Allow users to easily name DMA channels

[Dominaezzz]

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Pull Request Details 📖

Description

Fixes part of #1767.

With this PR users can now consistently name the dma channel in use in a driver.

let i8080: I8080<_, GdmaCh<0>, _>;
let cam: Camera<_, GdmaCh<2>, _>;
let spi: Spi<_, GdmaCh<4>, _>;
let spi: Spi<_, Spi2DmaCh, _>;

This PR introduces a public trait called DmaChannel which describes the RX and TX parts of a channel in associated types.

The existing ChannelTypes trait which is really similar just didn't cut it (maybe a skill issue). With this PR at least, the "private but public for technical reasons" traits Tx, TxPrivate, Rx and RxPrivate can be removed and ChannelTx and ChannelRx can just have pub(crate) methods on it.

There will be a follow up PR to simplify the InstanceDma trait (based on this) in the SPI driver, which was my main motive for making this PR.

Testing

No actual code changes, I just shuffled the generics about so CI should be fine.

[bugadani]

This is totally bike-shedding, but to be honest, I'm not sure how, as a user, I would react seeing a type like GdmaCh. I would probably prefer not renaming Channel<N>, but instead renaming the other, shadowing Channel. That being said, the ChannelN type aliasses are probably better - if they were instead newtypes, they would show up in rustdoc and it would be obvious how many channels users may choose from.

[Dominaezzz]

I added Gdma to the name because Channel is a common type in the repo and it make imports a little more tedious in the IDE. Thought this is something that can be fixed with an import alias.
I also wasn't sure what I would rename the shadowing Channel type to if I went that direction instead. Adding "Gdma" felt straightforward.

The type aliases kinda suck for discoverability like you said. The DmaChannel implementations are meant to be the new types at least.

Having said that, I don't care about the names (someone else can decide), I just want the solution to not use type aliases, it should be consistent across drivers and it shouldn't get in the way of splitting a channel into an Rx half and tx half.

[bjoernQ]

I also think names like GdmaCh are not ideal (but I see why just Channel is a bit too generic). Maybe something like DmaChannel as the user-facing name?

Maybe having DmaChannelN new-types in addition would make sense since I agree that it would make it easier to know how many channels are available

[Dominaezzz]

Alright I'll replace the const generic with a simple type.
GdmaCh<0> -> DmaChannel0.

Spi1DmaCh, Spi2DmaCh, I2S1DmaCh stay the same?

[bjoernQ]

Spi1DmaCh, Spi2DmaCh, I2S1DmaCh stay the same?

Probably naming them like Spi1DmaChannel, Spi2DmaChannel etc. looks more consistent since those are the ones which are actually used in user code - but would be probably good to hear another opinion on this

[bjoernQ]

LGTM - Thanks!

[JurajSadel]

LGTM, thanks!