of_mut!(TX_BUFFER) };

gets rid of the error, but I'm not convinced this is not undefined behaviour. The reason being that multiple &mut references are created to both TX_BUFFER and RX_BUFFER, and used via Input and Output.

This pull request gets around the issue by wrapping a raw pointer to the buffer instead of storing the references directly. *mut pointers are allowed to alias. References are created whenever the buffer is accessed, but these are short lived.

I have verified this works in practice: https://github.com/ErikNatanael/daisy-blank/tree/example-sine-wave

The first commit is the changes in PR #40

[mtthw-meyer]

I need to look into the correct way to do this myself and see what I dig up

[mtthw-meyer]

Not seeing anything better. The example in stm32h7xx-hal was updated but still has this warning.

[ErikNatanael]

Thanks for taking a look. "anything better" about or than what do you mean? Sorry, don't quite understand.

I had a look at the stm32h7xx-hal dma example which uses MaybeUninit as well as core::ptr::addr_of_mut!. That might be better. But that example doesn't store multiple &mut references to the buffers, it sends them via raw pointers, so I still think using raw pointers is required instead of multiple &mut to avoid UB.

Since I wrote this code, the new &raw mut syntax was also stabilized, equivalent to core::ptr::addr_of_mut! IIUC.

[mtthw-meyer]

I tried using the MaybeUninit from the stm32h7xx-hal crate but it still gave the same warning with the assume_init() method. Raw pointers seems correct but not sure exactly how to use them yet. Maybe a combination of the two. Use Maybeuninit to handle the Init (does it even need to be initialized for DMA?) and then convert to the &raw mut

[mtthw-meyer]

Cleans up nicely with
let tx_buffer = DmaBufferRawRef { ptr: &raw mut TX_BUFFER, };

[mtthw-meyer]

Need a similar wrapper for usb_midi and we can merge it.

[ErikNatanael]

Great, I'll get on it later this week!

[ErikNatanael]

So UsbBus::new requires a &'static mut so &raw mut cannot be used. Until they change their API, I suppressed the warning and added a comment documenting that any additional references to the EP_MEMORY is UB. Also fixed some other simple warnings while I was at it.