Allow dma::Transfer.peripheral() for serial::Rx

[qwerty19106]

• Impl Transfer.is_idle() and Transfer.is_rx_not_empty() for serial::Rx and uart::Rx.
• Relax trait bounds this methods.

See #586 for details.

[qwerty19106]

It is ready for review.
I will add serial-dma-rx.rs a little later.

[qwerty19106]

I add rtic-serial-dma-rx-idle example

[burrbull]

I'm not happy in existence of fn peripheral which was introduced in #396 as it potentially break API safety.
I still think this was a mistake.

cc @therealprof your comments?

[qwerty19106]

The fn peripheral problem seems to be delayed.
I can remove fn peripheral part of this PR to expedite the acceptance of this PR.
@burrbull

[rursprung]

FYI: i had introduced the API you've marked as deprecated in #556 (see #550 for background). the original ticket also describes why i did this with a ReadBuffer instead of a WriteBuffer (it was a read use-case for me). if there was something wrong with that then i'm sorry (i'm absolutely no expert on STM32, HALs or this specific repo). but it'd just be good if the use-case i described there will still work one way or the other with a clean API afterwards.

[qwerty19106]

@rursprung
A WriteBuffer is

Trait for buffers that can be given to DMA for writing.

Moreover the WriteBuffer::write_buffer method takes mutable reference unlike ReadBuffer:

unsafe fn write_buffer(&self) -> (*mut Self::Word, usize);

Thus DMA read word from peripheral and write to WriteBuffer.
We should use WriteBuffer for PeripheralToMemory, and ReadBuffer for MemoryToPeripheral.

Probably your example compiles with this bug because your buffer implements both WriteBuffer and ReadBuffer.

[burrbull]

Maybe it is better to implement RxISR for Transfer instead of using inherent methods.

[qwerty19106]

I removed the controversial peripheral() code and add RxISR implementation for Transfer.

[burrbull]

bors r+

[bors]

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